How to Measure a Bike Tire (2026)
Bike tire sizes can be confusing. Here's how to read the sidewall, what ISO/ETRTO numbers mean and how to measure if the markings are worn.
Read the Sidewall First
Almost every bike tire has its size printed on the sidewall. You'll usually see a traditional size (like 700x25c, 29x2.3 or 26x1.95) and an ISO/ETRTO size (like 25-622). The ISO number is the most reliable because it shows the tire width and the exact bead seat diameter that must match your rim.
Common Bike Tire Sizes
| Common Name | ISO Bead Seat Diameter | Typical Bikes |
|---|---|---|
| 700c | 622 mm | Road, gravel, hybrid |
| 29" | 622 mm | Mountain bikes |
| 27.5" (650b) | 584 mm | Mountain and gravel bikes |
| 26" | 559 mm | Older mountain bikes, cruisers, fat bikes |
| 24" | 507 mm | Kids' and some BMX bikes |
| 20" | 406 mm | BMX, kids' and folding bikes |
Some sizes have more than one variant (for example, other 26" and 20" standards exist), which is why the ISO number matters.
How to Read an ISO/ETRTO Number
In 25-622, 25 is the tire width in millimeters and 622 is the bead seat diameter in millimeters. A tire fits your rim when the bead seat diameter matches; width should suit your rim width and frame clearance.
The "26-Inch" Trap
Several different, non-interchangeable tire sizes have all been called "26 inch." Bicycle mechanic Sheldon Brown documented them by their ISO bead seat diameters:
| ISO Bead Seat | Common Markings | Typical Bikes |
|---|---|---|
| 559 mm | 26 x 1.75, 26 x 2.1 (decimal widths) | Most 26" mountain bikes, cruisers, fat bikes |
| 571 mm | 650C, 26 x 1 | Some small road and triathlon bikes |
| 584 mm | 650B, 26 x 1 1/2, "27.5" | Modern 27.5" MTB, some touring bikes |
| 590 mm | 650A, 26 x 1 3/8 | Older English 3-speeds |
| 597 mm | 26 x 1 1/4, Schwinn 26 x 1 3/8 | Older Schwinns and British club bikes |
His rule of thumb: a tire marked with a decimal width and one marked with a fraction usually won't fit the same rim, even if the math looks equal. Always match the ISO number.
French Sizes (700c, 650b)
The letters in 700c or 650b are a leftover French system that once indicated tire width. Today, 700c means a 622mm bead seat diameter regardless of tire width, and 650b means 584mm.
Measuring Without Markings
- Check the rim; many rims have the ISO size printed on them.
- Check your bike's spec sheet or the manufacturer's website.
- Measure the tire width across its widest point with calipers.
- As a rough guide, measure the wheel's outer diameter, but confirm the bead seat diameter before buying.
Buying Tubes and Tires
- Match the bead seat diameter.
- Choose a width your rim and frame can handle.
- Match the valve type (Presta or Schrader) and length.
- Check whether your rims are tubeless-ready.
Pressure
Use the pressure range printed on the sidewall. See mountain bike tire pressure and bike pumps.
Upgrading tires? See mountain bike tires and mountain bike wheels.
Frequently Asked Questions
How do I find my bike tire size?
Look on the tire sidewall for sizes like 700x25c, 29x2.3 or an ISO/ETRTO number such as 25-622.
What does ISO/ETRTO 25-622 mean?
25 mm tire width and a 622 mm bead seat diameter. The bead seat diameter must match your rim.
Are 700c and 29-inch the same?
Both use a 622 mm bead seat diameter, so they fit the same rims. 29-inch usually refers to wider mountain bike tires.
Can I measure a tire without markings?
Measure the rim's bead seat diameter or the tire's outer diameter as a rough guide, but checking the rim's markings or the bike's specs is more reliable.
Do tubes need to match the tire size?
Yes. Match the diameter and choose a tube width range that covers your tire, plus the right valve type.