Check the sidewall of your current tire

The fastest way to find your tube size is to look at the sidewall of the tire already on your bike. The size is printed there in a standardized format. You will see something like 700c × 25c, 26 × 1.95, or 27.5 × 2.1. Write down both numbers — the wheel diameter and the tire width.

If your tire is worn down or the printing is faded, move to the next step. But if you can read it clearly, you have what you need to buy a replacement tube that will fit.

Measure your wheel diameter if the tire is unreadable

The first number in a tire size is the wheel diameter. You can measure this yourself with a ruler or tape measure. Place the ruler across the center of the wheel from one edge to the other, measuring the metal rim itself, not the tire. Common sizes are 20 inches, 24 inches, 26 inches, 27.5 inches, and 700 centimeters (which is about 27.5 inches and used on road bikes).

Write down this measurement. It is the most important number because a tube that is too large or too small for your wheel will not seal properly and will fail under pressure.

Find the tire width range on the sidewall or rim

The second number in the tire size is the width. Look again at the sidewall — you may see two numbers listed, like 26 × 1.5–2.0. This means the tire width can range from 1.5 inches to 2.0 inches. Tubes are sold to fit a range of widths, not a single width, so you need to know the range your tire falls into, not the exact width.

If the sidewall shows only one width number, that is the nominal width. Tubes sold for that size will cover a range around it. For example, a tube marked 26 × 1.75–2.125 will work with most 26-inch tires between about 1.5 and 2.25 inches wide.

Key Takeaways

  • The tire size is printed on the sidewall of your tire in two numbers: wheel diameter and tire width.
  • If you cannot read the sidewall, measure the wheel rim across its center with a ruler to find the diameter.
  • Tubes are sold to fit a range of widths, so you need the wheel size and the width range, not exact measurements.
  • Bring the complete size (like 700c × 25–28c or 26 × 1.5–2.0) to the bike shop or use it when ordering online.
  • A tube that is too large or too small for your wheel will not hold air properly and can fail while riding.

Match the tube size to your wheel and tire at the shop

Once you know your wheel diameter and tire width range, go to a bike shop or search online for tubes in that size. Tubes are labeled with both numbers. A tube marked 700c × 23–28c fits a 700-centimeter wheel with tires between 23 and 28 millimeters wide. A tube marked 26 × 1.5–2.125 fits a 26-inch wheel with tires between 1.5 and 2.125 inches wide.

The shop staff can confirm the size in seconds if you bring the wheel or the tire size written down. If you are ordering online, double-check the product description to make sure the diameter and width range match what you measured.

Account for valve type when you order

Tubes come with different valve types: Schrader (the thick valve that looks like a car tire valve), Presta (the thin, long valve with a small lock nut at the top), or Dunlop (less common, found on some older or utility bikes). The valve type is determined by the hole drilled in your rim, not by the tire size.

Look at the valve on your current tube or at the rim itself. If the hole is small and narrow, you need Presta. If it is wide and round, you need Schrader. When you buy a new tube, make sure the valve type matches. A tube with the wrong valve will not fit through the rim hole.

Use an online size converter if you have an old or unusual size

Older bikes sometimes use sizes that are no longer common, like 27 × 1.25 or 650c. If you cannot find a tube labeled with your exact size, use an online tire size converter to translate it to a modern equivalent. Many vintage 27-inch wheels, for example, can use 700c tubes because the diameters are nearly identical.

A bike shop can also help with this. Bring the bike or the old tube, and the staff can tell you what modern size will work. Do not guess — a tube that is close but not quite right will still fail.

Frequently Asked Questions

Can I use a tube that is slightly larger than my tire width?

Yes. Tubes are designed to fit a range of widths. A tube marked for 1.5–2.125 inches will work fine on a 1.75-inch tire. The tube will stretch slightly to fill the tire. What matters is that the wheel diameter is exact — a 26-inch tube will not work on a 27.5-inch wheel.

What happens if I use the wrong wheel size?

The tube will not seal properly. It will either be too loose and slip around inside the tire, or too tight and stretch beyond its limits. Either way, you will lose air pressure while riding and risk a blowout. Always match the wheel diameter first.

Do I need to know the brand of my tire to buy a tube?

No. Tubes are universal — a Schwalbe tube works in a Kenda tire, and vice versa. The size and valve type are all that matter. Brand does not affect fit.

Can I reuse a tube if I patch it?

Yes, if the patch holds. A patched tube works the same as a new one, as long as the size and valve type are still correct. Keep the old tube as a spare and carry a patch kit when you ride.