I got tired of doing rollouts so I switched to the old tried and true: Circumference = pi x diameter.

With the bike in a vertical position, measure straight up from the floor to the center of your axle in millimeters. That is your radius. Multiply by 2 to get your diameter then multiply that by 3.14 to get your circumference. The result will be very close and you can adjust it from there depending on your level of OCD.