Select the size of the bike wheels: 26 “, 28” or 29 “?
One of the most important parts of a bicycle is its wheels. over, it is their size that matters more than weight. There was a time when the bicycle market offered adult cyclists one standard size. 26 inches. The modern market is more diverse and therefore the question arises: which wheel diameter to choose?
Let’s say one thing: there is no definite answer to this question. But we will speculate on this topic. And each of you will take a grain of reason from this reasoning and make your choice.
And let’s make a few more useful Комментарии и мнения владельцев before we get down to the discussion:

the size of the wheels is determined not only by the diameter of the rim, but also by the height of the tire;
size units. English inch. It is designated. ““ ”;
modern manufacturers produce adult bicycle wheels in sizes 26, 27, 27.5, 28 and 29 inches. At the same time, with a wheel size of 26 ”, the rim diameter is 559 mm and the thickness of the tire. In sizes 27 “, 28”, 29 “, the rim diameter is the same and is 622 mm. Only the height of the tire is different;
The 29 ”wheel has only recently come into use. Models with wheels of this size are called 29ers or tuniners.
wheels of different diameters
 As already mentioned, 26 “is the most common size. It is used mainly on standard mountain models and walking city bikes. 27 ”is used on road bikes. 28 ” on hybrid and road. The newest 29 ”size is versatile as it is ideal for both smooth asphalt and problematic dirt roads.
The larger the diameter of the wheels, the more smoothly obstacles in the form of pits, bumps and stones on the road are overcome. Large diameter absorbs small impacts more easily.
The larger the diameter, the greater the grip between the wheel and the road surface, and the safer the ride (especially on slippery and muddy roads). The size of the adhesion area is also influenced by the width of the rim. Widest rim. at 29 “.

the bike is heavier. Long spokes, wider rims, taller frames and heavier tires
the dynamics of acceleration is lower, since due to the greater inertia and weight during acceleration, you have to apply much more effort;
It should also be noted that for the 29 “29er, the choice of some components is limited: rims, tires, forks. This does not apply to the selection of transmission and other little things.
The larger the diameter of the wheels, the more harmoniously the wheels are combined with tall people. 29 “. size ideal for cyclists taller than 180 cm.
Diameter and radius
The very first question is how to measure the diameter of a bicycle wheel? There are several proven ways to measure the radius or circumference of a tire. To measure the radius, place your vehicle level and, to avoid injury, fix it with a footboard. Then attach a tape measure (it is better to take a metal or wooden ruler, elastic centimeters can give an error when stretched) with one edge to the ground, and fix the other near the bushing on the axis. Thus, we measured the radius, multiply by 2 and get the diameter.
Circle
The second required dimension is the circle. There are several surefire ways to measure the circumference of a bicycle’s rim:
 We take a nonelastic thread, wrap it around the top of the rubber. We lay it out exactly on the surface, take a ruler and op!. we get the length of our tire.
 From the course of school mathematics according to the formula: circumference = πr 2.where π is a constant number 3.14, and r is the radius (the method of measurement is above).
 With a drop of paint. Place a dot on the tread, then run your bike on a level mowing line and on a smooth (eg asphalt) surface. The point will be printed twice, the distance between them will be the circumference.
A bit of math
The article is designed more for beginners, since experienced cyclists have probably already encountered similar mathematics. There can be many reasons for this: they punched a tire on the road and urgently needed a replacement, they just decided to change the tires. So how do you know the size of a bicycle wheel? First you need to say that tire sizes are measured in:
 centimeters;
 inches;
 millimeters (IS0 standards, these are worldwide standards of quality, units, measurement values, etc. All global bicycle manufacturers use and adhere to these standards).
From math lessons, we can convert one unit to another, and vice versa:
That is, if the value 20 is indicated in the tire description, then we know that it is in the English counting system. We multiply this value by 2.54, get 50.8, and then convert to millimeters, multiplying by 10. Answer: 20 inches = 508 mm.
Sizes of bicycles and tires
We measured the rim, how to find out the diameter of a bicycle wheel. we found out, you can get acquainted with the standard sizes of your tires, as well as find out what type of bike they are mounted on.
Choose the size of the rubber for your twowheeled friend, not for beauty and personal sympathy, but based on the environment and road surface.
Find Angular Speed of Wheel
 On small pieces, for example, for children, folding, BMX. wheels with a radius of 20 inches and a rim of 406 mm are installed.
 For teenagers and folding bicycles for adults, a “shoe” with a radius of 24 inches and a rim of 507 mm is recommended.
 Mountain models are equipped with rubber with a diameter of 26 inches and a rim of 559 mm. These tires feature an aggressive tread pattern to aid traction in offroad conditions.
 If you are a fan of city cycling, as well as participating in races and competitions, then most likely you are the owner of either a road or road twowheeled partner. Your manufacturer has supplied you with a 28 ” rubber with a rim circumference of 622 mm.
 Finally, the owners of mountain 29ers “put on” the largest tires with a diameter of 29 inches and a circumference of 622.
A few words about the exact size when buying a new rubber, you should know the rim rim diameter. You will need it when buying new wheels.
HOW TO MEASURE BIKE WHEEL AND FRAME DIAMETER?
Today, the popularity of cycling has increased dramatically, this is due to the fact that people have begun to pay more attention to their health. Many experienced cyclists know everything from “a” to “z” in this business, but beginners sometimes do not even know what their wheel diameter is. Next, we will talk about how to measure the diameter of a bicycle wheel and how to measure the size of the frame.
WHEEL DIAMETER
Before answering the question of how to measure the diameter of a bicycle wheel, let’s talk a little about the wheels themselves, their dimensions, whether the speed of the bicycle depends on the diameter of the wheels.
A bicycle usually has two wheels. As a rule, bicycles use spoke wheels with steel spokes, there can be from 12 to 42, but, as a rule, there are 32 or 36.
Some sports bicycles are fitted with 46 pairs of flat carbon fiber spokes. Disc wheels are used, most often for cycling, have the greatest rigidity and aerodynamics.
The wheels, along with the frame, are the most important structural elements of a bicycle, it is the wheels that withstand all the load created by the cyclist. Also, the wheel must withstand all shock loads that occur with unevenness on the road.
Modern wheels are both very strong and lightweight at the same time. A wheel usually consists of spokes, a hub, a tube with a tire, a rim.
The size of the wheel is of great importance, with large wheels you can reach higher speeds, and with small ones, the bike is more compact. The smallest 20 “wheels are used in folding models, 26” wheels are most often used for mountain bikes, and the largest 28 “wheels are common for road bikes.
How to Check a Bike Wheel Size
Now, after all, let’s answer the question, how to measure the diameter of a bicycle wheel? And this is easy to do. The size from the highest point of the wheel to the lowest point will be the diameter of the bicycle wheel.
FRAME SIZE
The size of the bike frame is selected based not on the height of the cyclist, but on the length of his legs, so you first need to know how to measure the size of the bike frame so as not to become a “victim” of the wrong purchase.
The bike frame is the main body to which the bike components attach. A bicycle frame is usually welded from profiles that form two socalled triangles. the front axle and the rear axle.
The bicycle frame is selected solely on the basis of the capabilities of both the material and physical data of the cyclist, as well as on what you are going to do on this bike, just ride in the park or on the slopes of the mountains, or can jump from high slopes and steep jumps.
Still, how do you measure the size of a bike frame? If you decide to buy a sports bike, then you need to stand on a flat floor without shoes and measure the height of your crotch, a tape measure will help you with this, multiply this figure by a factor of 0.665. for sports or road bikes.
And those who buy a bike for aggressive riding on hills and slopes, that is, a mountain bike, then multiply your figure by the coefficient for mountain bikes. this is 0.226, as a rule, mountain values go in inches, the resulting figure must be multiplied by 2.54 and then we get the size in inches.
With an inelastic cord
To do this, pull a thread, cord or cable around the bike tire. Its thickness does not matter, the main thing is that it fits snugly against the rubber. Next, you need to make a mark at the place of its contact with the initial end. Then remove the cord, spread it on the ground and use a ruler (tape measure) to measure the distance from the starting end to the marked mark.
Bicycle tire markings
And now a little about tires and their meanings. According to the classical system, the dimensions are applied in the following order: wheel diameter in inches / tire profile width.
According to the international ISO system, the same parameters are stuffed onto rubber in the opposite order. first the profile height, then the rim landing diameter. Values are in millimeters. For the same tire or tube 20 / 1.75 in ISO it will look like this: 44.4 / 406.
The table below shows the ranges of the profile heights of tires for the landing diameters of the rim:
Standard markings  Rim, mm 
20 / 1.5. 2.125  406 
24 / 1.5. 2.125  507 
26/1. 2.35  559 
27/1. 1.75  622 
28/1. 175  622 
29 / 1.75. 2.35  622 
The listed tires can be selected for one fit size. However, it is better to take into account the recommendations of the bike manufacturer in the passport and adhere to the recommended parameters. Tires that are too narrow will puncture more often, while tires that are too high increase resistance to movement and can damage the sides. Each rim has its own tires. Optimum matching of tires to wheel sizes will allow you to ride longer, without punctures or damage.
ISO
The same materials are required for measurements.
Tire width
Measurement, as in the above case, is made with a tape measure. It is placed on the tire perpendicularly and the distance from one edge to the other is measured. The tape should be held strictly horizontally (that is, across the bike tire) so that the result is as reliable as possible.
The width of a bicycle tire purchased to replace a damaged one may slightly differ from the original one (be slightly wider or slightly narrower). This fact will not affect the ability to install it.
By calculation
The required value can be calculated using the formula, knowing the diameter of the tire. The diameter can be indicated on the tire itself or determined using the previously described method. The circumference of the wheel is equal to the product of its diameter by the number Pi: C = D π. Unit of measure inches.
For example, 20×3.14 = 62.8 “, where 20 is the diameter of the tire in inches, and 3.14 is the pi number (constant).
Wheel circumference
This value is no less important than the previous ones. After replacing an old or damaged bicycle tire with a new one, you need to adjust the exact operation of the instruments (speedometer, odometer, navigator). To do this, the size of the circumference of the wheel is entered into the cycling computer.
You can find out this parameter in several ways.
By labeling
A mark is applied to the tire. it can be a drop of paint or chalk. After that, you need to drive the bike on a flat and smooth surface, for example, on asphalt. So that the wheel completes one full revolution. After the mark on the tread of the tire touches the road surface, at least 2 tracks will remain on the ground. This is the beginning and end of the path traveled by the wheel. The distance between the spots imprinted on the road section is the wheel circumference. Measurement is made using a metal tape measure.
For more accurate results, the bike with the paint on the tread is rolled in a straight line a little longer. For example, so that not 2, but 4 tracks remain on the ground. Next, the distance between the points is measured in turn (from 1 to 2, from 2 to 3, from 3 to 4) and the arithmetic mean is calculated. The resulting number is the desired value.
When there is no cable or chalk, you can drive the wheel through a puddle or apply a few drops of water to the tire.
What are the sizes of bicycle wheels?
The geometric parameters of bicycles differ according to a strict pattern. this also applies to the size of the frame and the diameters of the wheels. First, let’s present a size chart:
The first column shows the traditional wheel diameters in inches and millimeters. The wheel is measured with the tire. To find out the size of the rim, you need to subtract the height of the tire, multiplied by two. Standard rim sizes are shown in the second column.
Bicycle wheel measurement systems:
 classic, in inches;
 in centimeters;
 ISO standard, in millimeters.
The International Organization for Standardization (ISO) has implemented a universal wheel measurement system to avoid confusion. All bicycle manufacturers adhere to it. The dimensions of the wheels are made up of the diameter of the “clean” rim and the width of the tire profile.
When choosing new tires, you need to know the rim landing diameters. these numbers on the tires must correspond to them. This will come in handy when changing to a different tire profile. In everyday life, a wheel is measured in inches, and it is more convenient to give dimensions in inches. And by the size of the wheel, you can also determine the type of bike. walking, mountain or road.
Wheel sizes are important to know when choosing a child’s bike. There is a direct relationship between the numerical values of the diameters with the height and age of the child, the socalled “wheel size”. Below is a table of standards:
Wheel sizes for kids ‘bikes are separate from adults: kids’ range ends on 24 “wheels, while for adults it just starts (excluding 20” BMX). 24inch wheels can be safely attributed to teenage and universal (for small adults). Of course, this data will only help you choose a bike, otherwise you need to try it on.
How to determine the diameter of a wheel
First of all, we find the exact value of the wheel diameter, and then we go to buy the appropriate tires. As far as you do it correctly, your car will behave correctly and steadily on the road. The diameter of the wheel is an integral quantity, which is why care is required in the calculations. How to determine the diameter of a wheel is our topic today.
Determine the diameter and choose the right tires!
How to find out the size of the rim
The correct choice of wheel rims depends on the technical characteristics, denoting all parameters, namely, width, diameter, overhang, as well as DIA (wheel rim diameter) and PCD (drilling parameters).
You also need to know the designation of the marking. It indicates the standard parameters of all types of wheeled products:
Markings are indicated on the inside. Usually manufacturers duplicate them in accompanying documents and on packaging if the product is new.
Disc departure
This indicator is marked with the English letters ET. What is ET in disks and why is it? The indicator denotes the distance from the plane of the wheel product to the middle zone of the rim. The seating surface of the wheel product indicates the pressing plane of the disc to the hub.
 with a zero indicator;
 with negative;
 with positive.
Zero overhang indicates that the plane of the disc corresponds to its middle zone. Thus, the lower the value, the more the wheel product protrudes from the outside of the vehicle. If the departure indicator is increased, this means that the disc is deepened into the inner zone of the car.
It is also necessary to take into account the fact that depending on the width of the product, the departure indicators differ. Manufacturers indicate in the accompanying documentation for the vehicle a lower offset value for wide discs.
Diameter and other parameters of the disc schematically
Marking
As an example, consider the marking of a 9J x20H PCD 5 × 130 ET60 DIA 71.60 wheel rim:
 The number 9 indicates the width, measured in inches. To convert inches to centimes, multiply the total by 25.4.
 The letter J indicates structural elements: the shape of the disc flanges. This parameter does not play an important role when choosing.
 X denotes an inseparable disk.
 The number 20 denotes the diameter of the fit of the wheel product. This indicator corresponds to the landing of a car tire.
 The letter H indicates the presence of one hump or protrusion on the rim.
 PCD abbreviation 5 × 130, where the number 5 indicates the number of holes for fastening nuts or bolts and the number 130 indicates their PCD diameter in millimeters.
 ET60 marking indicates a disc overhang. In this situation, the indicator is 60 mm.
 The DIA value 71.60 indicates the center hole diameter. Usually, DIA corresponds to the fit of the hub and is indicated in millimeters. If the DIA is greater than the hub diameter, then a centering ring is used to fit the disc.
Information is also attached to the label:
ISO, SAE, TUV. these abbreviations indicate the organizations that checked the wheeled product, similar to the Russian GOST. The standards that correspond to the wheel marking are also indicated.
Maxload indicates the permissible load on the vehicle wheel. This indicator is indicated in kilograms and pounds.
How to find out the size of a bicycle wheel?
If it becomes necessary to replace a punctured tire or purchase a spare bicycle tire, you first need to determine the size of the wheel. For this, it is important to know how this indicator is measured correctly. Taking into account all the nuances, finding the size of the tire seems to be a fairly simple task.
How to find out the size of a bicycle wheel?
To correctly determine the size of a bicycle wheel, you must take into account that the measurement is made in English inches and is denoted by “”.
Today there are the following basic sizes of bicycle wheels:
 twenty”. Children’s bicycles or those models that can be folded.
 24 “. Teenage and stunt models.
 26 “. This standard is the most widely used. Wheels of this size are installed on mountain and walking models.
 27 “. Medium road bikes.
 27.5 “. Extreme mountain bikes.
 28 “and 29”. This standard is applicable to mountain and hybrid models.
 36 “. Bicycles with such wheels easily overcome stones, mud and other road imperfections.
When choosing the size of bicycle wheels, most often buyers give their preference to 2629 inch models. When it comes to a child or teenager, it is better to consider wheels 20 and 24 inches.
How to find out the diameter of a bicycle wheel?
Measuring the diameter of bicycle wheels can be done in the following ways:
 Radius measurement.
 ISO method.
 Circumference measurement.
The easiest way is to use the first method. The tape is drawn to the edge of the tire from the very beginning of the hub axis. The resulting figure is doubled. To convert a digit to inches, the number is multiplied by 2.54.
How to determine the ISO size of a bicycle wheel? The rim and profile diameters are measured here. This definition is the most accurate if you need to install a tire of a different profile. Consequently, the same rim diameter allows for a selection of different tires.
Western Bikeworks Workshop: Tire Size Explained
What diameter of bicycle wheels to choose?
The differences between the permissible wheel diameters are obvious. When choosing a size, first of all, you need to rely on the type of bike and the personal preferences of the owner:
 Bicycles with 29 “wheels are excellent obstacle course and are easy to handle. But, if you compare them with the 26 “models, additional physical efforts may be required to develop high speed here.
 26 “wheels are more agile, pick up speed and stop quickly. But, if we consider inertia, the primacy is undoubtedly given to 29 “models.
 What diameter of bicycle wheels should a child choose? Only 20 “and 24” wheels are considered here. Such cycling will not require a significant expenditure of energy and will be perfectly combined with the small growth of the baby.
Traditional markings of bicycle wheel sizes
The traditional wheel sizing system is built around the outside diameter of the entire wheel, which is the outside diameter of the tire, not the rim. Most often, it is the inch that is the unit of measurement and is indicated when marking, less often the wheel diameter is indicated in millimeters. The most common sizes are 20, 24, 26, 27, 28 and 29 inches. With the traditional marking system, only the diameter is indicated on the rims. Tires and tubes indicate the diameter and width of the tire, for example 26×1.75 or 26×2.35. It is important to keep in mind that sometimes the width can be marked with a simple fraction, for example, 26×3 / 4, although this size is mathematically equal to 26×2.175, such pakryshki are not interchangeable.
Interestingly, there are six types of tire diameters, there are only four types of rims for them (406 mm, 507 mm, 559 mm and 622 mm). For 27 ”, 28 ” and 29 ” tires, only 622mm rims are available.
Bicycle wheels: dimensions and markings
Without exaggeration, the wheel for a bicycle is the most important detail, because without it the bicycle simply cannot go. A bicycle wheel has two main indicators that are required for selection before buying. Weight is not so important, but still a lot depends on it. With weight, everything is simple, the less the weight of the wheel, the better. But with the second parameter. the size of the bicycle wheel, everything is different. There is no wheel size that is absolutely better than others. The choice of wheel size depends on the type of bike and its application.
The wheel consists of several parts. hub, rim, spokes, tube and tires. Each of the details is worthy of a separate article. In the same article, we will discuss the issues. how to find out the size of a bicycle wheel, how to choose the size, what are the sizes of a bicycle and talk about the standards for marking wheel sizes.
Bicycle wheel device
To determine the size of the wheel and in order to select the tire to the rim of the bicycle, there are the following values  the diameter of the rim, the diameter of the entire wheel, the width of the rim and tires. Sizes are most often indicated in inches, less often in millimeters.
There are a huge number of standards for marking bicycle wheels, but we will consider only a few, only those that we can encounter in Russia.
How to choose a wheel and tire size
When choosing, the most important thing is that the landing dimensions of the tire and rim are the same. Any combination of rim and tire widths can be used. But it’s worth noting that ideally the width should be the same. If the tires are too narrow, they will puncture more often and the rim may be damaged. If, on the contrary, the rim is narrow, and the tire is too wide, then the sidewalls will be easily damaged.
ISO Marking Standard
In order to avoid confusion in the bicycle wheel market with dimensions, the International Organization for Standardization (ISO) has developed its own universal marking system, which all manufacturers must adhere to.
The system is based on a marking consisting of two numbers. the width of the tire and the diameter of the rim. It is the rim landing diameter that is the value that guides when choosing a tire, these dimensions must completely coincide
For example, mountain tire. 38622 and road tire 20622 are interchangeable, despite the difference in width, since they have the same bore diameter.
It was these designations that were used on the wheels of popular Soviet bicycles. Salyut, Kama, Aist, Minsk, Ural and others. And they are still used on the marking of Russian rubber for bicycles.
Below we provide a table in which we indicate the correspondence between the traditional marking and the ISO standard. Also in the table we have indicated the most popular types of bicycles for these sizes.
It is worth noting that the dimensions of the wheels of a bicycle are not selected according to height, but are guided by the riding style and the purpose of the bicycle. This does not apply to children’s bicycles, which will be discussed below.
Choosing the size of a bicycle wheel for a child by height and age
In addition to the usual wheel sizes discussed above, there are also sizes for children’s bicycles, which are shown in the table below. Ideally, a child needs to choose a bike by trying it on. To do this, you need to put the child on the bike, if the heel of the child’s foot reaches the bike pedal in the lower phase, then the bike fits perfectly. It is not worth buying a bike to grow, it can lead to injury.
If you do not have the opportunity to try on a bike, you can follow the table below.
When choosing a bike for a child, pay special attention to the bicycle’s possibilities in terms of adjusting the saddle and handlebars in the plane of height and tilt. The wider these adjustments are, the longer the child will be able to ride the bike. For example, if you take a bike with 12inch wheels for three years, then with the adjustments the child will be able to ride it for 2. 3 years.
Children’s Stealth bike 18 inches
Methods for calculating the size with improvised means
There are several ways to determine the diameter of a bicycle wheel without resorting to complex methods.
By calculation
It is easy to find this value by knowing the circumference. Having set this value in mm with a tape measure, divide it by the socalled. number “pi” = 3.14. The result obtained corresponds to the diameter of the wheel. To express measurement in inches, divide by 25.4.
Determining the size (diameter) of a bicycle wheel
When replacing tires during repairs, you need to know how to measure a bicycle wheel. Its diameter can be different.
Common method for measuring wheels and tires
The classic method is to measure the maximum size including the bicycle tire in mm, cm and inches. Place the bike perpendicular to the ground. To do this, we lean it against a vertical surface so that it does not fall. We measure the radius of the wheel with a ruler (tape measure), i.e. the length of the segment from its center to the extreme point of the tire. To avoid errors, it is better to measure from the wheel axis to the ground. For the same purpose, you need to measure with a metal or, in extreme cases, a wooden ruler.
Place the ruler exactly in the middle of the sleeve. Multiplying the resulting value by 2, we find out the required diameter. It becomes especially important when choosing a child’s bike, because there is a relationship between the parameters of the bicycle and the height (age) of the child. There are tables of correspondence of these values.
Often the wheel size is indicated on the outside of the tire. First, its diameter is in inches, and then its width, for example: 26 × 1.25.
If there is a slight deviation in the tread size compared to the previously installed one, it can be used. At the same time, do not forget that if the tire circumference changes, all devices installed on the bike will have to be recalibrated.
Using a lace
When repairing your bike, you can take a long rope or thick thread to determine the circumference and place it flat against the middle of the tread. Having fixed the starting point of measurement, we circle the surface of the wheel with a cord until it touches the beginning and mark the resulting point. We remove the lace, determine its length to the mark. This is the desired value. You also need to know it in order to calibrate the speedometer, bicycle computer device and other devices. To calculate the diameter of the wheel, divide the resulting value by 3.14.
Determination of the diameter according to the ISO system
With different tread widths, the inner size may not match the rim. Wide tires are produced for driving on bad roads, and narrow ones for highways. Sports bike treads have the smallest width and are used for the fastest riding possible. Correctly fitted wheels will increase its lifespan, so it is best to use previously installed tires designed for this type of bike.
To avoid mistakes when choosing the right part, the International Organization for Standardization (ISO) has developed a method for determining the parameters of a wheel by its inner (landing) diameter. It is measured by applying a tape measure to the center axis and rim of the bicycle. The resulting number is multiplied by 2.
While the rim always remains the same for a given tire size, the tire width can be slightly altered when changing. However, tires that are too narrow are more likely to be punctured, and high ones slow down the movement, creating additional stress.
Inches are often used as the unit of measure. To convert a value from mm to inches, divide the quantity by 25.4.
The ruler is installed vertically, and not along the spoke, strictly from the middle of the central axis hub.
Also, according to the ISO system in millimeters, the tire is measured.
Unlike the previously described calculation method, here, when marking, the tread width is first indicated, and then the wheel diameter in mm, for example, 56 × 550.
Thus, manufacturers apply designations to the tire in different formats. Possible options:
 2 numbers separated by dashes indicate the thickness of the tire and its inner diameter in mm (ISO method), for example 38669;
 2 or 3 numbers in inches, one of which means the maximum size of the wheel, and the next. the width and thickness of the tire;
 2 numbers in mm, with the 3digit being the inner diameter of the rim and the width of the tire.
Standard wheel sizes
Each type of bike corresponds to certain parameters of the wheels, depending on its brand and purpose. So, for a children’s bike, this size can be 508 mm, for a teenager. 609 mm, for an adult road bike. 622 mm, for a mountain bike. 660 or 736 mm. The indicated value corresponds to the distance measured on the outside of the tire.
Traditionally, measurements are taken in the English numeral system, in inches (1 inch = 25.4 mm). Rounding is done to the nearest whole number. For an adult bicycle, it varies between 2429 inches, or 622736 mm.
There are 2 ways to determine the wheel diameter:
 French (standard, on the outer edge of the tire);
 method according to ISO requirements (on the inside of the rim).
In many cases, the tire size is indicated in mm or inches.