Fire load, also called fire loading, refers to the amount of flammable material and the amount of heat that can be generated by a substance if ignited within a given area. It is most commonly used to refer to the amount of heat that can be generated by the materials in an enclosed area, such as a compartment or room. The fire load of a room or other area can be used to quantify the potential severity of a fire in that location and so is an important concept in fire safety, firefighting, and construction.
A room's fire load is quantified as the amount of heat that would be generated per unit of area in the room if all combustible materials present were burned. In Imperial or United States customary units, this is given as British thermal units (BTUs) per square foot, while in metric units it is in kilojoules (kJ) per square meter. A single BTU equals about 1055 joules, or 1.055 kJ. A BTU is formally defined as the amount of heat needed to raise the temperature of 1 pound of water by a single degree Fahrenheit less than 1 atmosphere of pressure, which is roughly the average air pressure at sea level.
A room's fire load can be calculated in customary units by multiplying the number of pounds of flammable materials in the room by the average BTUs generated per pound and then dividing the result by the number of square feet in the room. The same procedure can be done using kilograms, kilojoules, and square meters. Less precisely and more informally, the term can also refer to the amount or mass of flammable materials within a given area, quantified as pounds per square foot or kilograms per square meter, though this is a cruder because it does not include the amount of heat generated by different materials. The fire load of an area can vary greatly depending on what is stored there. For example, burning dry wood produces about 7,000 BTUs per pound, while burning propane produces 15,000 per pound.
Knowing the fire load of rooms in a burning structure is important information for fire safety, because it indicates how destructive fires in different rooms or compartments can be and gives an idea of how likely a fire is to spread from one area to another. Firefighters use this information to identify the most vulnerable or dangerous areas of burning buildings. It is also one consideration taken into account when a building is being constructed. For example, concrete does not contribute to fire load because it does not burn, and so is often used to construct rooms or building where highly flammable materials are kept. Fire codes and building codes often include regulations restricting where and how highly flammable materials such as fuel may be stored, because they greatly contribute to the local fire load and present a heightened risk of an out-of-control fire if kept in places with fire-control measures that were not designed to deal with the amount of heat they can generate.