Find how many smoke or heat detectors a room needs and where they go, using the NFPA 72 0.7 × spacing rule or the BS 5839-1 coverage radius — with ceiling-height derating for heat detectors and a layout drawing.
Smooth, flat ceilings only. Beams, joists, sloped ceilings, high air movement, stratification and open-grid ceilings change the spacing; follow the full requirements of the adopted standard and the detector's listing.
Both standards come down to one geometric test: every point on the ceiling must be within a set distance of a detector.
For a detector at the centre of a grid cell of size a × b, the farthest point is the cell corner, so the test is (a/2)² + (b/2)² ≤ R². The tool tries every layout and returns the one with the fewest detectors.
In a narrow corridor the cell is thin, so its corner is close to the detector across the width and the spacing along the corridor can stretch. With NFPA's 30 ft guide in a 10 ft corridor, (a/2)² + 5² ≤ 21² gives a ≈ 40.8 ft — the reason the NFPA annex shows corridor detectors about 41 ft apart. BS 5839-1 applies the same geometry to its 7.5 m radius, which allows about 15 m spacing in a corridor up to 2 m wide.
| Ceiling height | % of listed spacing | For a 50 ft listed detector |
|---|---|---|
| up to 10 ft (3.0 m) | 100% | 50.0 ft (15.2 m) |
| 10–12 ft (3.0–3.7 m) | 91% | 45.5 ft (13.9 m) |
| 12–14 ft (3.7–4.3 m) | 84% | 42.0 ft (12.8 m) |
| 14–16 ft (4.3–4.9 m) | 77% | 38.5 ft (11.7 m) |
| 16–18 ft (4.9–5.5 m) | 71% | 35.5 ft (10.8 m) |
| 18–20 ft (5.5–6.1 m) | 64% | 32.0 ft (9.8 m) |
| 20–22 ft (6.1–6.7 m) | 58% | 29.0 ft (8.8 m) |
| 22–24 ft (6.7–7.3 m) | 52% | 26.0 ft (7.9 m) |
| 24–26 ft (7.3–7.9 m) | 46% | 23.0 ft (7.0 m) |
| 26–28 ft (7.9–8.5 m) | 40% | 20.0 ft (6.1 m) |
| 28–30 ft (8.5–9.1 m) | 34% | 17.0 ft (5.2 m) |
Hot gases cool as they rise, so a heat detector on a high ceiling responds later. Its listed spacing is reduced to make up for it. BS 5839-1 instead caps the ceiling height — generally 10.5 m for point smoke and 9 m for point heat detectors.
Office 24 × 15 m, smoke detectors to NFPA 72 (S = 9.1 m, R = 6.37 m).
NFPA 72 uses 30 ft (9.1 m) as a guide on smooth ceilings, with every point on the ceiling within 21 ft (0.7 × 30 ft) of a detector. BS 5839-1 requires every point within 7.5 m of a smoke detector. Narrow corridors allow wider spacing along their length.
There is no fixed area per detector. A square layout at NFPA's 9.1 m spacing covers about 84 m² each, but room shape changes the count, which is why the tool calculates the actual grid instead of dividing by an area.
Under NFPA 72 the 0.7 × S rule controls the corners, and a detector may not be closer than 4 in (100 mm) to a wall on a ceiling. BS 5839-1 requires at least 500 mm from walls and partitions. The tool reports the distance from the walls to the first row and column.
NFPA 72 does not reduce spot smoke detector spacing with height, but stratification on high ceilings must be considered. Heat detector spacing is reduced above 10 ft. BS 5839-1 limits point smoke detectors to about 10.5 m ceiling height.
Every point on the ceiling must be within 5.3 m of a heat detector, compared with 7.5 m for a smoke detector, so heat detectors need roughly twice as many units for the same area.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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