Fire load density: the formula, the calorific values and the traps

Updated: 2026-08-13 · ISKRIT

Fire load density is one division, and almost every dispute about it comes from the three inputs rather than from the arithmetic: which masses were counted, which calorific value was used, and what area sat in the denominator.

The formula

Fire load density is the total combustible energy in a compartment divided by the floor area: qf = sum over materials of mass times net calorific value, divided by the area, in MJ per square metre. The same expression appears in EN 1991-1-2 for structural fire design and in SP 12.13130 annex B for assigning a room category, and the two produce the same number from the same inputs.

Net against gross calorific value

Gross calorific value counts the heat recovered when the water vapour from combustion condenses back to liquid. In a fire it does not condense, it leaves through the openings, so the heat available is the net value. The difference is small for materials with little hydrogen and reaches ten percent for hydrocarbons, and it always runs the same way: using the gross figure inflates the fire load.

Common net values are wood and paper around 17 MJ/kg, textiles around 21, rubber around 32, general plastics around 38 with a wide spread underneath (PVC near 18, polyethylene near 46), and liquid fuels around 44. A single line of a schedule that says "plastic" therefore carries a factor of two and a half of uncertainty, which is why the material data sheet beats the table.

Which area goes in the denominator

The area is the one the load actually sits on, not the whole floor of the building. A compartment of 400 square metres with all of its stock on a 40 square metre pallet run has a load density ten times the average, and averaging it over the compartment hides the very concentration that decides the outcome. SP 12.13130 puts a floor under this and takes the area as at least 10 square metres, so that a small dense stack cannot be driven to an arbitrary figure by shrinking its footprint.

When a benchmark may stand in

Design codes publish characteristic densities by occupancy: offices near 511 MJ/m2, retail near 600, hotels near 310, hospitals near 230, warehouses near 1180, taken as the 80th percentile of surveys. They are a legitimate input at the design stage, when the contents are not yet known, and they are not a substitute for an inventory in an existing building. A warehouse of plastics sits several times above the warehouse benchmark, and the benchmark will not say so.

Reading the result

The number by itself decides nothing. It is an input to fire resistance requirements, to sprinkler design and, under SP 12.13130, to the category of the room: above 2200 MJ/m2 is V1, 1401 to 2200 is V2, 181 to 1400 is V3 and 1 to 180 is V4. Near a boundary the sensible check is the one on the inputs, because a plastics line entered at the wrong calorific value moves a room a whole category.

Questions

Do the structural elements count? Combustible parts of the structure and finishes count; a concrete frame does not.

Does packaging count? Yes, and in a warehouse the pallets and film are often a larger share than the goods.

Enter material masses and calorific values and get the fire load density, the explosion overpressure and the resulting category in the fire load density calculator.