Quick answer
The fastest way to learn your wheel size is to read the numbers printed on the tire sidewall. If they are unreadable, measure the tire width and the rim diameter, then compare against a size chart; how to find and measure your bike tire size goes deeper on the tire side. Do not rely on overall wheel diameter alone, because several different rims share similar outer diameters.
Step 1: Read the tire sidewall
Look along the side of the tire for a set of numbers. You will usually find one or both of these formats:
| Marking | What it means |
|---|---|
| 700 x 35c | Old French style: roughly 700 mm outer diameter, 35 mm width |
| 29 x 2.1 | Inch style: roughly 29 inch outer diameter, 2.1 inch width |
| 26 x 1.95 | Inch style: roughly 26 inch outer diameter, 1.95 inch width |
| 37-622 | ISO/ETRTO: 37 mm tire width, 622 mm rim bead seat diameter |
| 54-559 | ISO/ETRTO: 54 mm tire width, 559 mm rim bead seat diameter |
When an ISO number is present, trust it. The second number identifies the rim size, and it is the one that has to match when you buy a tire or tube. Our bike wheel size chart translates the common rim diameters into names.
Step 2: Measure it yourself if the markings are gone
- Tire width: with the tire inflated, measure across the widest point of the tread, or use calipers.
- Outer diameter: measure across the wheel through the center, from one outer tire edge to the other. Treat this as a rough check only.
- Rim diameter: if you remove the tire, the bead seat diameter is the figure you want. Measuring this at home is difficult without taking the tire off, so many people compare the outer diameter and width with a chart, or ask a shop.
Common rim diameters you might land on are 622 mm, 584 mm, 559 mm and 406 mm.
Watch out for lookalike sizes
- 26 inch is not one size. The common mountain bike version is 559 mm, but older or city bikes may use a different 26-inch size with a larger rim. They are not interchangeable.
- 24 inch also has more than one rim size, including 507 mm and 540 mm versions.
- 27.5 inch and 650b are the same 584 mm rim.
- 29 inch and 700c share the same 622 mm rim, so a 700c wheel is not 26 inch, but it is the same rim as a 29er.
- 20 inch is commonly 406 mm on BMX and kids’ bikes, while some folding bikes use a different 20-inch size of 451 mm.
If your bike seems to be 24 or 26 inch, check the ISO number rather than guessing. See also who a 26-inch wheel bike is for.
Measuring rolling circumference for a bike computer
Bike computers and speed sensors often ask for wheel circumference. Tables give a starting value, but a roll-out measurement is usually more accurate.
- Inflate the tire to your normal riding pressure and sit on the bike or load it with similar weight.
- Mark the ground at the point directly below the valve or a tape mark.
- Roll the bike forward in a straight line until the mark returns to the ground.
- Measure the distance in millimeters and enter it in your device.
What to do with the result
Once you know the ISO rim diameter, you can pick a matching tire and tube, or look up the wheel on the bike wheel size chart. If you are matching a child to a bike, continue to kids bike wheel size by age.
FAQ
How do I tell what size my bike wheel is?
Read the sidewall markings. The ISO number, such as 622, tells you the rim diameter.
Is my bike 24 or 26 inch?
Check the sidewall. A 559 mm rim is the typical mountain bike 26 inch, while 507 mm is a common 24 inch kids’ size. If numbers are missing, take the bike to a shop to confirm.
What is a 20 inch bike?
It is a bike with wheels of roughly 20 inches in diameter, often a 406 mm rim size, commonly sized for children around 5 to 9.
Is a 700c wheel 26 inches?
No. 700c uses a 622 mm rim, which is the same as a 29-inch wheel. A typical 26-inch mountain bike wheel uses a 559 mm rim.