How do bike tire sizes work?
Three systems are printed on tires, often all at once. A common road tire reads 700 × 28C (the French system), and 28-622 (ISO, maintained in Europe by the ETRTO), and older or European tires add an inch size as well. They all describe the same tire. The inch and French names began as rough outside diameters, and they stopped being accurate decades ago: a "700" tire is not 700 mm tall, and a "26-inch" tire can sit on any of five different rims. Only the ISO number is a measurement of the thing that decides fit.
The first number is the tire's width in millimeters, as designed on a standard rim. The second is the bead seat diameter (BSD): the diameter of the ledge on the rim where the tire's bead sits, in millimeters. The second number is the whole fit question. A tire and a rim with the same BSD will mount; any difference, even 5 mm, and they won't. Everything else on the sidewall is a nickname for one of these two numbers.
Same tire, three names: the conversion table
Each row is one tire, with every name it's sold under and its real outside height, computed as the rim diameter plus the tire's height twice over:
| On the sidewall | ETRTO / ISO | French | Inch (decimal) | Outside diameter |
|---|---|---|---|---|
| 700 × 23C | 23-622 | 700 × 23C | — | 26.3″ |
| 700 × 25C | 25-622 | 700 × 25C | — | 26.5″ |
| 700 × 28C | 28-622 | 700 × 28C | — | 26.7″ |
| 700 × 32C | 32-622 | 700 × 32C | — | 27.0″ |
| 700 × 40C | 40-622 | 700 × 40C | 28 × 1.57 | 27.6″ |
| 700 × 45C | 45-622 | 700 × 45C | 28 × 1.77 | 28.0″ |
| 650 × 47B | 47-584 | 650 × 47B | 27.5 × 1.85 | 26.7″ |
| 29 × 2.25 | 57-622 | 700 × 57C | 29 × 2.25 | 29.0″ |
| 29 × 2.4 | 61-622 | 700 × 61C | 29 × 2.40 | 29.3″ |
| 27.5 × 2.35 | 60-584 | 650 × 60B | 27.5 × 2.35 | 27.7″ |
| 26 × 2.1 | 53-559 | — | 26 × 2.10 | 26.2″ |
| 26 × 1 3/8 | 35-590 | 650 × 35A | — | 26.0″ |
| 27 × 1 1/4 | 32-630 | — | — | 27.3″ |
| 20 × 1.75 | 44-406 | — | 20 × 1.75 | 19.5″ |
Diameter = bead seat + 2 × width, taking tire height as roughly equal to width (true within a few millimeters for modern tires). Fractional sizes (26 × 1⅜, 27 × 1¼) have only a nominal width; the ETRTO number on those sidewalls is exact.
Is 700c the same as 29-inch? And what about 28-inch?
Yes: 700c and 29-inch are the same rim, bead seat 622 mm. "29er" is what mountain bikers called 700c wheels once tires got fat enough to make the whole wheel about 29 inches tall: a 29 × 2.4 stands 29.3″. A road tire on the same rim is much shorter, from 26.1″ on a 700 × 20 to 28.0″ on a 700 × 45. That's why "700c in inches" has no single answer; the popular "27.5 inches" is true only of a 38 mm tire. Most European "28-inch" tires (28 × 1.60, 28 × 1⅝ × 1⅜) are also 622. The exception is the old roadster size 28 × 1½, which is a 635 mm rim.
Is 27-inch the same as 700c?
No, and it's the most common mistake with older road bikes. A 27-inch rim has a bead seat of 630 mm, 8 mm larger than 700c's 622, so a 700c tire won't seat on a 27-inch rim, and a 27-inch tire falls off a 700c rim. Swapping the whole wheels is possible, because 700c wheels are only 4 mm smaller in radius. The catch is the brakes: the pads have to reach 4 mm further down, which many old calipers can't do. Before buying tires for a bike from the 1970s or early '80s, read the ETRTO number: …-630 means 27-inch.
650b vs 650c vs 27.5: which is which?
650b and 27.5-inch are identical, 584 mm, the same rim as the old French 26 × 1½. Mountain bikes adopted it under the 27.5 name, and gravel and "road-plus" bikes use it as 650b with fat road tires. 650c is a different, smaller rim at 571 mm, used on small road bikes and triathlon bikes. They are 13 mm apart and never interchangeable. If a tire just says "650" without a letter, the converter above won't guess, because 650a (590), 650b and 650c all exist.
Why don't 26 × 1.75 and 26 × 1¾ fit the same rim?
Because the two sizes come from different families, even though 1.75 and 1¾ are the same number. Decimal inch sizes (26 × 1.75, 26 × 2.1) grew up on American mountain bikes and all use the 559 mm rim. Fractional sizes are older and each one mapped to its own rim: 26 × 1¾ was Schwinn's cruiser size (571), 26 × 1⅜ the English three-speed (590), 26 × 1½ the French 650b (584), 26 × 1¼ the British sport bike (597). Five different rims all get called "26-inch."
26 × 1.75 and 26 × 1¾ are mathematically the same width, and their rims are 12 mm apart in diameter. A 559 tire on a 571 rim won't stretch over the edge; the reverse falls off at the first corner. Sheldon Brown's rule for the whole family: if two sizes are equal but one is written as a decimal and the other as a fraction, they are not interchangeable. Match the second ETRTO number and ignore the rest.
Tire width: millimeters to inches
Width converts at 25.4 mm to the inch. Mountain bike tires are sold in inches and road and gravel tires in millimeters, so a 2.25″ trail tire and a 57 mm gravel tire are the same width:
| Inches | mm |
|---|---|
| 1.50″ | 38.1 |
| 1.75″ | 44.4 |
| 1.95″ | 49.5 |
| 2.10″ | 53.3 |
| 2.25″ | 57.1 |
| 2.35″ | 59.7 |
| 2.40″ | 61.0 |
| 2.50″ | 63.5 |
| 2.60″ | 66.0 |
| 2.80″ | 71.1 |
| 3.00″ | 76.2 |
| mm | Inches |
|---|---|
| 23 | 0.91″ |
| 25 | 0.98″ |
| 28 | 1.10″ |
| 30 | 1.18″ |
| 32 | 1.26″ |
| 35 | 1.38″ |
| 38 | 1.50″ |
| 40 | 1.57″ |
| 42 | 1.65″ |
| 45 | 1.77″ |
| 50 | 1.97″ |
Labeled widths. On a rim wider than the label assumes, a tire measures wider than its label, about 0.4 mm per millimeter of extra rim width. The tire pressure calculator uses that measured width.
Kids bike wheel sizes in ETRTO
Kids bikes are sold by nominal wheel size, and each size maps to one rim in its decimal form:
| Wheel size | Rim (BSD) | Found on |
|---|---|---|
| 12½″ / 12″ | …-203 | Kids and balance bikes, scooters |
| 14″ | …-254 | Kids bikes |
| 16″ (16 × 1.5 to 16 × 2.4) | …-305 | Kids bikes, some folders |
| 20″ (20 × 1.5 to 20 × 4) | …-406 | BMX, folders, kids bikes, cargo and e-bikes |
| 24″ (24 × 1.5 to 24 × 2.8) | …-507 | Kids MTB, small adult MTB |
The fractional 20 × 1⅜ (451) and 16 × 1⅜ (349) are different rims from the kids' 20″ and 16″. Which size fits which child is the kids bike size calculator's job.
Will a different size fit my bike?
Two separate questions. Rim: the bead seat must match exactly, and the rim's internal width has to suit the tire's width. Frame: the tire needs clearance at the fork crown, the chainstays and the seatstay bridge, at least 4–6 mm all around for mud and wheel flex. For a bigger tire on the same rim, measure the gap at the tightest point with the current tire inflated, and add half the width increase on each side. For the effect on your bike computer, the wheel circumference calculator takes the same sizes, and the gear ratio calculator shows how a taller tire raises every gear.