Fire extinguisher calculator

How fire extinguisher provision is worked out

The number of portable fire extinguishers a floor needs is driven by its area and the kinds of fire that could start there. The recognised UK code of practice is BS 5306-8, which deals with the selection and positioning of portable extinguishers. This calculator applies its Class A rule and adds the extra extinguisher types your risks call for.

The Class A area rule

Class A fires are the everyday ones — wood, paper, card, textiles and most general site materials. BS 5306-8 sizes Class A cover from floor area: each storey needs a minimum aggregate Class A rating of 0.065 × the floor area in square metres. There is also a floor of two extinguishers per storey with a combined rating of at least 26A, although a very small storey of 50 m² or less may be covered by a single extinguisher.

Extinguishers carry a fire rating — a typical water or foam unit (9 litres of water, or 6 litres of foam) is rated 13A. One 13A extinguisher counts 13 toward the aggregate and covers roughly 200 m². So a 300 m² floor needs an aggregate of about 20A, which two 13A extinguishers (26A) comfortably meet, and the per-storey minimum of two applies anyway. No one should have to travel more than 30 metres to reach a Class A extinguisher, so spread them along escape routes and near exits rather than clustering them.

Matching the extinguisher to the risk

Floor area only covers the Class A fire. Other risks need the right medium next to them, and the wrong one can make things worse:

• Electrical equipment — a CO₂ extinguisher. Never water or foam on or near live electrics.
• Flammable liquids, paints and solvents — a foam (AFFF) extinguisher with a Class B rating at the storage or decanting point.
• Cooking in welfare areas — a wet chemical extinguisher and a fire blanket for cooking-oil (Class F) fires.
• Hot works — welding, grinding, cutting and torch-on work need a fire point right at the work position, a fire watch and a hot-works permit, in line with HSG168.
• Flammable gases / LPG — isolating the supply and removing cylinders comes first; dry powder is the usual provision where gases are present.

What this tool is — and isn't

This calculator covers portable extinguisher provision only. It is not a fire risk assessment, and it does not size fixed systems, detection, alarms or means of escape. On construction sites, fire safety in temporary buildings, site accommodation and hot works is governed by HSE's HSG168 and the site fire plan. Use the result as a planning aid, then have a competent person confirm the provision against your fire risk assessment.

Sources: BS 5306-8 (selection and positioning of portable fire extinguishers); HSE guidance on general fire safety in construction and HSG168 Fire safety in construction; the Regulatory Reform (Fire Safety) Order 2005, Article 13 (fire-fighting equipment).