Occupant capacity calculator (floor space factors)
Occupant capacity drives almost everything downstream in a means-of-escape design: the number of exits, the width of every door on the route, the width of the stairs, and whether you can accept a single direction of travel at all. Get it wrong at the start and every number after it is wrong too.
Add each area of the storey, pick its use, and the calculator applies the floor space factors in Approved Document B Volume 2, Table D1 (Appendix D) and totals the occupancy. It then returns the minimum number of escape routes that occupancy requires under Table 2.2, and the minimum width each one needs under Table 2.3.
144 people, 1,050 mm per exit
With the largest exit discounted, each remaining exit must be at least 1,050 mm clear width. Clear width is measured between the door stops with the door open 90°, not the structural opening.
Working
| Step | Value |
|---|---|
| Office500 m² ÷ 6 m²/personApproved Document B, Volume 2, Table D1 (Appendix D) | 83.3 people |
| Committee, common, conference, dining, licensed betting (pub…60 m² ÷ 1 m²/personApproved Document B, Volume 2, Table D1 (Appendix D) | 60 people |
| Rounded occupant capacity143.3 rounded up | 144 people |
| Escape routes required144 people: Table 2.2 requires 2Approved Document B, Volume 2, Table 2.2 | 2 (2 provided) |
| Exits available after discounting the largest2 provided − 1 discountedADB paragraph 2.21 | 1 |
| Minimum width of each remaining exit144 people falls in the 1050 mm bandApproved Document B, Volume 2, Table 2.3 | 1,050 mm |
What this means
- Escape routes must also satisfy the minimum widths for corridors and doors, and the 1050 mm minimum where an escape route is used by wheelchair users in a place of assembly.
- Table 2.3 note 3: widths below 1050 mm should not be interpolated. Note 5: the 5 mm per person rule does not apply to an opening serving fewer than 220 people.
Sources
- Approved Document B, Volume 2, Table D1 (Appendix D)Floor space factors, in square metres per person, by use of the space. Car parks are given as two persons per parking space rather than as an area factor.
- Approved Document B, Volume 2, Table 2.2Minimum number of escape routes and exits from a room, tier or storey: one up to 60 people, two up to 600, three above 600.
- Approved Document B, Volume 2, Table 2.3Minimum width of escape routes and exits: 750 mm up to 60 people, 850 mm up to 110, 1050 mm up to 220, then 5 mm per person.
- Approved Document B, Volume 2, paragraph 2.21Where multiple storey exits are available, fire might prevent one from being used, so when using Table 2.3 the largest exit should be discounted.
- Approved Document B, Volume 2, paragraph 2.9The building design should be based on the number of occupants; where that number is not known, use the floor space factors in Appendix D.
This is an engineering aid, not engineering advice. The result above is produced from the inputs you gave and the guidance cited. It does not account for everything a competent fire engineer would consider on a real building, and it is not a substitute for reading the current published standard. Check every number before it goes into a design.
Questions people ask
What is the floor space factor for an office?
6 m² per person. Table D1 of Approved Document B Volume 2 gives Office as a single row at 6.0; there is no separate 'open plan over 60 m²' row in this edition, and the 5.0 row is art galleries, dormitories, factory production areas, museums and workshops. If you have seen 5 m²/person quoted for offices, it comes from older guidance or from a misreading of that 5.0 row.
How do you calculate occupant capacity?
Divide the net floor area of each space by the floor space factor for its use, then add the results, plus any areas where occupancy is known directly: fixed seating, for example. Approved Document B Volume 2 Table D1, in Appendix D, gives the factors. Exclude stair enclosures, lift shafts, service risers and toilet cores from the area. Paragraph 2.9 makes clear the factors are a fallback for when the number of occupants is not otherwise known.
How is a car park's occupancy worked out?
Not by area. Table D1 gives car parks as two persons per parking space, so you count spaces rather than dividing the floor area by a factor. It is the only row in the table that works this way.
Why is the largest exit discounted?
Because a fire can block an exit. Paragraph 2.21 says that where multiple storey exits are available, fire might prevent one from being used, so the remaining exits need to be wide enough for all occupants and the largest is discounted when using Table 2.3. A storey with two equal exits therefore needs each one sized for the whole occupancy.
How many exits does a storey need?
Table 2.2 gives the minimum: one exit up to 60 people, two up to 600, and three above 600. That is a floor, not a target; the number is likely to be increased by travel distance limits and by the requirement that alternative escape routes are far enough apart that one fire cannot block them all.
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