To quickly determine diameter of any cable (up to 3/8" or 10 mm) this simple gauge does the trick. Merely slip it over a cable and when the cable fits snugly in the "V" slot, note the diameter printed on the gauge. A very inexpensive insurance to eliminate using or sending out the wrong size cable.
Easily fits in a pocket or wallet. Lightweight and corrosion resistant. It's a item you can't afford to be without.