Read the ETRTO number
The ETRTO size is the most reliable, because every system maps onto it. It's printed on the sidewall as width-diameter, in millimetres.
| ETRTO diameter | Also called | Typical bikes |
|---|---|---|
| 622 | 700c, 29er, 28" | Road, gravel, hybrid, 29er MTB |
| 584 | 27.5", 650b | MTB, some gravel |
| 571 | 650c | Small road and triathlon bikes |
| 559 | 26" | Older MTBs, some hybrids and kids' bikes |
| 406 | 20" | BMX, kids' bikes, some folding bikes |
Example: 28-622 is a 28mm-wide tyre for a 622mm rim, sold as 700 x 28c. A 29 x 2.4 MTB tyre is about 60-622.
Pick a width that suits your rim
The rim's internal width limits how narrow or wide a tyre can safely go. As a rough guide:
| Rim internal width | Usual tyre width |
|---|---|
| 15–19mm | 25–32mm road |
| 19–25mm | 28–45mm road and gravel |
| 25–30mm | 2.1–2.4" MTB |
| 30mm+ | 2.3–2.6" MTB |
Check your rim maker's chart for the exact range. On a wider rim a tyre often measures a little wider than its label.
Check your frame clearance
Measure the tightest gap around your current tyre at the fork crown, the seatstays and the chainstays. Leave a few millimetres each side for mud and wheel flex, more if you ride in winter or off-road.
FAQs
Is 700c the same as 29er?
Yes, the diameter is the same (622). 29er tyres are just much wider.
Can I fit a 28mm tyre where a 25mm was?
Usually, if your frame and brakes have room. Check clearance first.
What does the inch size mean?
It's an approximate outside diameter. Always go by the ETRTO number.