How is occupant capacity calculated for fire strategy?

Occupant capacity is calculated by dividing the floor area by an occupant density factor based on the building use. Typical densities are: offices 6m2/person, shops and retail 2m2/person, assembly standing 0.5m2/person, assembly seated 1m2/person, industrial 5m2/person, storage 30m2/person. These figures come from AD B Table 1 and BS 9999. The calculated occupancy determines exit widths, number of exits and fire alarm requirements.

What are the maximum travel distances in commercial buildings?

Maximum travel distances depend on the building use and whether escape is in one direction or two. For offices: 18m one direction, 45m two directions. For shops: 18m and 45m. For assembly: 15m and 32m. For industrial and storage: 25m and 45m. These are per AD B Table 3. With sprinkler protection, distances may be increased by up to 50 percent per BS 9999 Table 15.

How does sprinkler protection affect fire strategy?

Sprinkler protection provides significant trade-offs in fire strategy. Travel distances may be increased by up to 50 percent. Maximum compartment floor areas may be doubled. Fire resistance periods may be reduced by 30 minutes (to a minimum of 30 minutes) for some building types. These benefits are detailed in BS 9999 and recognised in AD B as alternative compliance approaches.

How many exits does a commercial building need?

The number of exits depends on the occupant capacity: up to 60 persons may have a single exit (subject to travel distance limits), 61-600 persons need minimum 2 exits, 601-1000 persons need minimum 3 exits, and over 1000 persons need minimum 4 exits. Each exit must provide sufficient width at 5mm per person, with a minimum clear width of 750mm per leaf.

What fire resistance is required for commercial buildings?

Fire resistance periods depend on the building height and purpose group. Single storey commercial buildings typically need 30 minutes. Buildings of 2-3 storeys need 30-60 minutes. Buildings of 4-5 storeys (up to 18m) need 60 minutes. 6-10 storeys (18-30m) need 90 minutes. Over 30m height needs 120 minutes. These apply to structure, compartment walls and compartment floors per AD B Table B3.

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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.

Purpose Group

Building use classification per AD B

Number of Storeys

Affects fire resistance period

Floor Area

Total usable floor area per storey

Occupant Density

Floor area per person (offices: 6, shops: 2)

Number of Exits Provided

Actual number of exits in your design

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: Determines occupant density factors, travel distances and compartment sizes.
Occupancy: Floor area divided by density factor gives the design occupancy for exit sizing.
Exit width: 5mm per person minimum, with at least 750mm per door leaf.
Sprinkler trade-off: Can increase travel distances, double compartment areas and reduce fire resistance periods.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.