Battery standby calculator
Check standby capacity against Annex E.
Detector spacing calculator
Work out how many point smoke or heat detectors a rectangular room needs, and where they go, using the coverage radii in BS 5839-1:2025.
Based on BS 5839-1:2025 · Last reviewed October 2026
How the layout is calculated
Every point on the ceiling must be within a set distance of the nearest detector: 7.5 m for point smoke detectors and 5.3 m for point heat detectors.
| Rule | Point smoke | Point heat |
|---|---|---|
| Coverage radius | 7.5 m | 5.3 m |
| Corridor spacing | 15.0 m | 10.6 m |
| Max ceiling height | 10.5 m | 7.5 m |
Open areas
The calculator tries each possible number of columns along the room's length, works out how many rows are needed so that no point on the ceiling is further than the coverage radius from a detector, and keeps the arrangement that needs the fewest detectors.
Detectors sit at the centre of equal grid cells, so the distance from a wall to the first detector is half the spacing between detectors. Wall offsets are calculated automatically (minimum 0.5 m).
If removing one row or column would leave the corners less than 5% of the radius outside coverage, it's listed in the notes as an option for the engineer's discretion.
Corridors
Rooms less than 2 m wide are treated as corridors, as in BS 5839-1: a single row of detectors, up to 15 m apart for smoke or 10.6 m for heat, centred along the length. In Category L systems, escape-route corridors normally need smoke rather than heat detection.
High ceilings
Ceilings above 10.5 m (smoke) or 7.5 m (heat) aren't supported. BS 5839-1 allows more in some cases — class A1 heat detectors up to 9.0 m, and over no more than 10% of the ceiling area, point smoke detectors up to 12.5 m and heat detectors up to 10.5 m — but the calculator uses the general limits. For higher ceilings, consider aspirating detection, beam detection, or another suitable detection method.
What it doesn't cover
This is for rectangular rooms with flat ceilings. It doesn't account for pitched ceilings, beams, obstructions, partitions, voids or manufacturer-specific guidance — see the detector siting rules in the BS 5839 reference.