Start with the sidewall
The easiest and most reliable way to find your size is to read it off the tire. Rotate the wheel and look along the sidewall for a printed size. Most tires have one or more of these:
- ETRTO / ISO, such as
37-622(width then bead seat diameter in mm) - Inch, such as
26 x 1.95 - French metric, such as
700x28c
If you have an ETRTO number, you are done: the second number is the bead seat diameter that must match your rim. If you only have inches or French sizing, the bike tire size chart can help you match it to a bead seat diameter. For how the systems relate, see bike tire sizes explained.
If the sidewall is unreadable
Markings wear off, and some tires are printed on the inner side only. Try these in order.
- Look at the other side of the tire and the other wheel. Front and rear are often the same size, but not always.
- Check your bike’s spec sheet, manual or the manufacturer’s website using the model and year.
- Check the rim. Some rims have the size or ERD printed on them.
- Measure it yourself, as below.
Measuring width
With the tire inflated and fitted, use a ruler or caliper across the widest part of the tire, from sidewall to sidewall. Note that actual width can differ from the printed number, depending on the rim width and pressure, so treat your measurement as approximate. Record it in millimeters for road and gravel, or inches for mountain bikes.
Measuring diameter
Stand the wheel on the ground and measure the height from the ground to the top of the tire, or measure from the axle to the outer tread and double it. The result is the outside diameter. Because tires vary in height, this only tells you the rough size class. For example, roughly 26 inches suggests a 26 inch wheel and around 29 inches suggests a 29er or 700c wheel. For the wheel itself rather than the tire, see how to measure your bike wheel size.
The more precise value is the bead seat diameter, which is only visible with the tire removed. Measuring the bare rim across its inside, from bead seat to bead seat, is the most reliable manual method but needs care. When in doubt, compare your measurements against the chart and confirm with a bike shop before buying.
Is my bike 24 or 26 inch?
The outside diameter will be near 24 inches or near 26 inches. A 24 inch wheel has a 507 mm bead seat and a 26 inch mountain wheel has 559 mm, which is a visible difference. Be aware that some older bikes use 26 inch labels with other rim sizes, so the sidewall ETRTO number is the final check.
Is a 28 inch tire the same as 700c?
On most modern bikes, yes: 28 inch and 700c both refer to a 622 mm bead seat. There are older 28 inch tires, for example 28 x 1 1/2, that use a 635 mm rim and are not the same. See what 700c really means.
What about a 20 inch bike?
Most 20 inch bikes, including BMX and many kids’ bikes, use a 406 mm rim, but a 20 inch folding bike may use 451 mm. Look for 406 or 451 on the sidewall. For child sizing, see choosing a tire size for your height.
Quick checklist
- Read the sidewall first.
- Note ETRTO if present; the second number must match.
- Measure width in mm or inches.
- Check frame and fork clearance before going wider (see gravel tire width and clearance for how to measure it).
- Use conversion help if your size is in a different system than the shop’s.