Exit Width Calculator
The total exit width and the number of exits a space needs for its occupant load — from the width-per-person factor your code uses — with the minimum width of each exit.
Building codes allot egress width per occupant: about 5 mm per person for doors and level routes and 7.
How the exit width calculator works
Building codes allot egress width per occupant: about 5 mm per person for doors and level routes and 7.6–8 mm for stairs (the IBC figures), or a flow of 100 persons per 550 mm unit of width in older methods. Multiply the occupants by the factor for the total width, split it across at least two exits above the threshold occupancy (typically 50), and check each exit is at least the minimum clear width, usually 850–900 mm. The occupant load itself comes from the floor area and use.
Formula: total width = occupants × mm per person; exits = max(minimum by occupancy, ceil(total ÷ max width per exit))
Worked examples
| Inputs | Total egress width required | Note |
|---|---|---|
| 320 occupants, 5 mm each | 1,600 mm | 1,600 mm across 2 exits of 900 mm |
| 800 occupants on stairs at 7.6 mm | 6,080 mm | 6,080 mm across 3 exits of 2,050 mm |
| A small room of 30 | 150 mm | 150 mm — one 850 mm exit |
FAQFrequently asked questions
Where do the width factors come from?
From measured crowd flow: about 1.3 people per second through a meter of doorway, giving 5 mm per person for a 2.5-minute evacuation in the IBC; stairs are slower, so 7.6 mm. Other codes use similar figures or the 550 mm unit-of-width method.
Why at least two exits?
So that a fire blocking one still leaves a way out. Most codes require two above about 50 occupants, three above 500 and four above 1,000, placed far enough apart that one event cannot block both.
Does the widest exit count fully?
Usually not — codes assume the widest exit may be the blocked one and require the remainder to carry at least half the occupants. Check the local rule when one exit is much wider than the others.
How is the occupant load found?
From the floor area divided by an occupancy factor for the use — 1 m² per person for standing assembly, 2 for dining, 10 for offices. The occupant load calculator in this category gives it.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- International Code Council — publisher of the model codes most US jurisdictions adopt
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.