Place the bike upright
Measure the wheel diameter from one outer edge of the tire to the opposite outer edge through the center
If needed, remove the wheel for easier measuring
Measure the tire width from side to side at the widest point
Check the sidewall for the tire size marking
Use a tape measure for accuracy
Measure the rim diameter if you need the wheel size without the tire
Record the measurement in inches or millimeters
Repeat the measurement to confirm accuracy