The maximum and minimum concentration of a combustible gas or vapour in air or oxygen that is capable of propagating a flame at a specified temperature and pressure with the aid of an ignition source such as a spark or open flame. The concentrations are generally expressed as the per cent fuel by volume. Above the upper flammable limit (UFL), the mixture is too rich to burn, while below the lower flammable limit (LFL), the lack of oxygen does not allow the fuel to burn. Any concentration between these limits can result in ignition or explosion. For example, the LFL for hydrogen in air is 4 per cent ^V/_V and UFL is 75 per cent ^V/_V. Concentrations below or above these limits therefore do not result in ignition or explosion.

The bounds of the flammability envelope expand with increased temperature and pressure. That is, the minimum oxygen level and the LFL decreases and the UFL increases. For example, diesel oil has a very low vapour pressure in which a fuel–air mixture at room temperature is too lean. That is, it is below the lower flammable limit. Once the oil is warmed to above its flash point it will ignite.