Place the wheel on a flat surface
Measure the diameter from one outer edge to the opposite outer edge
Measure straight across the center
For tire size, read the numbers on the tire sidewall
For rim size, measure the bead seat diameter, not the outer lip
Use a tape measure or ruler for the diameter
Measure the width from one inside edge to the other inside edge
Record the measurements in inches or millimeters
Check the wheel markings if visible
Verify the size against the vehicle or bike specifications