How wide does a fire exit door need to be?

The minimum clear width of a fire exit is calculated at 5mm per person using that exit. The absolute minimum width is 850mm for any fire exit door. For most commercial buildings with moderate occupancy, a standard 926mm single leaf door is sufficient. Double doors (1800mm+) are needed for larger occupancies.

How do you calculate occupant numbers?

Approved Document B Table C1 provides floor space factors: offices 6m²/person, shops 2m²/person, assembly (seated) 1m²/person, assembly (standing) 0.5m²/person. Divide the floor area by the factor to get the maximum occupant capacity.

What is the discount factor for multiple exits?

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.

Do fire exits need to open outwards?

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.

How many fire exits does a building need?

A single exit is acceptable for small premises (up to 60 occupants in most cases, or up to 600 at ground level with short travel distances). Two exits are needed for most buildings. Three or more exits are required for larger occupancies or where travel distances demand it.

TradeCalculator

Fire Exit Width Calculator

Calculate the minimum fire exit door widths based on occupant numbers per Approved Document B. Uses the 5mm per person rule for UK commercial and residential buildings.

Building Type

Determines occupant density

Floor Area

Usable floor area of storey

Known Occupants

Leave 0 to auto-calculate from area

Number of Exits

Fire exit doors provided

Price per Exit Door

Fire door supply and install

How We Calculate This

This calculator determines minimum fire exit widths using the Approved Document B method.

The formula

Exit width = Number of persons × 5mm (minimum 850mm)

Occupants = Floor area ÷ Floor space factor
5mm per person minimum width
Absolute minimum: 850mm clear width
Discount largest exit when 2+ exits provided

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.