Calculate fire load density in MJ/m² from material masses and floor area, with typical calorific values for wood, paper and plastics, or your own values.
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Fire Load Formula
Fire load density is the total combustible energy in a space divided by its floor area.
- 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)
Choose a material inventory (one to four materials) or enter known total energy for density and area calculations. Total-energy mode sums the inventory. Results represent nominal complete-combustion energy. Include combustible structural materials when required by the assessment; the arithmetic does not determine which design factors or protected components apply.
Reference Values
Historical survey examples reproduced in NIST TN 1681, Table 3.3(c), citing ECCS (2001). The mean and 80th percentile are different statistics. These are not current code limits or automatically applicable design values:
| Occupancy | Mean MJ/m² | 80th percentile MJ/m² |
|---|---|---|
| Office | 420 | 511 |
| Dwelling | 780 | 948 |
| Hotel | 310 | 377 |
| Shopping center | 600 | 730 |
| Schools | 285 | 347 |
| Hospitals | 230 | 280 |
Illustrative calorific assumptions retained for examples and presets follow. Actual values vary with composition and moisture; obtain material-specific net calorific data on a consistent basis for assessment:
| 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 = 215.33 MJ/m² (rounded)
This illustrative inventory gives 215.33 MJ/m². Comparing it with a historical occupancy statistic does not establish that the room is safe or compliant. Fire behavior also depends on ventilation, fuel arrangement, burning rate and other conditions.
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.
What is an 80th percentile? It is a statistical value with approximately 80% of the modeled observations below it. A mean is different. Selecting a fire-engineering design value also requires the applicable assessment method, factors and jurisdictional requirements.
What is the equivalent in wood? Using an assumed reference of 17.5 MJ/kg, divide MJ/m² by 17.5 to express equivalent reference-wood mass per m². For example, 420 MJ/m² corresponds to 24 kg/m² on that basis. This is not an actual wood mass or a universal wood heating value.
