Needed Fire Flow Calculator (NFA & ISO Formulas)

How it works

NFA: NFF = (L × W ÷ 3) × floors × % involved, + 25% per exposure; ISO: NFF = 18 × F × √(effective area) × O × (1 + exposure charge), rounded to 250 gpm, 500–12,000

The National Fire Academy formula is a fireground quick estimate: a building's floor area in square feet divided by three gives the gallons per minute needed for full involvement of one floor, scaled for the number of floors burning and the fraction of the structure involved, with a quarter more for each exposure that must be protected. It assumes offensive interior attack in ordinary construction and is meant to answer "how many lines and engines do I need right now". The ISO formula is the insurance-rating and planning method: it scales with the square root of effective area (fire flow does not grow linearly with size) and adjusts for how well the building resists fire and how combustible its contents are, with a required duration that sets the water-supply volume a hydrant system or shuttle must provide.

Worked example

A 60 × 40 ft two-storey ordinary-construction house half involved needs (2,400 ÷ 3) × 2 × 50% = 800 gpm, plus 200 gpm for one exposure — 1,000 gpm, or seven 1¾″ lines, so realistically 2½″ lines and master streams. The ISO planning figure is 18 × 1.0 × √4,800 × 1.0 × 1.1 ≈ 1,372 → 1,250 gpm (nearest 250) for 2 hours, 150,000 gallons.

Frequently asked questions