Fire Load Density Calculator

The fire load density is the amount of combustible energy per unit floor area, and it drives the likely fire severity, required fire resistance and compartment sizing. This calculator sums the energy of the combustible contents — wood and paper, plastics and rubber, textiles and liquid fuels — using typical calorific values, then divides by the floor area to give the fire load density in MJ/m² and a qualitative severity band.

Fire load density underpins the equivalent time of fire exposure and the fire-resistance rating in performance-based design (for example the Eurocode parametric approach). Calorific values vary with material, and only the combustible contents and linings count. Typical occupancy values range from around 200-400 MJ/m² for offices and dwellings up to much higher for storage. Use representative values and confirm the method against the adopted code for any life-safety decision.

A 100 m² store holds 800 kg wood (17 MJ/kg), 300 kg plastics (40 MJ/kg) and 100 kg textiles (20 MJ/kg). Energy = 13,600 + 12,000 + 2,000 = 27,600 MJ. Fire load density = 27,600 / 100 = 276 MJ/m² — a moderate load typical of light commercial use.

Why it matters

The fire load density sets the potential severity of a fully developed fire. Together with ventilation and compartment geometry it drives the equivalent time of fire exposure and hence the fire-resistance rating needed for structure and compartmentation.

Fixed vs movable load

Design methods often split the load into the fixed (building) component and the variable (contents) component, with characteristic values and combustion factors applied. This tool gives the raw contents figure; a full assessment applies the code factors.

Use with judgement

Calorific values and occupancy data carry scatter, so treat the result as indicative. For any life-safety or structural fire decision, follow the adopted code method and have it reviewed by a fire engineer.