Fire Flow Requirement Calculator (ISO Method)

Fire Flow Requirement Calculator (ISO Method)

How much water must the distribution system deliver to fight a fire in this building? Estimate the ISO needed fire flow from construction, occupancy, and exposures.





Needed Fire Flow
Item Value

How the calculation works

ISO method (nonsprinklered buildings)
Construction factor C = 18 × F × √A, rounded to the nearest 250 gpm (minimum 500 gpm)
NFF = C × O × [1 + (X + P)], rounded to the nearest 250 gpm
F = construction coefficient, A = effective area (ft²), O = occupancy factor, X = exposure factor, P = communication factor
Duration comes from the ISO needed-fire-flow duration table (see below).
Needed fire flow (gpm) Duration (hours)
2,500 or less 2
3,000 3
3,500 – 4,000 4
4,500 – 5,000 5
5,500 – 6,000 6
6,500 – 7,000 7
7,500 – 8,000 8
8,500 – 9,000 9
10,000 and up 10

Notes & limitations

Formulas per ISO’s “Guide for Determination of Required Fire Flow” (NFFi = (Ci)(Oi)[1.0 + (X + P)i]; C = 18F√A). The ISO guide addresses nonsprinklered buildings; the full method includes detailed exposure and communication tables.
  • Effective-area rules vary by construction class — fire-resistive buildings use the two largest adjoining floors plus 50% of floors above (up to eight) when vertical openings are inadequately protected. Confirm the area basis with your AHJ.
  • The exposure (X) and communication (P) factors need an adjacent-building analysis (construction type, separation distance, openings). Enter their combined decimal value, or 0 for a standalone screening estimate.
  • Sprinklered buildings follow different reductions not covered here. One- and two-family dwellings have separate ISO provisions.
  • This is a planning estimate for water-system sizing. The authority having jurisdiction (fire marshal / ISO field representative) makes the final determination.

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