Fire Flow Calculator

Find the required fire flow with our fire flow calculator to easily estimate the amount of water needed to suppress fires effectively.

Needed fire flow (ISO)1,000 gal/min3,785 l/min

Iowa State formula360 gal/minvolume ÷ 100 — a knock-down rate for the interior attack, not a supply figure

NFA rule of thumb1,333 gal/minarea × storeys ÷ 3

Effective area3,000 sq ftthe largest floor plus a share of the others

Construction coefficient1

Before the exposure charge1,000 gal/min

Required duration2 hoursISO sets 2 hours up to 2,500 gpm, 3 to 3,500, then 4

Total water needed120,000 gallons454 m³ on hand

Hydrants needed1reckoning about 1,000 gpm from each

Building volume36,000 cu ft

The method

Iowa: GPM = volume ÷ 100. ISO: NFF = 18 × C × √(effective area)

Three formulas, three different questions

The ISO needed fire flow is a water supply figure — what the mains and hydrants must be able to deliver, and what insurance ratings are based on. It scales with the square root of the effective area and is multiplied by a coefficient for how combustible the construction is.

The Iowa State formula (volume ÷ 100) answers something quite different: the flow needed to knock down a fire filling that volume, applied for about 30 seconds. It is a fireground attack rate, not a supply requirement, and the two are routinely confused.

The NFA rule — area × storeys ÷ 3 — is a rapid mental estimate for the first-arriving officer. All three are planning tools; a real water-supply analysis accounts for the mains, the pressure and the exposures in detail.