Place the bicycle upright on a flat surface
Measure the wheel diameter from one outer edge of the tire to the opposite outer edge
Measure the tire width if needed
Check the sidewall for the tire size marking
Use the ISO/ETRTO number on the tire sidewall for the most accurate size
Measure the rim diameter if the tire is removed
Use a tape measure for direct measurement
Measure in millimeters for ISO/ETRTO sizing
Measure both wheels if you need to confirm consistency