Fire Alarm Zone Calculator

BS 5839-1 limits each fire alarm zone to a maximum floor area of 2000m². Each storey should generally form a separate zone unless the total area is less than 300m² and the search distance from the panel is acceptable. Zones should be arranged so that the seat of fire can be quickly identified — the zone indicator on the fire alarm panel should allow firefighters to identify the general area within 2 minutes.

BS 5839-1 defines system categories for different protection objectives. L categories protect life: L1 is full building coverage, L2 covers escape routes plus high-risk rooms, L3 covers escape routes only, L4 covers circulation spaces, L5 is engineered. P categories protect property: P1 is full coverage, P2 covers defined areas. The system category is determined by the fire risk assessment and building type. Most offices need L2 or L3, HMOs need L2 minimum. Note that the current edition, BS 5839-1:2025 (Section 20), no longer recommends heat detectors in rooms where people sleep for L1, L2 or L3 systems — smoke, multi-sensor or carbon monoxide detection should be used in sleeping rooms unless the fire risk assessment specifically justifies heat detection.

BS 5839-1 requires a manual call point at every exit from each storey to a protected stairway, at every final exit to the open air, and at any other point where a person might need to raise the alarm. In practice, this means at least one call point per exit on each floor. Call points should be positioned on exit routes at 1.4m above floor level, and the travel distance to the nearest call point should not exceed 45m.

BS 5839-1 requires a minimum of 65dB(A) throughout general and all accessible areas, or 5dB(A) above any ambient noise that persists for more than 30 seconds — whichever is higher (this may reduce to 60dB(A) in stairwells and enclosed rooms under 60m²). In sleeping areas a HIGHER minimum of 75dB(A) is required at the bedhead with all doors closed, because a normal door loses about 20dB(A) of sound and a fire door about 30dB(A) — so a sounder is normally needed inside each sleeping room. A common mistake is to reverse these two figures; specifying 65dB(A) at the bedhead instead of 75dB(A) can leave sleeping occupants unroused. Visual alarm devices (VADs) achieving a minimum 0.4 lux must be provided where hearing-impaired persons may be present.

BS 5839-1:2025 Annex A sites point detectors to fixed maximum spacing rather than a coverage area that shrinks with ceiling height. On flat ceilings up to 6m, optical or ionisation smoke detectors are spaced no more than 10.6m apart and 5.3m from any wall (7.5m radius), which works out to about 100m² per detector. Heat detectors are spaced no more than 7.5m apart and 3.8m from a wall (5.3m radius), about 50m² per detector. These spacings are constant up to a 6m ceiling — they do not step down at 3m. Above the point-detector maximum mounting height (roughly 10.5m for smoke, lower for heat) point detectors are no longer suitable and beam or aspirating detection should be specified instead. These figures assume flat, unobstructed ceilings; pitched ceilings, beams and partitions require additional detectors. BS 5839-1 applies to commercial / non-domestic premises only.

The current edition is BS 5839-1:2025, published 29 April 2025, which superseded BS 5839-1:2017 (withdrawn 30 April 2025). The key zoning, audibility and detector-spacing figures used here are unchanged from the 2017 edition — each zone is still capped at 2000 m², a building under 300 m² may form a single zone, manual call point travel distance stays at 45 m, and the audibility targets remain 65 dB(A) general / 75 dB(A) at the bedhead. The main 2025 changes are a recommendation against heat detectors in sleeping rooms (Section 20), L-category signal-monitoring timings (90 seconds for activation, 3 minutes for faults), red fire-alarm mains cable for identification, and a new Section 7 covering extensions and modifications. Always confirm the design against a fire risk assessment and the full BS 5839-1:2025 standard.

No. Dwellings, HMOs, sheltered housing and other sleeping premises are covered by BS 5839-6:2019, which is significantly stricter than BS 5839-1. Clause 11.2 h) ("Location and siting of fire detectors") states that within rooms no point should be further than 7.5 m from the nearest smoke detector, or 5.3 m from the nearest heat detector — which works out at roughly 50 m² per smoke detector (about 25 m² per heat detector). This is roughly half the commercial BS 5839-1 coverage. Using BS 5839-1 commercial spacing in a dwelling will under-provision detectors by about 2x and can leave bedrooms outside the detection radius. Select "Dwelling / HMO" in the premises type input to apply BS 5839-6 spacing.

Conventional systems group detectors into zones with a zone indicator on the panel. When a detector activates, the panel shows which zone is in alarm. Addressable systems give each detector a unique address, so the panel can identify the exact detector that has activated — pinpointing the fire location. Addressable systems are more flexible for zone configuration, easier to maintain, and provide better diagnostics. They are standard for commercial buildings and larger residential schemes.