Ohms is a measure of the electrical resistance of a circuit.

To calculate the resistance of a firing cable you need to know the length of the cable and the number of ohms per 100m.

Example if a firing cable is 1,600 meters long with a resistance of 1.4 ohms for every 100m the resistance is 16 x 1.4 = 22.4 ohms

The symbol for ohms is Ω