Kerova

Calculator · Fire alarm · BS 5839-1

Fire alarm detector spacing calculator

Choose the detector type and enter the room's length, width and ceiling height. You get a planning count for a flat ceiling, the grid it works out to, and whether the ceiling height is inside the limit for that detector. It is an estimate, not a design.

Your figures

m
m
m

A flat ceiling. Sloping ceilings need their own design.

Result

Enter the room size and ceiling height to see the result.

Working

detectors = roundup(length ÷ (R × √2)) × roundup(width ÷ (R × √2))

  • A smoke detector covers any point up to 7.5 m away. A heat detector covers up to 5.3 m.
  • The largest square one detector covers has sides of the radius × √2: 10.6 m for smoke, 7.5 m for heat.
  • The count is the number of those squares along the length times the number across the width, rounded up.
  • Ceiling height limits: smoke and CO 10.5 m (12.5 m over up to 10% of the area); heat class A1 9 m (10.5 m); other heat 7.5 m (10.5 m). A multi-sensor that works on heat and smoke only takes the heat limit.
  • A corridor under 2 m wide is different: the coverage areas need not overlap, so smoke detectors can be about 15 m apart and heat detectors about 10.6 m. Over 2 m wide, treat it as a room.

Source: BS 5839-1, as set out in the FIA guide to the 2025 edition and in three detector and panel makers' design guides that agree. The reach, ceiling heights and 500 mm figures are unchanged from the 2017 edition. The number of detectors also has to allow for walls, beams, obstructions and voids, which this tool does not.

For the zone a room sits in, use the zone checker. For the standby battery, use the battery calculator.

For guidance only. Always check against the current edition of the standard and the manufacturer's data.

Built by the engineer behind Kerova, certificate software for fire and security firms.