Heat Density, heat flux or thermal flux is the rate of heat energy transfer through a given surface per unit time. The SI derived unit of heat rate is joule per second (J/s), or watt (W). Heat density is a scalar quantity, while heat flux is a vectorial quantity. This Heat Density converter allows you to convert Btu (IT)/sq. foot (Btu(IT)/ft²), Btu (th)/sq. foot (Btu(th)/ft²), calorie (th)/sq. centimeter (cal(th)/cm²), joule/sq. meter (J/m²), langley, among other units related to Heat Density.