Read the sidewall of your tire first

The easiest way to find your tire size is to look at the sidewall — the outer edge of the tire itself. Manufacturers print the size in large numbers, usually near the bead (the edge that sits on the rim). You will see a format like 700x25c, 26x2.0, or 29x2.2. Write down exactly what you see, including letters.

If the sidewall is worn, faded, or missing the size, you can measure the tire yourself. This takes a few minutes and a ruler or measuring tape, and it is the only way to know the size if you have an older tire or one with unclear markings.

Key Takeaways

  • The tire size is printed on the sidewall in a format like 700x25c or 26x2.0, and this is your fastest source of truth.
  • If the sidewall marking is missing or unreadable, measure the tire's outer diameter and width with a ruler or tape measure while the tire is mounted on the rim.
  • Tire sizes use two main systems: ISO (metric, like 700x25c) and ETRTO (the full technical standard), and knowing which one your bike uses matters when you buy a replacement.
  • Width is measured in millimeters or inches depending on the system, and even a 2mm difference can affect how the tire fits and handles.
  • The rim diameter (the number before the x, like 700 or 26) must match your wheel, or the tire will not fit at all.

Measure the outer diameter if the sidewall is unreadable

Place the bike upright on a flat surface or hold the wheel in your hands. Use a ruler or measuring tape to measure straight across the tire from the very top to the very bottom, passing through the center. This is the outer diameter. Write the number down in millimeters if possible — it will be easier to match to a standard size.

Common outer diameters are 700mm (road and hybrid bikes), 26 inches (older mountain bikes), 27.5 inches (modern mountain bikes), and 29 inches (larger mountain bikes). If your measurement falls between standard sizes, round to the nearest one and use that as your starting point.

Measure the width of the tire

Width is the second number in the tire size. To measure it, place the tire on a flat surface and use a ruler to measure the distance from one outer edge of the tire to the other, straight across the widest part. Measure in millimeters if you can.

Road bike tires are usually 20–32mm wide. Mountain bike tires range from 2.0 to 2.5 inches (roughly 50–65mm). Hybrid and commuter tires fall between them, usually 35–45mm. If your measurement does not match a standard width, round to the nearest half-millimeter or 0.1 inch.

Understand the two tire sizing systems

Road and hybrid bikes use the ISO system, which looks like 700x25c. The 700 is the rim diameter in millimeters, and the 25 is the width in millimeters. The letter (c, b, or a) indicates the tire profile and is less important for replacement, but write it down anyway because some shops use it to narrow down options.

Mountain bikes use a simpler system: 26x2.0, 27.5x2.2, or 29x2.1. The first number is the rim diameter in inches, and the second is the width in inches. This system is older and less precise than ISO, but it is still common on many bikes sold today.

A third system, ETRTO, is the technical standard used by manufacturers and appears on some tire sidewalls as a five-digit number like 25-622. The first number is width in millimeters, and the second is the rim diameter in millimeters. You do not need to use ETRTO to buy a tire, but it is useful if you want to cross-reference sizes across systems.

Check the rim diameter matches your wheel

The rim diameter is the most critical measurement because a tire that does not fit the rim will not work at all. If you measured 700mm across the tire, your rim is 700c. If you measured 26, 27.5, or 29 inches, use that number as your rim size.

The rim diameter is printed on the sidewall of your current tire and is also stamped on the wheel rim itself, usually on the inside edge. If you are replacing a tire, the new tire must have the same rim diameter as the old one. Mixing sizes — for example, putting a 26-inch tire on a 29-inch rim — will cause the tire to slip or fail.

Write down your complete tire size before shopping

Once you have measured or read the sidewall, write down the complete size in the exact format you found it. If the sidewall says 700x28c, write that. If it says 26x2.0, write that. If you measured and rounded, note that you rounded — for example, "measured approximately 700x27" or "measured approximately 29x2.1".

Bring this information with you when you shop for a replacement tire, or use it to search online. Tire shops and online retailers use these exact formats to organize their inventory, so matching the format makes it much faster to find the right tire for your bike.

Frequently Asked Questions

Can I put a different width tire on the same rim?

Yes, within limits. The rim diameter must stay the same, but you can usually go up or down by a few millimeters or tenths of an inch. A 700x25c rim can usually fit a 700x28c or 700x32c tire. Check your bike's frame and brakes to make sure there is clearance, because a wider tire takes up more space.

What if my tire size has a letter like 700x25c, and I see 700x25b for sale?

The letter indicates the tire profile — how the sidewall is shaped. It affects how the tire sits on the rim and how it handles, but it is not a hard requirement. A 700x25b will fit a 700c rim, though it may feel slightly different. If you want the exact same tire, match the letter too, but if you cannot find it, the width and rim diameter are what matter most.

My tire sidewall is completely worn away. How do I know what size to buy?

Measure the outer diameter and width as described above, then round to the nearest standard size. You can also check your bike's manual or the manufacturer's website if you know the model — they usually list the stock tire size. If you are still unsure, bring the wheel to a bike shop and they can tell you the size in minutes.

Is the size on the sidewall always accurate?

Usually, yes, but not always. Some tires are manufactured slightly larger or smaller than the marked size, especially older or budget tires. If you measure and the result is 2–3mm different from the sidewall marking, the sidewall is still your best guide for buying a replacement. If the difference is larger, measure again to be sure.

Can I use a tire size from a different system on my bike?

No. A 700c tire will not fit a 26-inch rim, and a 29-inch tire will not fit a 27.5-inch rim. The rim diameter must match exactly. If you are not sure which system your bike uses, measure the rim diameter first — that is the non-negotiable number.