To quickly determine diameter of any cable (up to and including 3/8" or 10 mm) this simple gauge does the trick. Simply slip it over a cable, and when the cable fits snugly in the "V" slot, note the diameter printed on the gauge.
One side reads in fractions of an inch, the other side in millimeters.
Very inexpensive insurance to eliminate using wrong size cable.
Easily carried in pocket or wallet.
Lightweight and corrosion resistant.
Hole for lanyard or carrying loop
Note: This gauge is made from relatively thin sheet metal, and is not meant for heavy duty or professional use - which will distort the V-groove.