The "caloric content" (energy content combustion) of firewood depends mainly on how dry it is. "Green" wood is about 10 MJ/kg (megajoule per kilogram), air-seasoned wood about 16 MJ/kg, while kiln dried wood is about 19 to 20 MJ/kg. The potential heat content per kilogram of wood is roughly equal for all wood varieties.
However, the heating potential of firewood per cubic meter or per log varies widely, depending upon the species of tree from which the wood is cut and thus the density of the log. In the United States, firewood is typically broadly classified into two categories: "hardwood" (any broadleaf tree) and "softwood" (any species of conifer). These labels are often misleading, as some species of conifer have harder wood than some species of broad-leaf tree. Generally, the harder the wood (which results from slower growth), the denser it is and the greater the amount of biomass per unit volume. Such woods, when well-seasoned, produce hot, long-burning fires with relatively little particulate emissions. Varieties of wood such as Oak, Hard Maple, Hickory, and most of the fruit woods (apple, cherry, etc.) have the hardest, most dense wood, and are most desirable for firewood. Broad-leafed varieties such as willow, aspen, or poplar have less-dense wood and require a greater volume of wood to produce the same amount of heat. In areas where broad-leafed trees do not grow, varieties such as slash pine, Western Larch, and Yew are desirable varieties of firewood.