Escape Stair Width Calculator — AD B Capacity & Discounting
An escape staircase is sized by how many people it must evacuate, not by the general staircase rules. This calculator applies the Approved Document B capacity method for simultaneous evacuation — the P = 200w + 50(w − 0.3)(n − 1) formula — together with the discounting rule (where a building has more than one escape stair, the widest is assumed blocked by the fire), and the statutory minimum widths. Enter your occupancy, storeys and stair count, and get the required clear width per stair. Built by Continox — EN 1090 certified fabricators of steel fire escape staircases.
Escape Stair Width Calculator
Occupancy + storeys + stair count in — required clear width per stair out.
Escape stair width in England is driven by capacity: for simultaneous evacuation, Approved Document B gives P = 200w + 50(w − 0.3)(n − 1), where P is the number of people served, w the clear stair width in metres and n the number of storeys. Minimums: 1,000mm for any escape stair (800mm is possible only in limited low-occupancy cases), 1,100mm where the stair serves more than 220 people, and final exit doors at 5mm per person (min 750mm up to 60 people). Where a building has two or more escape stairs, the widest stair is discounted — assumed lost to the fire — and the remaining stairs must evacuate everyone. A stair can't be "roughly wide enough": it's a calculated number, and Building Control will ask to see the calculation.
The Capacity Formula — How Escape Stairs Are Sized
The logic: the first term is the flow capacity of the stair itself (200 persons per metre of width), and the second term credits the standing capacity of the stair enclosure on multi-storey buildings — people queue on the stairs while evacuating, so a stair serving more storeys holds more. Solve for w given your occupancy and storeys, round up to the next practical width, and apply the minimums. The calculator does exactly this, then checks the result against 1,000mm / 1,100mm floors.
The AD B capacity method: the formula sizes the stair, the statutory minimums set the floor.
The Discounting Rule — Why Two Stairs Doesn't Halve the Width
Where a storey has more than one escape stair, Approved Document B requires the design to assume the fire has made one of them — the widest one — unusable. The remaining stairs must evacuate the entire occupancy on their own. The practical consequence surprises people: adding a second stair doesn't halve the required width of each, because after discounting, one stair still carries everything. Real relief only starts at three or more stairs, where the load after discounting splits across two. This is why the calculator asks for the stair count and shows the load per stair after the discount — it's the number the design actually has to meet.
Clear width means clear widthThe calculated width is measured between walls or balustrades, with handrails allowed to intrude only up to 100mm each side. A "1,000mm staircase" measured over the strings with handrails eating 90mm per side is a ~900mm escape route and fails. On steel external fire escapes this is a fabrication decision — Continox details the stringers and handrails so the clear width matches the fire strategy, and it's stated on the drawings Building Control sees. See the fire escape price calculator for how width drives cost.
Minimum Widths — Reference Table
| Situation | Minimum Clear Width | Notes |
|---|---|---|
| Escape stair — general | 1,000mm | The baseline for any designated escape stair |
| Escape stair serving >220 people | 1,100mm | And wider still as the formula demands |
| Low-occupancy exceptions | 800mm | Limited cases (<50 people) — confirm with Building Control |
| Final exit doors | 5mm / person | Minimum 750mm up to 60 people; 850mm above |
| Firefighting stairs | 1,100mm | Buildings requiring firefighting shafts |
BS 9999 can give a different answer — deliberately. Approved Document B is the prescriptive route; BS 9999 is the risk-based alternative, where the required width flexes with the building's risk profile (occupancy type × fire growth rate) and management level, and can come out narrower or wider than AD B for the same building. If your project is on the margin — the formula says 1,180mm and the structure wants 1,100mm — a BS 9999 assessment by a fire engineer is often the answer. This calculator uses the AD B method because it's the common reference point; the two routes must not be mixed within one building.
Escape Stair Width — FAQ
Common questions about sizing escape staircases in the UK.
Built to the Strategy, Documented for Sign-Off
Steel escape staircases fabricated to your approved fire strategy — clear widths as calculated, guarding to BS 6180, EN 1090 UKCA documentation included. Free site survey, fixed-price quotation within 24 hours.