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Commercial Fire Strategy Calculator
Determine fire strategy requirements for commercial buildings including travel distances, exit widths, fire resistance and compartmentation per Approved Document B.
Total usable floor area per storey
Floor space factor per AD B Appendix D Table D1 (office 6, shop sales 2, assembly hall 0.5, storage 30)
Actual number of exits in your design
Enables sprinkler trade-offs for distances and areas
Mechanical or natural smoke ventilation provided
Your measured maximum travel distance for comparison
Safety notice
Fire safety measures must satisfy Approved Document B and often need building control approval. Confirm the final specification with building control or a fire engineer before relying on it.
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How We Calculate This
This calculator determines the key fire strategy parameters for commercial buildings based on building use, size, height and installed fire protection systems. It follows Approved Document B and BS 9999 guidance.
How it works
- Purpose group: Sets the floor space factor (Table D1), travel distances (Table 2.1) and compartment size limits (Table 8.1).
- Occupancy: Floor area divided by the floor space factor gives the design occupancy used for exit sizing.
- Exit width: Banded per Table 2.3 — 750mm up to 60 people, 850mm up to 110, 1050mm up to 220, then 5mm per person above 220 (the 5mm rule does not apply below 220).
- Number of exits: Table 2.2 — 1 up to 60 people, 2 up to 600, 3 above 600; larger occupancies are governed by exit-width discounting, not a higher count.
- Fire resistance: Appendix B Table B2, by purpose group and height of the top floor above ground.
- Sprinkler trade-off: Does not extend AD B travel distances; it can enlarge compartment areas (Table 8.1) and lower fire-resistance periods (Table B2 sprinklered column). Longer travel needs a BS 9999 risk-profile assessment.
Frequently Asked Questions
Occupant capacity is calculated by dividing the floor area by a floor space factor based on the building use. From Approved Document B Volume 2 Appendix D Table D1, typical factors are: offices 6m²/person, shop sales area 2m²/person, assembly hall / dance floor 0.5m²/person, restaurant / dining / bar 1.0m²/person, factory production area 5m²/person, storage and warehousing 30m²/person. The calculated occupancy then drives exit widths (Table 2.3), the number of exits (Table 2.2) and fire alarm provision.
Maximum travel distances depend on the building use and whether escape is in one direction or more than one, per AD B Volume 2 Table 2.1. For offices: 18m one direction, 45m more than one direction. For shop and commercial: 18m and 45m. For assembly and recreation with seating in rows: 15m and 32m; assembly elsewhere: 18m and 45m. For industrial and storage at normal hazard: 25m and 45m (higher hazard 12m and 25m). These AD B figures are fixed: sprinklers do not extend them. A longer travel distance is only justified by a BS 9999 risk-profile assessment.
Sprinklers do not extend the AD B Table 2.1 travel distances directly; those figures are fixed. Under BS 9999:2017 a building fitted throughout with sprinklers can have its risk profile lowered by one level (for example B3 to B2), which in the standard’s worked example raises one-direction travel from 16m to 20m and two-direction from 40m to 50m (about +25%, not a flat +50%). Sprinklers do allow maximum compartment floor areas to be increased (broadly doubled per AD B Table 8.1) and reduce the required fire-resistance period in Appendix B Table B2 (use the sprinklered column).
Per AD B Volume 2 Table 2.2 the minimum number of escape routes depends on occupancy: up to 60 persons may have 1 exit (subject to travel distance limits), up to 600 persons need 2 exits, and more than 600 persons need 3 exits. There is no fixed 4-exit band: above 600 people the governing check is total exit width after discounting the largest exit (paragraphs 2.21 to 2.23), not a higher fixed count. Exit width itself is banded in Table 2.3, not a flat 5mm per person (see the next question).
AD B Volume 2 Table 2.3 sets exit widths in bands, not by interpolation: up to 60 persons need a 750mm minimum width, up to 110 persons 850mm, up to 220 persons 1050mm, and only above 220 persons does the 5mm-per-person rule apply. Note 5 states the 5mm-per-person figure does not apply to an opening serving fewer than 220 people, and Note 3 says widths below 1050mm should not be interpolated. So a 90-person office exit needs 850mm, not 450mm.
Fire-resistance periods come from AD B Volume 2 Appendix B Table B2, keyed on the purpose group and the height of the top floor above ground (single storey / up to 5m, up to 18m, up to 30m, over 30m), without sprinklers. Offices: 30, 60, 90 minutes. Shop and commercial and assembly and recreation: 60, 60, 90 minutes. Industrial and storage: 60, 90, 120 minutes. Single-storey commercial buildings other than offices therefore need 60 minutes, not 30. Above 30m these groups require sprinklers. These periods apply to the structure, compartment walls and compartment floors.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.