To begin, flatten out the heat shrink. Measure the width of the flattened piece with a set of calipers, if you have one, and multiply that figure by 2. Divide this figure by 3.1416 (pi) and the result will be the diameter of your heat shrink tubing.
In the First example picture, below, the heat shrink is measured Flat at 1.57" (40mm). Take this number 1.57 (40mm), then multiple by 2. 1.57 (40mm) x 2 = 3.14" (80mm). Then divide it by 3.1416, which comes out to 0.9994" (25.38mm)). Allowing for rounding and measurement errors, this example turns out to be a 1" (25.4mm) nominal diameter piece of heat shrink tubing.
The second example below is measured measured the same way. The flattened heat shrink’s width is 0.118" (3mm). The math breaks down as follows:
0.118(3mm) X 2 = 0.236 (6mm). Then, 0.236" (6mm) / 3.1416 = 0.075" (1.90mm). Nominal supplied diameter for this tubing is 0.075"
The third example below is showing the wrong way to measure the heat shrink tubing. Please don’t do this, it’s almost impossible to get an accurate measurement this way.