Escape route and stair width calculator
Two different rules govern width. Horizontal escape routes and final exits are sized from Approved Document B Table 2.3, a banded table that becomes 5 mm per person above 220 people. Escape stairs under simultaneous evacuation are sized from a capacity formula that also accounts for the people the stair itself can hold while the building empties.
For stairs 1100 mm wide or wider, paragraph 3.18 gives:
P = 200W + 50(W − 0.3)(N − 1)
where P is the number of people the stair can serve, W is the stair width in metres and N is the number of storeys served. There is no separate coefficient above 10 storeys: Table 3.2 is tabulated to 10 floors and its note 1 says capacities beyond that are obtained by linear extrapolation, which is exactly what this formula already does.
Width provided is not adequate
You need at least 1,400 mm and have 1,200 mm, a shortfall of 200 mm. Widen the stair, add a route, or reduce the occupancy the route serves.
Working
| Step | Value |
|---|---|
| Routes available after discounting one2 provided − 1 discountedADB paragraph 3.14 | 1 |
| People per remaining route500 ÷ 1 | 500 people |
| Width from the capacity formulaW = (P + 15N − 15) ÷ (150 + 50N) with P = 500, N = 5ADB paragraph 3.18(b) | 1,400 mm |
| Absolute minimum for this situationAny other building, serving an area with an occupancy of more than 50: Table 3.1 gives no fixed figure for this row, it directs you to Table 3.2Approved Document B, Volume 2, Table 3.1 | see note 3 |
| Capacity of the width providedP = 200(1.2) + 50(1.2 − 0.3)(5 − 1)Approved Document B, Volume 2, Table 3.2 | 420 people |
What this means
- Paragraph 3.10a requires the stair to be at least as wide as any exits giving access to it. Check that against the storey exit widths on your most heavily occupied floor; this tool sizes the stair on total population, so it cannot make that comparison for you.
- Table 3.1 gives no fixed minimum for this row: see note 3, which directs you to Table 3.2 for simultaneous evacuation and Table 3.3 for phased evacuation.
- Clear width is measured between walls, strings or balustrades. Handrails may intrude up to 100 mm each side and strings up to 30 mm without reducing the effective width.
- Paragraph 3.10c: the width must not reduce at any point on the way to a final exit. Paragraph 3.12: where the exit route from a stair is also the escape route from the ground or basement storey, the final exit may need to be wider still.
- Where a basement stair and an upper-floor stair merge at ground level, check the merging-flow provisions; the combined final exit usually needs to be wider than either stair on its own.
Sources
- Approved Document B, Volume 2, Table 2.3Minimum width of escape routes and exits.
- Approved Document B, Volume 2, Table 3.2Capacity of an escape stair for simultaneous evacuation. The underlying formula is paragraph 3.18, which applies to stairs 1100 mm wide or wider; note 1 covers extrapolation beyond 10 storeys.
- Approved Document B, Volume 2, Table 3.1Minimum widths of escape stairs by building situation.
- Approved Document B, Volume 2, Table 3.3Minimum stair width for phased evacuation, based on the occupancy of the most heavily occupied storey.
- Approved Document B, Volume 2, paragraph 3.10A stair should be at least as wide as any exits giving access to it, no less than Table 3.1, must not reduce on the way to a final exit, and must not exceed 1400 mm where taller than 30 m unless a central handrail is provided.
- Approved Document B, Volume 2, paragraphs 3.14, 3.15 and 2.21Discounting of stairs, the smoke-control and protected-lobby exceptions to it, and the storey exit that is discounted regardless.
- BS 9999:2017, clause 17.4Stair widths and the merging-flow provisions where a basement stair joins an upper-floor stair.
This is an engineering aid, not engineering advice. The result above is produced from the inputs you gave and the guidance cited. It does not account for everything a competent fire engineer would consider on a real building, and it is not a substitute for reading the current published standard. Check every number before it goes into a design.
Questions people ask
How wide does an escape stair need to be?
It depends on how many people it serves and how many storeys it serves. For simultaneous evacuation, Approved Document B paragraph 3.18 gives P = 200W + 50(W − 0.3)(N − 1), where P is the number of people, W is the width in metres and N is the number of storeys. That formula only applies to stairs 1100 mm wide or wider, so a narrower result is uplifted to 1100 mm; Approved Document B's own worked examples do exactly that. Table 3.1 then sets absolute minimums by building situation: 1000 mm in a residential (institutional) building, 1100 mm in an assembly building, and 800 mm for a stair serving no more than 50 people.
Does the stair capacity formula change above 10 storeys?
No. A widely repeated version of this formula swaps the 200 coefficient for 150 above 10 storeys, but no such rule appears in Approved Document B. Paragraph 3.18 gives a single formula, and note 1 to Table 3.2 says capacities for stairs serving more than 10 storeys are obtained by linear extrapolation, which the formula already does, because the per-storey increment 50(W − 0.3) is constant.
What is the difference between simultaneous and phased evacuation?
In simultaneous evacuation everybody leaves at once, so the stair must hold and discharge the whole building population. In phased evacuation only the fire floor and the floor above leave first, so the stair is sized on the most heavily occupied storey instead, which usually gives a much narrower stair, but demands a voice alarm, sprinklers above a certain height, and firefighting lifts.
Is stair width measured to the handrail?
Width is the clear width between the walls or balustrades. Handrails may intrude up to 100 mm on each side, and strings up to 30 mm, without being counted as a reduction. Anything more and you lose it from the effective width.
What is the minimum width of an escape route?
Table 2.3 gives 750 mm for up to 60 people, 850 mm for up to 110, 1050 mm for up to 220, then 5 mm per person above that. Note 3 says widths below 1050 mm should not be interpolated, and note 5 says the 5 mm per person rule does not apply to an opening serving fewer than 220 people.
Can an escape stair be too wide?
Yes. Paragraph 3.10d says a stair taller than 30 m should not exceed 1400 mm unless a central handrail is provided, because an unbroken expanse of width is hard to use under crowd conditions. Where a central handrail is fitted, the width each side of it is assessed separately when working out capacity. Approved Document K separately requires a central handrail on stairs wider than 2000 mm in public buildings.
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