A method is presented for the prediction of the depth of lethal heat penetration into mineral soils beneath a spreading fire. The information required for this prediction is the total amount of heat transferred to the soil by the fire, the time the fire heats the soil, the physical properties of the soil, and the Newtonian cooling coefficient at the soil surface. The depth of lethal heat penetration can be determined from a diagram using dimensionless parameters. The predicted values are compared with measured values from a number of test fires conducted in our laboratory.