Making sure that a bike has the correct size tires on it is an important safety measure. When replacing tires, it's vital to make sure that the new tires are the same size as the old tires. If you're not sure what size tires your bike needs, it's easy to determine the bike tire size with a tape measure

1. Step 1

   Use a bicycle tire air pressure gauge to make sure the tires are completely inflated.

2. Step 2

   Give the tires a bit more air with an air pump or let some air out, if necessary. Overinflated or under-inflated tires will give an inaccurate reading.

3. Step 3

   Prop the bike up against a wall or other supportive surface so that it is standing up straight on both wheels.

4. Step 4

   Measure the diameter of the bike tire with a tape measure. Starting at the center of the base of the tire, measure the tire from the ground up in a vertical line. Measure the tire from the spot where the rubber touches the ground to the tread on top of the tire. The resulting number is the tire size.