Measuring the tire is easy, however it is CRITICAL to carefully follow the steps below so it is ACCURATE (if it is not accurate it is worthless).
Tire Measuring Instructions
We need the rim width and TWO DIFFERENT measurements from the tire #1 is the INSIDE SURFACE of the tires. #2 is the BEAD WIDTH. Inches is fine, if possible to providing measurements in mm is even better.
Last, we also need the RIM WIDTH that this tire is going on.
#1 INSIDE SURFACE of the Tire:
Press masking tape to the inside surface of the tire from bead to bead, as straight as possible.
Make sure the tape is making full contact with the inside surface of the tire, and then mark the tape at the INSIDE corners of the beads (per the picture below).
Then lay the tape out on a flat surface and measure it with a tape (per below).
#2 BEAD WIDTH
Hook the tape measure to the inside edge of the tire bead and measure the width of the bead, calipers work even better if you have them.
#3 RIM WIDTH
Rim width is typically stamped onto the side of the rim in most modern bikes.
If your rim is not stamped with the size, this is the dimension we need.