When calculating the length of cable on a drum, there is a specific formula that must be used. The formula is relatively simple, but it must be followed precisely in order to get the correct answer. The measurements and the formula must be exact.

Write down the length of the wire; measure the length of wire in feet, but make the remainder of measurements in inches. Measure the width of the reel and make note of it on a piece of paper. Do this for all measurements. Make note of the measurements, marking specifically what each measurement is for.

Measure the diameter of the core. Measure the diameter of the full reel. Measure the diameter of the wire. Add the diameter of the wire to the diameter of the drum. Multiply this figure by the diameter of the wire. Multiply this figure by the width of the drum.

Multiply the current figure by the diameter of the wire; for example, if the diameter of the wire is 1.5 inches, this figure would be multiplied by 1.5. The final figure should give the length of the wire.