The quickest way: check the tire sidewall

Your tire sidewall carries the wheel size in a format like 700c, 26", or 29". This number is printed on the rubber itself, usually near the rim. Spin the wheel so you can see the side of the tire clearly, and look for markings that start with a number followed by either the letter "c" or a quotation mark (inch symbol). That number is your wheel size.

This method works on any bike with the tire still attached. You do not need tools, and you will have your answer in seconds. If the sidewall is worn or faded, move to the next method.

Key Takeaways

  • The tire sidewall displays wheel size as a number with "c" (700c) or inches (26", 29") printed directly on the rubber.
  • If the tire is removed or the sidewall is unreadable, measure the wheel rim diameter with a ruler or tape measure across the center.
  • Common sizes are 700c (road bikes), 26" and 29" (mountain bikes), and 20" (BMX and folding bikes).
  • The size refers to the rim diameter, not the tire width, so two tires on the same rim size may look different in thickness.

Measure the rim if the tire is off or unreadable

Remove the wheel from the bike and lay it flat on a table. Use a ruler or tape measure to measure straight across the center of the rim from one edge to the other. This measurement is your wheel diameter. Compare it to the common sizes: 700c is about 29 inches, 26" is 26 inches, 29" is 29 inches, and 20" is 20 inches.

This method is accurate even if the tire is missing or the sidewall is too worn to read. The measurement you get is the rim size, which is what matters when you order a replacement tire or tube.

Check the bike's original documentation or frame

The owner's manual, sales receipt, or spec sheet from the manufacturer will list the wheel size. If you still have the paperwork, this is the most reliable source. Some bikes also have a sticker on the frame or fork that shows the wheel size as part of the model information.

This route works best if you are buying a bike secondhand and want to confirm the size before you ride it. It also helps if you are ordering parts online and want to be certain before you pay.

Understand the difference between rim size and tire width

Wheel size refers to the diameter of the rim, not how thick or wide the tire is. A 700c rim can accept tires that are 23mm wide (thin, for road bikes) or 40mm wide (thick, for gravel). Both are still 700c wheels. The first number on a tire marking like "700 x 23c" is the rim size; the second number is the tire width.

This matters because you need the rim size to buy the correct tire, but the tire width is a separate choice based on how you ride. Mixing up the two can lead to ordering the wrong part.

Common wheel sizes and what bikes use them

700c is standard on road bikes, gravel bikes, and hybrid bikes. It is also called 29 inches and measures about 29 inches in diameter. 26 inches was the standard for mountain bikes for decades and is still common on older models and some hardtail bikes. 29 inches is now the most popular mountain bike size, offering better rolling speed and obstacle clearance than 26".

20 inches appears on BMX bikes, folding bikes, and some children's bikes. 27.5 inches (also called 650b) is a middle ground between 26" and 29" and is used on some modern mountain bikes and gravel bikes. If your measurement does not match these common sizes exactly, you may have a less common size or a wheel that is slightly out of true.

What to do if your wheel size does not match a standard

Measure again to make sure your first measurement was taken straight across the center of the rim. A measurement taken at an angle will be larger than the true diameter. If the second measurement is still between two standard sizes, your rim may be slightly warped or you may have an older or specialty bike.

Take a photo of the tire sidewall (if readable) and the rim, then bring them to a local bike shop. The staff can identify the size and help you order the correct replacement tire or tube. Many shops will also measure the wheel for you if you bring the bike in.

Frequently Asked Questions

Can I put a different wheel size on my bike?

No. The frame, fork, and brakes are designed for a specific wheel size. Installing a different size will affect handling, braking, and frame clearance. Always replace wheels with the same size as the original.

What if the tire sidewall shows two different numbers?

You are likely seeing both the metric and inch measurements of the same wheel. For example, 700c and 29" refer to the same rim size. Use whichever format matches the replacement tire you are ordering.

Does wheel size affect how fast my bike goes?

Larger wheels roll faster over the same distance because they cover more ground per rotation. A 29" wheel will be slightly faster than a 26" wheel on flat ground, but the difference depends more on tire width and your own effort.

Why do road bikes use 700c instead of inches?

700c is a metric standard used in Europe and adopted by road bike makers. It means 700 millimeters nominal diameter. Inches are the older standard still used for mountain bikes. Both are just different ways of measuring the same thing.