How to Convert Bike Wheel Sizes

When a tire blows on your bike, it can be dangerous if you're on the road and annoying if you're off the road. Bike tires come in myriad sizes, and if you get the wrong size, it may appear to fit right but wear down faster or offer less control in a road incident. Master the math behind bike tire sizes by using the standard sizes, and get to know the folks at your local bike shop for help upgrading your ride.

Instructions

    • 1

      Check your bicycle tires for numbers written on the rubber sidewall. You'll see two numbers, which will be in either inches or millimeters. Write down what you find. A cruiser style bike might have tires sized 26 by 2.125, which is 26 inches in diameter by 2.125 inches wide. A beater road bike you found in a garage might have a diameter of 27 inches and a width of any size.

    • 2

      Find a tire sizing chart for help converting your measurements (see Resources). There's an ISO or International Organization for Standardation tire size chart, which lists tire measurements in width times diameter of the bead seat (which measures rim diameter using the tire's bead as a starting point), where both are listed in millimeters.

    • 3

      Note the corresponding bead seat value for your tire,using the conversion chart. Your cruiser bike with a size of 26 inches has a corresponding bead seat diameter of 571 mm, and that old road bike has a bead seat diameter of 630 mm.

    • 4

      Write down both the values from your tire, either in inches or millimeters, and the new ISO values from the conversion. Also jot down the make and model of your bike. Head to a nearby bike shop for help getting a replacement tire. Let the bike shop know how you arrived at the measurements and what type of bike you have.