You can calculate the voltage needed for electricity to jump across a spark gap with a simple formula: voltage equals the air gap length in centimeters times 30,000. Air is normally an electrical insulator; electricity can’t jump out of a wall socket and shock you because the surrounding air does not conduct it. But very high voltages have enough energy to turn air into a conductor, allowing electricity to jump the gap. To calculate a spark gap voltage, first measure the gap with a ruler, then use the distance formula mentioned above to find the voltage.
Turn off all power to the spark gap apparatus. For example, to measure a spark plug, turn off the engine and remove the plug.
Measure the distance in centimeters between the electrodes in the spark gap with the ruler. For a spark plug, slide the gapping tool into the gap until it fits snugly, then read the gap distance on the tool.
Key in the number of centimeters into the calculator. Press the multiply key. Enter 30,000. Press the equals key. The result is the voltage required to produce a spark in the gap. For example, if you measure 1 millimeter, first convert to centimeters. At ten millimeters per centimeter, you'll have .1 centimeter. Multiply .1 by 30,000 to get 3,000 volts.
TL;DR (Too Long; Didn't Read)
If you have a ruler or gapping tool that measures in inches, use 60,000 as the multiplier. Because of the complexities involving humidity, pressure and other factors, the result is a rough guide rather than an exact figure.
A shock from high-voltage equipment can be painful or even lethal. Carefully read and follow the safety procedures, labels and signage for your equipment. Do not touch or measure the spark gap when the equipment is turned on.