Bike Wheel Size Table
| Bead seat (mm) | ISO marking | Old marking | Common name |
|---|---|---|---|
| 406 | 47-406 | 20 × 1.75 | 20" BMX & folding |
| 490 | 32-490 | 22 × 1⅚ | 22" kids (old 550 A) |
| 507 | 47-507 | 24 × 1.75 | 24" BMX |
| 559 | 47-559 | 26 × 1.75 | 26" mountain (classic 26er) |
| 571 | 40-571 | 26 × 1½ C.S. | 650 C road racing |
| 584 | 44-584 | 26 × 1½ × 1⅛ | 650 B / 27.5" modern MTB |
| 590 | 37-590 | 26 × 1⅚ | 650 A old three-speed |
| 597 | 32-597 | 26 × 1¼ | 26" antique club |
| 622 | 28-622 | 700 × 28 C | 700 C / 29" / 28" - three names, one bead |
| 630 | 32-630 | 27 × 1¼ | 27" vintage road |
| 635 | 40-635 | 28 × 1½ × 1⅚ | 700 B Dutch & city bikes |
Bike wheels are sized by one number that actually matters and several that do not: the bead seat diameter in millimeters - the diameter of the rim seat where the tire bead locks in - standardized as ISO 5775. A 622 mm bead is sold as 700C on a road bike, 29er on a mountain bike, and 28 inch on a European city bike, all the same rim. The old inch markings measured something different (often the outer tire diameter), which is why the chart pairs every ISO size with its most common legacy names.
The ISO marking on a tire sidewall reads width first, bead second: a 47-559 tire is 47 mm wide and fits a 559 mm bead rim. Two tires marked "26 inch" can carry beads 559, 571, 584, 590 or 597 mm - up to 38 mm apart - so a tire bought by the inch name alone may not seat on your rim at all. The table below is the decoder: find your rim (or the number on your tire's sidewall) and read across.
How to use
- Read the numbers off your tire sidewall, not off the wheel: the ISO marking (for example 37-622) is printed near the valve or in small type on the carcass. The second number is the bead seat diameter - that is the only number that must match your rim.
- Match width to your rim's inner width and frame clearance: the same 622 mm rim accepts 23 mm road tires through 50 mm gravel tires, but the frame and brakes set the upper limit. When in doubt, replace like with like - buy the same ISO marking the old tire carried.
- When upgrading by feel, convert honestly: 27.5 inch mountain wheels are ISO 584 (650B), and 29ers are ISO 622 - the same bead as a road 700C wheel. A 26-inch-era frame usually cannot swallow a 584 or 622 wheel with a real tire on it, so check brake reach and clearance before buying.
Frequently asked questions
How big is a 700C wheel in millimeters?
622 mm at the bead seat - about 27 inches - which is why a 700C road wheel, a 29er mountain wheel and a 28-inch European city wheel can all be the same rim. The French 700C name dates from an old system that measured the outer tire diameter (700 mm with a nominal tire mounted), not the rim itself; the letter C marked the narrow width class in that system. What survives today is just the bead number: if your tire says 622 anywhere on it, it fits the same rim as any other 622.
Is 27.5 the same as 650B?
Yes - both are ISO 584 mm at the bead seat. 650B is the older French triathlon/touring name (650 = nominal outer diameter, B = the wide class), and 27.5 is the marketing name the mountain-bike industry revived it under around 2009. Same bead, same rims; a 584-marked tire fits either. The parallel revival gave 29-inch mountain bikes their name too - they are plain 622 road-size beads with fat tires.
Why do so many different wheels get called 26 inch?
The inch name usually referred to the approximate outer diameter of the tire that was mounted, and tires of different widths change that number - so five distinct bead diameters (559, 571, 584, 590 and 597 mm) all circulated as "26 inch" in different eras and countries. Classic mountain bikes are 559; old British three-speeds are 590 (650A); 650B mountain wheels are 584. A 26 x 1 3/8 tire from a three-speed will not fit a 26-inch mountain-bike rim - the beads are 31 mm apart. Only the ISO number is exact.
What does the tire marking 47-559 mean?
47 millimeters wide, 559 mm bead seat diameter - a typical 26-inch mountain-bike tire. The ISO system always writes width first, bead second, in millimeters, with a dash: common examples are 28-622 (a 700x28C road tire), 47-406 (a 20-inch BMX tire) and 57-584 (a 27.5 x 2.25 mountain tire). If the sidewall shows the old style instead (26 x 1.75), the table matches it to the ISO pair - but note fractional-inch and decimal-inch markings are different size systems too, so the ISO numbers remain the only safe cross-reference.