The quickest way: check the tire sidewall

Your wheel size is printed on the side of your tire in a format like 26 x 2.0 or 700 x 35c. The first number is the diameter in inches (26, 27.5, 29) or millimeters (700). The second number is the width. This marking is the fastest way to know what you have, and it's the number you'll need when buying a replacement tire or inner tube.

Look at the tire itself, not the rim. The sidewall — the curved part between the tread and the bead (where the tire meets the rim) — has this information printed in white or raised lettering. You may need to rotate the wheel to find the clearest spot. If the lettering is worn away, move to the other methods below.

Write down the full number exactly as it appears. Different manufacturers use slightly different formats, and getting it right matters when you order a replacement.

Key Takeaways

  • The tire sidewall shows your wheel size in a format like 26 x 2.0 or 700 x 35c, and this is the number you need for replacements.
  • If the tire marking is worn away, measure the rim diameter with a ruler or tape measure across the center, or check the rim itself for printed size.
  • Common road bike sizes are 700c; mountain bikes are usually 26, 27.5, or 29 inches; and children's bikes range from 12 to 20 inches.
  • The tire width (the second number) matters for fit, so don't ignore it when ordering a new tire.
  • When in doubt, bring the wheel to a bike shop and they can tell you in seconds.

Measure the rim if the tire is unmarked

If the tire sidewall is too worn to read, you can measure the rim directly. Use a ruler or tape measure and measure straight across the center of the rim from edge to edge, not including the tire. This gives you the diameter. For a 26-inch wheel, you'll get roughly 559 millimeters; for 700c, roughly 622 millimeters.

The measurement won't be exact because rims are measured in a specific way (the bead seat diameter, which is slightly smaller than the outer edge), but it will be close enough to identify which size you have. If your measurement falls between two standard sizes, you likely have one of the common ones: 26, 27.5, 29, or 700c.

For width, look at the rim itself. Many rims have the width printed on the side. If not, measure the inside width of the rim where the tire bead sits. This is harder to do accurately without removing the tire, so if you're stuck here, a bike shop can measure it in seconds.

Check the rim for printed markings

Many rims have the size stamped or printed directly on them. Look around the entire rim — check both the inside and outside surfaces. The marking might say something like "26 x 1.5" or "700c x 28" or even just "700c". This is the manufacturer's specification and is reliable if you can find it.

The marking is often on the side of the rim that faces outward when the wheel is on the bike. You may need to rotate the wheel or look from different angles. If the rim is dirty, wipe it down first — the marking might be there but hidden under grime.

Know the common sizes by bike type

Road bikes almost always use 700c wheels. This is a metric size (622 millimeters bead seat diameter) and is the standard for road cycling. Road tires on 700c rims range from about 23 millimeters wide to 45 millimeters wide.

Mountain bikes come in three main sizes: 26 inches (older bikes and some smaller models), 27.5 inches (mid-size, also called 650b), and 29 inches (larger frames and taller riders). Mountain bike tires are wider, typically 2.0 to 2.5 inches or 50 to 65 millimeters.

Children's bikes use smaller wheels: 12, 14, 16, 18, or 20 inches, depending on the child's age and height. These sizes are specific to kids' bikes and are not interchangeable with adult sizes.

Hybrid and commuter bikes often use 700c wheels like road bikes, but with wider tires (35 to 45 millimeters) for comfort and grip. Some older hybrids use 26-inch wheels.

Why the second number matters

The tire width (the second number in the size marking) is not just a detail — it affects how the tire fits on your rim and how the bike handles. A tire that is too narrow for your rim can slip or pinch. A tire that is too wide may not fit in your frame or brake area.

When you replace a tire, you can go slightly narrower or wider than the original (usually within 5 to 10 millimeters), but you should stay within the range that your rim and frame allow. The rim itself has a recommended tire width range printed on it, or you can find it in the bike manufacturer's specs.

For example, a 700c rim marked "25-28c" means it works best with tires between 25 and 28 millimeters wide. A 700 x 32c tire would be slightly wider than ideal but might still fit; a 700 x 45c would likely be too wide.

What to do if you still can't find the size

If the tire is unreadable, the rim has no markings, and you're not sure what type of bike you have, take a photo of the wheel and bring it to a local bike shop. They can identify the size by sight or by measuring, and they can also tell you what tire width range your rim supports. This takes a few minutes and costs nothing.

Alternatively, if you know the bike's brand and model, you can search the manufacturer's website or contact them directly. They keep specification sheets for older models and can tell you the original wheel size.

Frequently Asked Questions

What's the difference between 26 inches and 700c?

Both are wheel diameters, but they're measured differently. 26 inches is the outer diameter of the tire when inflated; 700c (622 millimeters) is the rim's bead seat diameter. A 26-inch tire is roughly equivalent to a 700c rim in overall size, but they are not interchangeable — the rim and tire must match.

Can I put a different size tire on my wheel?

You must use a tire that matches your rim size (26, 700c, 29, etc.). You can vary the width slightly — usually within 5 to 10 millimeters — as long as it fits your frame and brakes. Check your rim's printed width range before buying.

Why do some tires say 700 x 35c and others say 700 x 35mm?

Both describe the same tire width: 35 millimeters. The "c" is an older road bike convention; "mm" is the metric standard. They mean the same thing, and either tire will fit your 700c rim if the width is in your rim's range.

Is 27.5 inches the same as 650b?

Yes. 27.5 inches and 650b refer to the same rim size (584 millimeters bead seat diameter). The term 650b is older and used mainly in Europe; 27.5 inches is the modern standard in North America. Tires marked either way will fit the same rim.

What if my bike has one wheel size in front and a different size in back?

This is rare on modern bikes but does happen on some cargo bikes or custom builds. Measure or check each wheel separately — they may have different sizes, and you'll need to buy tires for each one individually.