Egress Width Calculator

How to work it out

1. Establish the occupant load. Crowded premises derive it from seats or floor area (retail at 0.5-0.85 persons per m² of sales area), while other buildings use actual occupancy. Get this wrong and everything downstream is wrong.
2. Look up the width per 100 persons by floor: 0.65 for ground and first floors in class 1 and 2, 0.75 in class 3, 1.00 in class 4; 0.75 or 1.00 for the third floor; 1.00 or 1.25 for the fourth and above.
3. Basements: 0.75 where the difference in level to the exit is 10 m or less, 1.00 above that, and never less than 1.00 in crowded basements.
4. Total clear width = occupants ÷ 100 × the per-100 figure. Five hundred people on the ground floor of a class 1 or 2 building gives 500 ÷ 100 × 0.65 = 3.25 m.
5. Check the minimum clear width of each individual exit: 0.90 m generally, 1.40 m in crowded premises. Sufficient total width with undersized individual exits still fails.
6. Check the exit count: at least two per fire compartment, dispersed with a separation of at least 5 m.

Where it goes wrong

• Treating structural opening width as clear width. Clear width excludes frames, handrails and finishes, so a 1.5 m opening may only give 1.3 m of passage.
• Using the general 0.65 figure for crowded premises. Such premises may require both a higher per-100 figure and a 1.40 m minimum per exit; both apply.
• Using above-ground values below ground. Basements over 10 m in level difference require 1.00 against 0.65 above — 54% more — and getting it wrong badly under-provides exits.
• Accepting adequate total width without checking individual exits. A 3.25 m total can be two 1.63 m exits or four at 0.81 m; the latter each fall below the 0.90 m minimum and fail.
• Ignoring stair and lobby widths. Adequate exit width with a narrower stair flight simply moves the bottleneck, and overall escape capacity is set by the narrowest point.

Material and cost

Each additional 0.1 m of exit width adds roughly 1-2 m² of stair and corridor area per floor. In a 20-storey building with four exits per floor, that is tens of square metres. The greater cost, though, is re-planning the core when concept-stage widths prove inadequate.

On site

• Occupant load is not simply the seat count. Retail uses sales area, exhibition halls use display area, and entertainment venues use licensed capacity — all with defined conversion factors.
• The total clear width of ground-floor external doors must exceed the combined width of the stairs above, or the ground floor becomes the bottleneck. This is frequently missed.
• Stair flight width is measured at the narrowest point, from the inside of the handrail to the wall, not the structural width.
• Escape corridor width is regulated separately, generally at least 1.4 m and 1.5-2.0 m in crowded premises. Do not size exits while ignoring corridors.

Basis

• GB 50016-2014 (2018 edition) Code for fire protection design of buildings, means of escape width
• GB 50016-2014 (2018 edition) Code for fire protection design of buildings, number of exits
• GB 50016-2014 (2018 edition) Code for fire protection design of buildings, clear width of escape routes