A method is presented for the evaluation of the heating efficiency required for ignition as a spreading fire closes with fuel. An array of thermocouples was implanted in the fuel ahead of the fire to obtain the heat absorbed by the fuel prior to ignition. The fraction absorbed compared to the total that would be absorbed if uniformly heated is the effective heating number. The effective heating number is represented in its final form as decreasing exponentially with the reciprocal of the surface area-to-volume ratio. The spreading fire is carried in a regular fuel crib array. Two fuel sizes are used in cross-sectional dimension: 0.6 cm. and l.3 cm. Isotherms are generated from the temperature field at ignition and presented for both fuel sizes to determine heat absorbed.