Calculate fire load density from room type, combustible material mass, or custom energy and floor area, in MJ/m² and total MJ for any room.
- All Construction Calculators
- Fire Flow Calculator
- Hose Flow Rate Calculator
- Fire Occupancy Calculator
- Heat Load Calculator
- Burn Time Calculator
Fire Load Formula
Fire load density is the total combustible energy in a space divided by its floor area.
q = (Σ mᵢ × Hᵢ) / A
- q = fire load density (MJ/m²)
- mᵢ = mass of each combustible item (kg)
- Hᵢ = calorific value of that material (MJ/kg)
- A = floor area of the compartment (m²)
The "By Room Type" mode skips the summation and uses a published density value for the occupancy. The "Custom" mode lets you enter the total energy directly if you have already summed it. Results assume all listed combustibles burn completely. Structural elements such as timber framing or wall linings should be included if they can contribute to the fire.
Reference Values
Typical fire load densities by occupancy (Eurocode EN 1991-1-2 and NFPA data, 80% fractile):
| Occupancy | MJ/m² |
|---|---|
| Classroom | 215 |
| Hotel room | 285 |
| Hospital room | 310 |
| Dwelling | 300 |
| Office | 420 |
| Warehouse (light) | 500 |
| Retail / shopping | 600 |
| Warehouse (heavy) | 1180 |
| Library | 1500 |
Calorific values for common materials:
| Material | MJ/kg |
|---|---|
| Cotton / textiles | 16 |
| Paper / cardboard | 17 |
| Wood / timber | 17.5 |
| Rubber | 24 |
| PVC | 25 |
| Plastics (mixed) | 40 |
| Diesel / heating oil | 43 |
| Petrol / polyethylene | 46 |
Worked Example
A 30 m² office contains 200 kg of timber furniture, 80 kg of paper, and 40 kg of mixed plastics.
- Timber: 200 × 17.5 = 3,500 MJ
- Paper: 80 × 17 = 1,360 MJ
- Plastics: 40 × 40 = 1,600 MJ
- Total: 6,460 MJ
- Fire load density: 6,460 ÷ 30 = 215 MJ/m²
That sits well below the 420 MJ/m² Eurocode reference for offices, suggesting the room is lightly loaded compared to typical office stock.
FAQ
Fire load vs. fire load density? Fire load is the total energy in MJ. Fire load density divides by floor area, giving MJ/m². Density is what fire codes use because it scales with compartment size.
Should I include the building structure? Include any combustible structure that can burn during the fire, such as timber floors, joists, or wall linings. Concrete, steel, and masonry are ignored.
Why use the 80% fractile? Eurocode tables give the value that 80% of comparable rooms fall below. It is a design value, not an average, and gives a margin for rooms that are more loaded than typical.
What is the equivalent in wood? Divide MJ/m² by 17.5 to get the kg of wood per m² that would release the same energy. 420 MJ/m² ≈ 24 kg of wood per m².
