Last updated: 2023-09-05
How can I calculate the tire circumference?
How do I measure the wheel circumference?
Where can I find a wheel circumference table?
The tire circumference calculator allows you to quickly and easily determine the circumference of a bicycle tire. Details of the calculation are explained below.
Tire circumference or wheel circumference (or perimeter) is the length of the distance a wheel travels when it makes one full revolution.
The following drawing illustrates the tire circumference:
In the following, different possibilities are presented how to determine the tire circumference. They differ in accuracy and required effort.
A simple and quick way to determine the tire circumference is to calculate it using the tire diameter.
Required tools: Measuring tape and calculator
Step 1: Measure the diameter of the tire. The following graphic shows what is meant by the diameter:
Step 2: Multiply the diameter by the number 3.14, which gives the wheel circumference.
Sample calculation: Assume you measure a tire diameter of 700 mm. You multiply this by 3.14 and get the wheel circumference
700 x 3.14 = 2198 mm.
The tire circumference calculator above does exactly the same thing and you don't even need a calculator.
Mathematical background: The exact formula for calculating the circumference is
diameter x π, where π (spelled out as "pi") means the constant 3.14159…. Most often, to calculate the circumference, the formula
U = 2πr is given, where r is the radius of the tire, i.e. half the diameter.
Rating: This method is quite accurate, it also takes into account the height of the tread lugs on the tire casing. However, a small inaccuracy results from the fact that the weight of the cyclist and the load are not included. The actual tire circumference is slightly smaller due to the tire deformation caused by the additional weight.
If you prefer to measure the tire circumference directly, you can proceed as follows:
Required tool: Measuring tape
Step 1: Place the measuring tape on the floor.
Step 2: Place your bike next to the measuring tape so that the tire valve is right down and right next to the beginning of the tape measure.
Step 3: Push the bike forward along the measuring tape until the tire has made one full rotation and the valve is right back down.
Step 4: Now read the distance traveled on the measuring tape at the location of the tire valve. This is the circumference of the tire.
The following figure illustrates the process:
If you want to know the tire circumference as precisely as possible for practical use (e.g. bike computer), inflate the tires to the normally used tire pressure beforehand and sit on the saddle while pushing the bike, as the additional weight will compress the tire a little and thus reduce the tire circumference a little.
Rating: This is the most accurate of the methods mentioned here to determine tire circumference. It takes into account the height of the tread lugs and the weight of the rider.
Another way to determine the tire circumference is via the sizing information on the tire casing.
Required tool: Calculator
Step 1: Read the ETRTO tire size on the tire casing, e.g. 42-622. The first number is the tire width in millimeters and the second number is the tire inner diameter, i.e. the nominal rim diameter, in millimeters.
Step 2: Now we calculate the tire diameter from this as follows: Since the tire casing is roughly circular when inflated, the tire width is approximately equal to the height of the casing above the rim. This results in the formula
tire outer diameter = 2 x tire width + tire inner diameter. So we add the tire width twice to the tire inner diameter to get the tire outer diameter.
Step 3: Finally, we calculate the tire circumference by multiplying the tire outer diameter by 3.14. This step has already been explained in more detail above.
Sample calculation: Suppose you read the size
42-622 on the tire casing. From this we calculate an outer tire diameter of
2 x 42 + 622 = 706 mm and ultimately the tire circumference as
706 x 3.14 = 2217 mm (rounded).
Rating: This method is somewhat inaccurate because it does not take into account tire pressure and the height of the tread lugs on the tire casing, which also decreases over time due to abrasion.
|Inch size||ETRTO size||Tire circumference in mm|
The above bike tire circumference chart contains an excerpt from the Bike Tire Sizes Chart and gives the corresponding calculated tire circumference. The tire circumference was determined using the above estimation method, i.e. using the formula
tire outer diameter = 2 x tire width + tire inner diameter.
Rating: This method is somewhat inaccurate because it does not take into account tire pressure and the height of the tread lugs on the tire casing, which also decreases over time due to abrasion. The advantage of the table is that you do not need a tape measure or calculator.
On the Internet there are several tire circumference tables with largely similar values, but most of them do not specify how the tire circumference was exactly determined, especially since for many ETRTO sizes there are several tires with different high profiles. Therefore, I calculated the tire circumference myself and also described the method.
The most common use case where you need to know the tire circumference is related to the display of the current speed in the bicycle speedometer. The speedometer counts the number of wheel revolutions per time interval by means of a magnetic contact on a spoke and calculates the speed of the bicycle from this. In order for the speedometer to know how far you have driven after one wheel revolution, you must set the tire circumference. This is typically done by entering a four-digit number – the tire circumference in millimeters.
Bicycle computers with GPS and GPS devices usually do not use a tire sensor; the current speed is determined via the change of the GPS coordinates.
How bad is it if I have not measured the circumference quite accurately? Does it really have to be so accurate, or is the value from the tire circumference table sufficient?
This question is about measurement error and how much of an impact it has in practice.
Example 1: Let's assume we have measured ourselves by a whole 10 cm (!), for a 28" tire which has a tire circumference of 2200 mm in reality. This gives an error of
100 / 2200 = 0.045, or 4.5 %. At a driving speed of 20 km/h, our speedometer would instead show about 19 or 21 km/h. On the other hand, if we are traveling at 40 km/h, it would read about 38 or 42 km/h. And after a driving distance of actually 100 km, our speedometer would deviantly show a covered distance of 95.5 or 104.5 km.
Example 2: Let's look at a more realistic example. Let's assume we are off by about 2 cm in our measurement. This gives an error of only
20 / 2200 = 0.009, just under 1 %. When driving at 20 km/h, our speedometer would read approximately 19.8 or 20.2 km/h, and when driving at 40 km/h, a value of 39.6 or 40.4 km/h. If the speedometer has no decimal places, you would not even see the deviation. After 100 km of travel, the speedometer would read 99.1 or 100.9 km.
As you can see from the two examples, a measurement error does not have as great an effect in practice as you might have thought. How high the measurement accuracy must ultimately be depends more on one's own expectations of the result.
The tire circumference, and thus the tire size, has a significant impact on ride comfort. Large tires from 26 to 29 inches roll effortlessly over small bumps in the ground away and are the standard for mountain bikes, trakking and city bikes.
Smaller tires like 20-inch tires on BMX bikes have the advantage of being faster to accelerate and more maneuverable. However, they would be much less pleasant to drive off-road and on forest paths.
Bicycle tires in 27.5 inch can have a tire circumference from about 2011 to 2306 mm, see the bicycle tire circumference chart above. This information alone is therefore not sufficient to determine a reasonably accurate value for the bicycle tire circumference. Use the bicycle tire circumference calculator above or apply one of the described ways to determine the tire circumference.
Bicycle tires in 29 inch can have a tire circumference from about 2268 to 2425 mm, see the bicycle wheel circumference chart above. The bike tire circumference calculator above will help you get a more concrete value. Alternatively, apply one of the ways described above to determine the tire circumference.
The number 700 stands for an tire outer diameter of 700 mm (the C in the French tire designation stands for a tire width of approximately 39 mm). Using the tire circumference calculator above, an approximate tire circumference of 2199 mm can be derived from this.