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 and letters on both sides of every tire. You do not need tools or special knowledge; you just need to be able to read what is already there.

Spin the wheel so you can see the sidewall clearly, or deflate the tire slightly if it makes the text easier to read. The size will be printed in one of two standard formats: either metric (like 700x28c) or inch-based (like 26 x 1.95). Write down exactly what you see, including all numbers and letters. This is the information you will need if you ever buy a replacement tire.

Key Takeaways

  • The tire size is printed on the sidewall of the tire itself in either metric format (700x28c) or inch format (26 x 1.95).
  • Metric sizes start with the wheel diameter in millimeters, then the tire width; inch sizes list wheel diameter first, then tire width as a fraction or decimal.
  • The rim size (the metal wheel underneath) must match your tire size exactly, or the tire will not fit or perform safely.
  • If the sidewall is worn or unreadable, you can measure the tire diameter with a ruler or tape measure, or check your bike's original documentation.

Understand metric tire sizing (700c and 650b)

Metric sizes appear as two numbers separated by an "x" — for example, 700x28c or 650x42b. The first number is the wheel diameter in millimeters (700 mm or 650 mm). The second number is the tire width in millimeters (28 mm, 42 mm, and so on). The letter at the end (c, b, or a) refers to the rim width and is less important for casual riding, but it does matter when you buy a replacement.

The 700c size is standard on road bikes and many hybrid bikes. The 650b size (also called 27.5 inches) is common on gravel bikes, some mountain bikes, and adventure bikes. Both are measured the same way: diameter first, width second. A tire marked 700x28c will fit a 700c rim and is 28 millimeters wide. A tire marked 650x42b will fit a 650b rim and is 42 millimeters wide.

Understand inch-based tire sizing (26, 27.5, 29)

Inch-based sizes appear as two numbers separated by an "x" — for example, 26 x 1.95 or 29 x 2.1. The first number is the wheel diameter in inches. The second number is the tire width, also in inches, usually written as a decimal (2.1) or sometimes as a fraction (1 7/8). This format is common on mountain bikes and older road bikes.

A tire marked 26 x 1.95 fits a 26-inch rim and is 1.95 inches wide. A tire marked 29 x 2.1 fits a 29-inch rim and is 2.1 inches wide. The width number can vary slightly between manufacturers even on the same rim size, so two tires marked 26 x 1.95 and 26 x 2.0 are both designed for 26-inch rims but have different widths. Always match the first number (rim diameter) exactly when you buy a replacement.

Check your bike's documentation if the tire is unreadable

If the sidewall is too worn, faded, or damaged to read, look for the original paperwork that came with your bike. The owner's manual or spec sheet will list the tire size. You can also check the manufacturer's website by entering your bike model and year — most brands keep detailed specifications online.

If you do not have the manual and cannot find it online, a local bike shop can measure your wheel and tell you the size in minutes. Bring the bike in, and a mechanic will use a ruler or calipers to measure the rim diameter and tire width. This is a quick, free service at most shops.

Measure the tire yourself if you need to verify

You can measure the tire diameter with a ruler or tape measure. Place the ruler across the center of the wheel from one edge of the tire to the other, passing through the hub. This measurement is the overall diameter. For a 700c wheel, you should get roughly 670 mm (about 26.4 inches). For a 26-inch wheel, you should get roughly 26 inches.

To measure width, place the ruler against the side of the tire where it is widest. The distance from one side of the tire to the other is the width. A 28 mm tire will measure close to 28 mm; a 1.95-inch tire will measure close to 1.95 inches. These measurements do not have to be exact — manufacturers allow some variation — but they should be close enough to confirm what the sidewall says.

Know why rim size matters more than tire width

The rim diameter (the first number in the size) is non-negotiable. A 700c tire will not fit a 26-inch rim, and a 29-inch tire will not fit a 27.5-inch rim, no matter how much you try. The tire will either be too large or too small, and forcing it on will damage both the tire and the rim.

Tire width, by contrast, has some flexibility. A 700c rim can usually accept tires ranging from about 23 mm wide to 40 mm wide, depending on the rim design. A 26-inch rim can accept tires from roughly 1.5 inches to 2.3 inches wide. Always check your bike's manual or ask a mechanic before going significantly wider or narrower than the original tire, because the frame and brakes need clearance for the new width.

Frequently Asked Questions

What does the letter at the end of a metric tire size mean?

The letter (c, b, or a) refers to the rim width standard. For most riders, it does not affect how the tire performs. When you buy a replacement, match the letter if possible, but if your local shop only has a different letter in the same size, it will usually work fine. Ask the shop mechanic if you are unsure.

Can I put a wider tire on my bike?

Maybe. Check your bike's manual first — it will say the maximum tire width the frame and brakes can handle. If you want to go wider than the original, measure the clearance between the tire and frame, and between the tire and brake pads. A mechanic can do this in seconds and tell you if it is safe.

What if I see two different sizes on my bike?

Your front and rear wheels can have different sizes. Write down both sizes separately. This is common on some gravel and mountain bikes. When you buy a replacement, make sure you are ordering the correct size for the correct wheel.

Is the size printed on the tire the same as the rim size?

No. The tire size includes both the rim diameter and the tire width. The rim size is just the first number. A 700x28c tire fits a 700c rim, but the rim itself is only 700 mm in diameter. The tire adds width on both sides.