The one number that matters: bead seat diameter
Bike tires are labelled in three old systems (inch decimal, inch fractional and French) plus the modern ISO / ETRTO size. Only the ETRTO size is unambiguous. In 37-622, 37 is the tire width in millimetres and 622 is the bead seat diameter (BSD) – the diameter of the part of the rim the tire hooks onto.
A tire fits your rim when the BSD matches. Width can usually change a few sizes either way, as long as it suits your rim width and clears the frame and brakes.
| Written as | System | ETRTO |
|---|---|---|
700x25c | French | 25-622 |
29 x 2.2 | Inch decimal | 56-622 |
27.5 x 2.4 / 650B | Inch decimal / French | 61-584 |
26 x 1.95 | Inch decimal | 50-559 |
26 x 1 3/8 | Inch fractional | 35-590 |
Notice the last two: both say “26 inch”, but the rims differ by 31 mm. That is the single most common reason people buy the wrong tire – see the 26-inch trap.
More size help
Bike tire size chartEvery common bike rim size in one chart: ISO/ETRTO bead seat diameter next to the inch and French names it is sold under, and what bikes use it.See the chart →Which inner tube fits my tire?Enter your tire size and see which inner tube fits: the rim size and width range to look for, plus the valve types explained.Find my tube →700c vs 29er vs 28 inch700c, 29 inch and 28 inch tires all fit the same 622 mm rim. What actually differs is width – here is what fits your frame and rim.Compare →