Among the major accidents that can occur in process plants and in the transportation of hazardous materials, those involving fire are the most frequent ones. And one type of fire which has been the origin of severe accidents are the jet fires, which originated by the ignition of a flammable gas/vapour or spray, released through a hole, a broken pipe, a flange, etc. Jet fires often occur in rather compact processes or storage plants, and the probability of flame impingement on other equipment is rather high. In such a case, very high heat fluxes can exist, with quick and severe effects on that equipment implying a probable domino effect. The present study aims to contribute to the knowledge and prediction of the main features of jet fires, focusing on the distance over which flames impingement can occur. New expressions are proposed to predict this distance for the case of horizontal subsonic and sonic hydrocarbon jet fires, from both experimental data and the mathematical and computational modelling of the main geometrical (size, reach, elevation) features of the flame.