How to Measure Tires for a Mountain Bike

Check the tire wall for any writing that is stamped onto it. Almost all tires have both their size and their recommended tire pressure recorded in this way. Only if this is absent or marred will you need to proceed to the remaining steps, but that is likely as mountain bike tires take a lot of abuse on rough terrain.

Take the wheel off the bike. Most mountain bikes will have a quick release handle, making this very easy. If not, use a wrench to unscrew the bolt holding the tire in place.

Check the tire's inflation. If you know what the pressure should be, use a pressure gauge. Otherwise, make sure the tire is not flat or almost flat. Tire sizes include both the wheel and the tire, so a flat or nearly flat tire could result in taking a bad measurement. Also, keep in mind that most mountain bike off-road tires use air pressures that are lower than what road-bike riders are accustomed to. Do not mistake that for underinflation.

Use a tape measure to take the diameter of the tire. This should be measured from the bottom outer edge of the tire to the top outer edge of the tire. Bike tire sizes are set in inches, in gradients of two or four inches. So, if you have a measurement of 19.5 inches, your tire is an under-inflated 20 incher, not a 16 incher.