Short answer: the number that decides whether a tyre fits is the ISO (ETRTO) size on the sidewall, written like 28-622: the first number is the tyre's width in millimetres, the second is the bead seat diameter of the rim it fits. 700c, 28in and 29in tyres all share the 622mm bead seat; 27.5in and 650b are 584mm; 26in mountain bike tyres are 559mm. Inch sizes with fractions and with decimals are not interchangeable, so 26x1 3/8 and 26x1.75 do not fit the same rim.
Tyre sizes are one of the most confusing things in cycling, because three different systems are printed side by side, and the same wheel can be called 700c, 28in or 29in depending on who is selling it. This reference explains each marking, lists the common sizes with their ISO equivalents, and covers the traps that lead to buying the wrong tyre or tube.
- The three systems on a tyre
- Reading the ISO size
- Common sizes and what they fit
- Inch sizes and the fraction trap
- What 700c and 650b mean
- Nominal width and measured width
- Other markings on the sidewall
- Matching an inner tube
- FAQ
- Finding the right tyre
The three systems on a tyre
| System | Example | Where you see it |
|---|---|---|
| ISO, also called ETRTO | 28-622 | On almost every modern tyre. The only system that reliably tells you what fits. |
| French | 700x28c | Road, gravel and hybrid tyres |
| Inch | 29x2.4, 26x1 3/8 | Mountain bike tyres, older roadsters, kids' and folding bikes |
When in doubt, ignore the French and inch markings and use the ISO numbers.
Reading the ISO size
An ISO size has two numbers, separated by a dash:
- The first is the tyre's nominal width in millimetres. 28-622 is a 28mm tyre.
- The second is the bead seat diameter in millimetres: the diameter of the part of the rim the tyre bead sits on. 28-622 fits a 622mm rim.
The second number must match the rim exactly. A 622 tyre will not fit a 584 rim, however close they look. The first number has some flexibility, within the range the rim and frame allow.
Rims carry a similar marking, such as 622x21, meaning a 622mm bead seat and a 21mm internal width.
Common sizes and what they fit
| Bead seat (ISO) | Also called | Typical bikes | Example sizes |
|---|---|---|---|
| 622mm | 700c, 28in, 29in, 29er | Road, gravel, hybrid and 29er mountain bikes | 700x25c (25-622), 700x40c (40-622), 29x2.4 (61-622) |
| 635mm | 28 x 1 1/2 | Traditional Indian roadster bikes | 28 x 1 1/2 (40-635) |
| 590mm | 26 x 1 3/8 | Older city and roadster bikes | 26 x 1 3/8 (37-590) |
| 584mm | 27.5in, 650b | 27.5in mountain bikes, some gravel bikes | 27.5x2.25 (57-584), 650x47b (47-584) |
| 571mm | 650c | Some small road and triathlon bikes | 650x23c (23-571) |
| 559mm | 26in mountain bike | Older and entry-level mountain bikes | 26x1.95 (50-559), 26x2.1 (54-559) |
| 507mm | 24in | Small mountain bikes | 24x1.95 (50-507) |
| 406mm | 20in | BMX and many folding bikes | 20x1.75 (47-406) |
| 349mm | 16in folding | Brompton-style folding bikes | 16 x 1 3/8 (35-349) |
Two things stand out. First, 700c and 29in are the same rim size; 29in mountain bike tyres are simply wider. Second, several sizes share a nominal name but not a rim: 26in alone covers 559mm and 590mm, and 650 covers 584mm and 571mm.
Inch sizes and the fraction trap
Inch sizes come in two forms, and they are not interchangeable:
- Decimal widths, such as 26x1.75 or 29x2.4, are mountain bike and BMX sizes. 26x1.75 is a 559mm tyre.
- Fractional widths, such as 26 x 1 3/8 or 28 x 1 1/2, come from older roadster sizing. 26 x 1 3/8 is a 590mm tyre, and 28 x 1 1/2 is 635mm.
So 26x1.75 and 26 x 1 3/8 are both "26 inch" tyres, but their rims differ by 31mm and neither fits the other. This catches people out often on older bikes. Check the ISO number before ordering, and if the old tyre is too worn to read, measure the rim or ask us.
To convert a decimal inch width to millimetres, multiply by 25.4: a 2.4in tyre is about 61mm wide.
What 700c and 650b mean
The French system began with the approximate outer diameter of the tyre in millimetres, 700 or 650, followed by a letter, a, b or c, for progressively wider tyres. To keep the outer diameter the same, wider tyres went on smaller rims. Over time only a few combinations survived, and the letters now simply identify a rim size:
- 700c means a 622mm rim, whatever the tyre width. A 700x23c and a 700x45c fit the same rim size.
- 650b means a 584mm rim, the same as 27.5in.
- 650c means a 571mm rim, a different size from 650b.
The number in the middle, such as the 28 in 700x28c, is the nominal width in millimetres.
Nominal width and measured width
The width on the tyre is a nominal figure. The actual width depends on the rim it is mounted on. The same 28mm tyre measures wider on a rim with a wide internal width than on a narrow one, often by a couple of millimetres on modern wide road rims. That matters for frame and brake clearance, especially on older road bikes with tight clearance.
Which tyre widths each rim width suits is covered in our tyre and rim width reference.
Other markings on the sidewall
| Marking | What it means |
|---|---|
| TLR, TL, Tubeless Ready | The tyre is designed to run tubeless with sealant. Tyres without it need an inner tube. |
| Hookless compatible | The tyre is approved for hookless rims, which limit pressure to 5 bar, about 72 psi. |
| Min and max pressure | The tyre's allowed pressure range. The rim's own limit may be lower. |
| Arrow or "Drive" | The direction of rotation. Fit the tyre so the arrow points forward at the top of the wheel. |
| TPI | Threads per inch in the casing. Higher usually means a more supple, lighter tyre, lower usually means tougher. |
For the pressure to actually run, see our guide to what tyre pressure to run.
Matching an inner tube
Inner tubes are sold for a range of sizes, such as 700x18-25c or 26x1.75-2.20. Choose a tube whose range includes your tyre's width and whose diameter matches the rim. Then check two more things:
- Valve type. Presta (thin, with a locknut) or Schrader (car-type). The rim's valve hole must suit it.
- Valve length. On deep rims, the valve must stick out far enough to attach a pump: as a rule of thumb, rim depth plus 15mm to 20mm.
Valve sizing for tubeless setups is covered in our tubeless valves, tape and sealant reference. Browse inner tubes and tyres.
FAQ
Is a 700c tyre the same size as a 29er tyre?
Yes, for the rim: 700c and 29in tyres both fit a 622mm bead seat diameter, so they mount on the same size rim, and the difference is width, since road and gravel tyres run from about 23mm to 50mm while 29er mountain bike tyres are typically 2.2in to 2.6in, or about 56mm to 66mm. Whether a given tyre fits your bike depends on rim width and frame clearance, so a 29er tyre will not fit a road frame even though the rim size matches.
Can I fit 27.5in wheels in a 29er frame?
Physically it is often possible on disc-brake frames, because a 584mm wheel is smaller than a 622mm wheel and fits inside the frame, but it lowers the bottom bracket, changes the steering and can bring the pedals closer to the ground, so most makers do not recommend it. Rim-brake frames will not work at all, since the brake pads no longer reach the rim. Check the frame maker's guidance before trying.
How do I find out what tyre size my bike uses?
Read the sidewall of your current tyre, where the ISO size appears as two numbers joined by a dash, such as 32-622 or 50-559, usually in small print near the larger French or inch size, and that pair tells you the width and the rim diameter you need. If the tyre is too worn to read, check the rim, which often carries a marking such as 622x19, or measure the bead seat diameter.
Are 26 x 1.75 and 26 x 1 3/8 tyres interchangeable?
No: a 26 x 1.75 tyre fits a 559mm mountain bike rim, while a 26 x 1 3/8 tyre fits a 590mm roadster rim, so although both are sold as 26 inch, the rims differ by 31mm and neither tyre will mount on the other's rim. The same applies to inner tubes. Always match the ISO number on the old tyre, such as 47-559 or 37-590, rather than the inch size.
Why does my tyre measure wider than the size printed on it?
Because the printed width is a nominal figure, measured by the maker on a standard rim, and a tyre mounted on a wider rim spreads out and measures wider, often by a couple of millimetres on modern wide-profile road and gravel rims, while pressure also makes a small difference. That is why a 28mm tyre may measure close to 30mm on a wide rim. Check your frame clearance against the measured width, not the label.
Finding the right tyre
If your tyre markings are worn or confusing, ask TBA Guide with your bike model or the markings on your tyre, and we will tell you which tyres and tubes fit.
