Check the sidewall for the tire size marking
Read the size in ETRTO format, such as 37-622
Read the size in imperial format, such as 700x35C or 26×1.95
Measure the tire diameter from one outer edge to the opposite outer edge
Measure the tire width across the widest part of the inflated tire
Measure the rim diameter if the sidewall marking is missing
Use a tape measure for diameter and width
Match the measurements to standard bicycle tire size charts
Verify the size against the rim and frame clearance
Confirm whether the tire is clincher, tubular, or tubeless if needed