What is fire load?
The fire load is the amount of heat energy that can be released when all the combustible materials in a space burn, including any combustible building elements. It is a quantity of energy, usually expressed in megajoules, that describes the available heat potential, not the probability of a fire occurring.
On its own, that figure doesn’t tell us much: the same value has different consequences depending on whether it’s concentrated in twenty square meters or spread over two thousand. That’s why the standard technical term is fire load density, that is, the fire load per unit area.
The fire load is part of the fire risk characterization , along with the probability of ignition, spread, occupancy and available means of protection.
Fire load, density, and weighted and corrected density
- Total fire load. Heat energy that can be released by all combustible materials in the space considered. It is an absolute value.
- Fire load density. Fire load per unit of built area, or per unit of the entire building envelope. Allows comparison of spaces of different sizes.
- Weighted and corrected fire load density. Used in industrial establishments, it incorporates a coefficient that weights the hazard by the combustibility of each material and another that corrects the hazard inherent to the activity carried out, such as production, assembly, transformation, repair or storage.
- Design fire load density. In the building sector, the value considered to determine thermal actions in a fire situation, taking into account uncertainties.
These quantities are not interchangeable. Citing a figure without specifying which one it is and what area it refers to renders the data useless.
What is it used for?
- Classify the intrinsic risk level of a sector or area of fire in industrial establishments.
- Determine construction requirements: compartmentation, fire resistance of the structure, maximum surfaces and evacuation conditions.
- Define the required fire protection systems and their scope.
- Delineate areas of special risk within non-industrial buildings.
- To provide a basis for fire safety engineering studies and structural calculations in fire situations.
How is it determined in industrial establishments?
Annex I of Royal Decree 164/2025 establishes that the weighted and corrected fire load density of each fire sector or area can be calculated by several methods: from the combustibility data of the materials present, from the fire load density data of the areas with manufacturing activities and similar processes, from the data corresponding to the storage areas, or by combining several of them.
In the first method, the formula considers the calorific value and mass of each combustible material, a dimensionless coefficient that weights the degree of hazard based on its combustibility, a coefficient that corrects for the inherent hazard of the activity, and the built area of the sector or fire zone. In the second method, instead of the mass of each material, average fire load density values for each activity are used, taken from the tables in the regulations themselves.
The standard states that the responsibility for the accuracy of the values used lies with the designer, who must adjust or increase them when the specific case requires it. This sheet describes the procedure and does not reproduce the tables from the regulations.
Intrinsic risk level
The intrinsic fire risk level of a sector or area reflects the risk derived from the quantity of combustible materials present, their flammability, their distribution, and the nature of the activities carried out there. It is classified as low, medium, or high and is further subclassified into eight levels based on the weighted and corrected fire load density.
The scale starts at values equal to or less than 425 megajoules per square meter, corresponding to the lowest level, and reaches over 13,600 megajoules per square meter at the highest level. The regulatory requirements regarding configuration, compartmentalization, materials, evacuation, and protective installations then depend on this classification.
Current Spanish framework
Royal Decree 164/2025, of March 4, approves the Fire Safety Regulations for Industrial Establishments. It was published in the Official State Gazette (BOE) on April 10, 2025, entered into force on May 10, 2025, and repealed Royal Decree 2267/2004, which had previously governed this matter. Any reference to the 2004 regulations as being in force is incorrect.
For non-industrial buildings, Basic Document SI of the Technical Building Code uses fire load density in the definition of special risk premises and in the annex dedicated to the fire resistance of structures, where it employs the design fire load density. The consolidated text of Basic Document SI includes among its modifications the one introduced by Royal Decree 164/2025 itself.
The methods, coefficients, and thresholds depend on the jurisdiction and the type of establishment, so the values from one country cannot be transferred to another without checking its applicable regulations.
Limits and common mistakes
- Presenting a single formula as universal: the regulations themselves allow for several methods depending on the type of zone.
- Cite Royal Decree 2267/2004 as the current regulation.
- Confusing the technical magnitude with the legal classification of risk: the fire load density is a fact, the intrinsic risk level is a regulatory consequence.
- Applying the industrial regulations to a building that is governed by the Technical Building Code, or vice versa.
- Calculate the fire load using one day’s inventory and do not revise it when the process or storage changes.
- Forgetting that the result does not replace the occupational risk assessment or the emergency plan .
Practical example
A furniture manufacturing company is expanding its factory with a storage area for boards and varnishes. Before construction begins, the project must recalculate the fire load.
- Compartmentation. It is decided to separate the storage area from the manufacturing area into distinct fire compartments.
- Calculation. For the manufacturing area, the method based on fire load densities per activity is used; for storage, the specific method for these areas is used, considering the stacking height.
- Result. The warehouse is classified as having a high intrinsic risk level, which determines the maximum area of the sector and the required protection facilities.
- As a consequence, the project adjusts the compartmentation and provides for the corresponding facilities, and the company updates its emergency plan with the new configuration.
Differences with related concepts
- Regarding the risk of fire. Risk includes probability and consequences; fire load describes only the available heat potential.
- Regarding fire resistance. This is a property of the building elements, not a characteristic of the contents of the premises.
- Regarding reaction to fire. It classifies the behavior of a material in fire, not the total energy it can release.
- Regarding occupancy. The number of people affects evacuation, but is not part of the fire load calculation.
Regulatory framework
- Royal Decree 164/2025, Annex I. Fire safety regulations in industrial establishments: calculation of the weighted and corrected fire load density and classification of the intrinsic risk level.
- Technical Building Code, Basic Document SI . Safety in case of fire in buildings: special risk premises and calculation fire load density.
- Royal Decree 513/2017 . Regulation of fire protection installations.
- Law 31/1995, article 20. Obligation to adopt emergency measures regarding fire fighting and evacuation.
