# How to Calculate Methane Number ••• gas pump image by jedphoto from Fotolia.com

An engine's spark plug should ignite fuel, but additional fuel pockets sometimes combust, creating "knocking" in the engine. A fuel's methane number describes how likely it is to combust uncontrollably. Hydrogen receives a methane number of "0," and methane receives a number of "100." Other fuels lie elsewhere on this scale. Engineers measure a fuel's methane number experimentally, using an adapted engine. But when a fuel has a carbon-hydrogen ratio of at least 2.5, you can use that ratio to calculate its methane number.

Determine your fuel's H/C ratio. You can calculate this with the chemical formula by dividing the number of hydrogen atoms by the number of carbon atoms, but heavier fuels will more likely be labeled by H/C ratio than by chemical formula. This example will use a fuel with an H/C of 3.72.

Multiply the H/C by 508.04. So 3.72 * 508.04 = 1,889.9

Square the H/C ratio. So 3.72 * 3.72 = 13.84

Find the cube of the H/C ratio:

3.72 * 3.72 * 3.72

\= 51.48