Summary

Small drive belts (used in tape players, CD players, turntables etc.) are designated by their (inner) diameter and cross-section dimensions. While the cross-section dimensions can be easily read with a caliper, the length or diameter are a little trickier to get especially on flat belts and larger square/round belts.
Of course, there is also the half-length (or folded) measuring method. It is common for thin flat belts but is less accurate for square and round belts.

This is my take on a dedicated belt measuring tool. It is based on the type used for measuring auto and machinery V belts etc. This measuring tool is capable of measuring flat, round and square section belts from diameter of 20mm and up.

The tool is designed to work with a simple 3m measuring tape which provides the initial measurement for calculating the belt’s length and diameter.

How to measure and calculate:
Insert the end tip of the measuring tape into the opening in the measuring tool base.
Place the sliding part of the measuring tool over the measuring tape.
Place the belt to be measured over the two half cylindrical stems, make sure the belt is not twisted and move the sliding part until the parallel parts of the belt between the stems are straight and parallel but lot stretched.
Read the scale of the measuring tape at the inner edge of the slide, this will be the X parameter (in mm).
Calculate the length of the belt (L) by the formula: L = X*2 + 40 (40mm is the sum circumference of the two half-cylindrical stems)
Calculate the diameter of the belt (D) by the formula: D = L / 3.14

For instance, in the attached picture the tape measure reads 57.5mm,
So the belt’s length is 57.5 * 2 + 40 which is 155mm, and the belt’s diameter is 155 / 3.14 which is 49 mm.