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Heat capacity is calculated from the density and the specific heat of a substance. Density is the weight of a unit of volume whereas specific heat indicates how much heat is required to increase the temperature of each unit of weight by one degree. Both density and specific heat vary widely in different substances; hence the heat capacities of these substances also vary widely. And since specific heat often varies with temperature, so also does heat capacity.

Materials with high heat capacities can absorb and lose large quantities of heat without much temperature change. Conversely, relatively little heat is needed to change the temperature of materials with low heat capacity. At ordinary temperatures, the heat capacity of air is about 0.017 Btu/ ft³/°F—little heat is needed to change its temperature. Under similar conditions, the heat capacity of dry soil and rock is 19 to 20 Btu/ft³/°F, and that of water is about 62 Btu/ ft³/°F. The high heat capacity of water is one of the reasons why the climate near oceans and large lakes is often more moderate than that of the climate further inland. The water absorbs and stores large quantities of heat during the summer without much change in temperature, and this tends to keep the air over the adjacent land relatively cool. In the winter the stored heat is released and warms the air over nearby land areas.

Because the specific heat of most wildland fuels varies over a relatively narrow span, differences in heat capacity of the fuels depend chiefly on their density. The differences in density of wildland fuels are quite large, and hence variations in heat capacity are also. Solid oak wood, for example, has a density of about 48 lb/ft³, while that of punky and decayed wood may be only 6 or 7 lb/ft³. The oak wood, then, requires a considerable amount of heat to raise its temperature to the ignition point, but decayed wood requires a relatively small amount. Thus, heat capacity is important in the ignition of wildland fuels.