Egress Width Calculator
Occupants times the per-100-persons index gives total width and exit count.
Inputs
Result
Formula
| Total width | B = occupants ÷ 100 × index |
| Index | ground-2nd 0.65/0.75/1.00; 3rd 0.75/1.00; 4th+ 1.00/1.25 |
| Basement | 0.75 at ≤10 m depth, 1.00 beyond; crowded rooms min 1.00 |
| Min per exit | 0.90 m general, 1.40 m crowded |
| Exit count | at least two per compartment |
3rd storey, grade II, 500 people: index 0.75 gives 3.75 m total; at 0.90 m minimum per exit that divides into 5 exits averaging 0.75 m, which is below the minimum, so 5 exits of at least 0.90 m each (4.5 m total) is what gets built.
FAQ
How is the occupant load calculated?
Not by dividing floor area arbitrarily - use the code density figures: retail halls around 0.5-0.85 persons/m² depending on storey and size, entertainment venues 0.5 persons/m², offices 4-8 m² per person, classrooms 1.0-1.2 m² per person. Getting this wrong is one of the most common reasons fire reviews are returned.
Why does a higher storey need a wider index?
Because egress is longer and depends on stairs, which become the bottleneck in a fire. A larger width per person effectively gives the stairs more capacity - hence 1.00 m per 100 persons at the 4th storey and above versus 0.65 at the lower floors.
The total is 3.75 m - can I just use four 1 m doors?
The total works but each door must also meet the minimum clear width (0.90 m, or 1.40 m in crowded occupancies), and the travel distance from the worst point must still comply. Remember clear width excludes the frame and the open leaf, so a 1 m opening often gives about 0.9 m of usable width.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.