Capacity calculation for business premises
This guide is for the calculation of capacity figures for a single room or simple premises comprising no more than a ground floor.
For multi-storey buildings or those with a more complex layout, you should ask an accredited fire risk assessor or a building control professional.
Fire safety and capacity
The term capacity refers to the number of people that a room, space, or building can safely accommodate. It takes account of:
- the function of the room and
- the number and width of fire exit routes available.
Where the number of occupants is such that they could not all leave the building within the required evacuation time, the safe capacity of the escape route has been exceeded and you must take steps to address this issue.
There must be enough exits to allow for the safe evacuation of all the occupants of a building within a reasonable time. This is fundamental to the fire safety provision of the building.
The starting point in determining whether there are enough exits is to consider how the building or space is used.
Where the number of people in a building is not fixed
You need to establish a maximum capacity to ensure that there are not more people than the exits can deal with. Examples would include a village hall, a community centre, a cafe or a simple shop.
Where the number of people in the building is known
This may be where fixed seating is provided such as cinemas, lecture theatres, or office blocks. This guide does not cover capacity calculations for these types of premises. These are potentially complex buildings, and their capacities will have been established during the build and certification process.
Calculating a maximum capacity figure
To determine the maximum number of people that a room can accommodate you must carry out two separate calculations:
- Calculate the maximum number of people based on the use of the room.
- Calculate the maximum number of people based on the number and width of exits.
You must take the lower of these two figures as your room capacity.
We have provided an example on this page of how to make these calculations.
You will need to refer to table D1 in the current edition of Approved Document B to the Building Regulations 2010: Volume 2.
Table D1 describes various types of rooms, and applies a floorspace factor. This is the minimum area required by a single person in a room of each type.
To establish a capacity for your room
- take the area of the room in square metres
- divide that figure by the floorspace factor appropriate to the use of the room as described in table D1.
A basic example
An office with a floor area of (5m x 5m) 25 sqm has a floorspace factor of 6 sqm per person. The capacity of the office is calculated as 25 divided by 6. The capacity (rounded down) is 4 people.
Where a room has areas of different use, it may be necessary to consider the floorspace in terms of zones with each zone calculated separately using the appropriate floorspace factor from table D1. The sum of each zone would be added together to give the capacity for the whole room.
A bar room measuring 5m x 10m could have these zones:
- A space for ordering from the bar: 4m x 2m x 3 people per square metre = 24 people
- 5 seating booths around the side of the room each having seats for 4 people = 20 people
- Main circulating area of the room: 4m x 4m x 1 person per square metre = 16 people
- Members of staff behind the bar = 5 people
- Total capacity based on room use = 65 people
You will need to refer to the 'escape routes and capacities' section on page 67 of the Fire Safety Risk Assessment Guide.
Each exit door from a room allows the room to accommodate a calculated number of people; the more exit doors a room has, the more people it can potentially hold. The width of each exit door also affects the number of occupants.
- Where a room has only one exit, regardless of its dimensions, the capacity is limited to 60.
- Where there are multiple exits from a room, the capacity is the sum of all the exit door capacities.
- For rooms with more than one exit, we assume that a fire has blocked the largest exit and we discount it from the calculation.
In our bar example, there are three exits and the Fire Safety Risk Assessment Guide gives the following numbers for each:
- The 750mm door can accommodate 100 people
- The 1050mm door can accommodate 200 people
- The 1500mm door can accommodate 290 people
- We discount the figure for the largest exit because we assume that a fire has blocked it. Therefore, we ignore the 1500mm door.
- Thus, the room capacity based on exits is 300.
You take the lower figure from the two calculations so in our example the room capacity is 65.
We have an example of a bar with different zones and five members of staff.
The capacity calculation based on room use is 65.
The capacity based on exits is 300.
You take the lower figure so the capacity in our example is 65.
Calculating the capacity for a room or building goes hand in hand with the amount of time needed to safely evacuate all the people.
The Risk Assessor must consider these factors to establish a suitable evacuation time that would allow everyone to get out of the building before the escape routes become unusable due to fire and smoke.
As a starting point there are three benchmark evacuation times depending on the construction of the building:
- Concrete buildings have a benchmark evacuation time of three minutes
- Traditional brick and timber buildings – two and a half minutes
- Wholly timber construction buildings – two minutes.
These times can be increased or decreased depending on the risks within the building. These risks may include:
- a higher fire loading than usual
- excessive travel distances to a fire escape
- occupants needing help such as the elderly, children or the disabled.
For the small to medium-sized premises covered by the guidance on this page:
- the furthest distance to a fire escape is less than 18 metres
- the occupants have reasonable mobility
- the fire risk is low/normal.
So, an evacuation time of two and a half minutes should be aimed for.
Where factors such as those mentioned above lead to a longer evacuation time being required, further measures must be taken to ensure that the escape routes remain protected from fire. In these cases, you should ask a competent fire risk assessor.
Where several rooms lead to a common escape corridor your final capacity figure must be further modified by applying the exit width calculation to the number of final exits from the building. For further reading on this subject see the 'escape routes and capacities' section of the Fire Safety Risk Assessment Guide appropriate to your type of premises.
The following points should also be addressed to ensure that occupants can use the escape routes without delay.
- Alternative exits should be suitably separated, so they cannot be affected by or lost to the fire.
- Exit doors must open in the direction of escape.
- Exit doors must have suitable emergency fastenings consisting of a simple device, which is easy to use in the event of an emergency.
- The width of escape routes should not reduce after leaving the room.