Fire Exit Width Calculator

AD B Vol 2 Table 2.3 gives the minimum clear width in bands: up to 60 persons needs 750mm, up to 110 persons 850mm, and up to 220 persons 1050mm. Only above 220 persons does the 5mm-per-person rule apply (and widths under 1050mm are never interpolated — you snap up to the band). These are clear opening widths measured with the leaf open at 90°, not leaf or structural-opening sizes — the leaf is always wider than the clear opening because the door stop and the leaf thickness on the hinge side eat into the gap. As a rule of thumb a single leaf loses roughly 50–80mm: an 838mm leaf gives only about 770–790mm clear (just the 750mm band), and a 926mm leaf gives roughly 860–880mm clear (enough for the 850mm band). Always confirm the clear opening from the manufacturer’s doorset schedule rather than the leaf size. Wider clear openings than a single leaf can give are achieved with a 1.5- or 2-leaf doorset.

Approved Document B Vol 2 Appendix D, Table D1 provides floor space factors: offices 6m²/person, shop sales area 2m²/person, restaurant/dining 1m²/person, assembly hall/dance floor (standing, no fixed seating) 0.5m²/person, and bar/standing spectator areas 0.3m²/person. Divide the floor area by the factor to get the maximum occupant capacity. Areas with fixed seating are counted by the actual number of seats.

When there are two or more exits, the design assumes the largest exit is unavailable. The remaining exits must accommodate all occupants. For two equal exits, each must handle 100% of occupants. For three exits, the two remaining must handle all occupants.

Fire exits serving more than 60 persons must open in the direction of travel (outwards). Doors serving fewer than 60 people can open inwards if they are not on an escape route. Sliding and revolving doors are not acceptable as fire exits unless they have hinged panels that open in the escape direction.

AD B Vol 2 Table 2.2 sets the minimum number of escape routes/exits from a room or storey by occupancy: up to 60 people need 1 exit, up to 600 people need 2 exits, and more than 600 people need 3 exits. A single exit is therefore only acceptable up to 60 people (and subject to the single-direction travel-distance limits in Table 2.1). Travel distance, inner rooms and small-premises rules can require more exits than the headcount alone suggests.