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 rubber. Manufacturers stamp the size directly onto it, usually in white lettering. You do not need tools or math; you just need to know where to look and what the numbers mean.

Spin the wheel so the sidewall faces you clearly, and look for a series of numbers and letters. Common formats are 700c × 28c, 26 × 2.0, or 27.5 × 1.95. These numbers tell you the wheel diameter and tire width — everything a bike shop needs to sell you a replacement.

If the sidewall is worn or faded and you cannot read it, you can measure the tire yourself. This takes a few minutes and a ruler or measuring tape, but it gives you the information you need.

Key Takeaways

  • The tire size is printed on the sidewall in a format like 700c × 28c or 26 × 2.0, where the first number is wheel diameter and the second is tire width.
  • If you cannot read the sidewall, measure the tire width with a ruler while it sits on the ground, and measure the wheel diameter by running a tape measure across the center of the wheel.
  • Tire width is measured in millimeters or inches depending on the tire type — road tires use millimeters, while older or mountain bike tires may use inches.
  • When buying a replacement tire, bring the size numbers with you or take a photo of the sidewall so the shop can match them exactly.

Measure tire width with a ruler

Place the bike upright on the ground so the wheel sits flat. Take a ruler or measuring tape and hold it horizontally across the tire at its widest point — usually somewhere in the middle of the tread. Measure from the outer edge of the rubber on one side to the outer edge on the other side. Write down this number in millimeters.

If your ruler only shows inches, measure in inches and note that. Most modern road and gravel tires are labeled in millimeters (like 28mm, 32mm, or 40mm), while some older or mountain bike tires use inches (like 2.0 or 2.5). The sidewall will tell you which unit the manufacturer used.

This measurement is the tire width — the second number in the size code. It matters because a tire that is too narrow or too wide for your rim can be unsafe or perform poorly.

Measure wheel diameter across the center

The wheel diameter is the distance from one edge of the rim to the opposite edge, passing through the center. To measure it, place the bike upright and run a measuring tape or ruler straight across the wheel, from the outer edge of the tire on one side to the outer edge on the other side, passing through the hub in the middle. This gives you the total diameter of the wheel with the tire mounted.

Write down this measurement in millimeters or inches. Common wheel diameters are 26 inches, 27.5 inches, 29 inches, or 700 millimeters (which is about 27.5 inches). The sidewall will show which one your tire is designed for.

If you want just the rim diameter without the tire, you can measure the rim itself by removing the tire, but for ordering a replacement tire you do not need to do this — the sidewall size is what matters.

Understand the two main tire sizing systems

Inch-based sizing is common on mountain bikes and older road bikes. It looks like 26 × 2.0 or 29 × 2.2, where the first number is wheel diameter in inches and the second is tire width in inches. A 26-inch wheel is about 660 millimeters in diameter; a 29-inch wheel is about 735 millimeters.

Metric sizing is standard on modern road and gravel bikes. It looks like 700c × 28c or 700c × 32c, where 700c is the wheel diameter (about 622 millimeters of rim diameter, or 700 millimeters with the tire mounted) and the number after the × is width in millimeters. The letter c is a historical designation and does not change — you will see it on nearly every road tire.

Some tires show both systems on the sidewall. If yours does, use whichever one is clearer or more legible. Either way, the information is the same; it is just written in different units.

Check your rim width if you are replacing a tire

When you order a new tire, the shop will ask for the size, and the sidewall number is usually enough. However, if you are switching to a much wider or narrower tire, the rim width matters. Rims have a recommended tire width range printed on them, usually on the inside where the tire sits.

If you cannot see the rim marking, measure the rim width by removing the tire and measuring across the inside of the rim from edge to edge. Most road bike rims are 17 to 21 millimeters wide; gravel and mountain bike rims are wider. A tire that is too narrow for the rim can be unstable; one that is too wide may not fit in the frame.

If you are staying with the same tire size as the original, you do not need to check this — the manufacturer already matched them. It only matters if you are trying something different.

Take a photo of the sidewall for the bike shop

Once you have read or measured the size, take a clear photo of the sidewall with your phone. This photo is useful if you forget the numbers, if the shop wants to verify the size, or if you are ordering online. Make sure the numbers are in focus and readable.

You can also write the size on a piece of paper or in your phone notes. Either way, having the information written down or photographed saves a trip back home if you are at the shop and realize you did not memorize it.

Frequently Asked Questions

What if the sidewall shows two different sizes?

Some tires list both metric and inch-based sizes. They are the same tire, just described in different units. Use whichever format is clearest to you. The shop will understand either one.

Can I use a tire that is a different width than the original?

You can go slightly wider or narrower — usually within 5 to 10 millimeters — as long as it fits your rim and frame. Check the rim width range on the inside of the rim, and make sure the new tire does not rub the frame or brakes. If you are unsure, ask the shop before you buy.

What does the letter c in 700c mean?

The c is a historical designation from the French tire sizing system and does not affect how you use the tire. All modern road tires use 700c, so you will see it on nearly every road bike tire. It straightforward means the tire is designed for a 700-millimeter wheel.

Do I need to measure if I can read the sidewall?

No. If the sidewall is legible, write down or photograph what it says and you are done. Measuring is only necessary if the sidewall is too worn or faded to read.

What if my tire has three numbers instead of two?

Some tires show an extra number for bead seat diameter or other specifications. The first two numbers — diameter and width — are what you need to order a replacement. The shop can look up any additional details if needed.