National Fire Academy. NFF = (length × width) ÷ 3 × % involvement, multiplied by the number of floors involved. Exposure protection adds 25 % of the calculated flow for each side of the fire building with an exposure. The formula came out of the NFA's Preparing for Incident Command course in the late 1970s and early 1980s, built from a study of flows that actually controlled working fires. Its own authors' limits: it is designed for aggressive offensive interior attack, it becomes increasingly inaccurate above about 50 % involvement, and it was never validated empirically — it is a fireground estimating tool, not a measured relationship.

Iowa State (Royer / Layman). Required flow = volume of the involved space ÷ 100, i.e. (length × width × height) ÷ 100. Developed during and after the Second World War from studies of fog streams in enclosed spaces, on the principle of converting water to steam to absorb the heat being produced and keep oxygen away from the fire. It is applied to the largest fire area, it deliberately does not scale with percentage involvement, and its assumptions — space buttoned up, no venting, total flow on scene before the attack begins, simultaneous coverage from several points — are the opposite of how most departments fight a structure fire. Effectiveness is generally regarded as capped around 1,000 gpm, and it is not recommended where there is a life hazard in the building.

ISO. C = 18 × F × √A, where F is the construction factor and A the effective area in square feet; then NFF = C × O × (1 + X + P), where O is the occupancy factor and X + P the exposure and communication factor, capped at 1.75. This is the pre-incident-planning and insurance-rating method. It is deliberately conservative, far too complex to run at an incident, and it ignores percentage involvement entirely.

Worked example. A 60 × 40 ft single-storey frame mercantile building, 50 % involved, no exposures. NFA: (60 × 40) ÷ 3 = 800, × 0.50 = 400 gpm. Iowa: (60 × 40 × 10) ÷ 100 = 240 gpm. ISO: A = 2400 ft², √A = 49.0, C = 18 × 1.5 × 49.0 = 1323, × 1.00 × (1 + 0) = ≈1,320 gpm (1,323 unrounded). Three correct answers to the same building, because they are answering three different questions — and at 250 gpm a line, the NFA figure means two attack lines while the ISO figure means six.

Where all three fail. None of them knows about compartmentation, contents, wind, ventilation, construction collapse, stair access or the water you actually have. Estimating floor area and percentage involvement from the street is the largest source of error and no formula fixes it. Above roughly 50 % involvement the NFA figure falls away from what a defensive operation needs, which is exactly when you need a number you can trust. Flow above the calculated minimum puts fires out faster; flow below the minimum, however it was calculated, does not. And a number on a screen never beats the size-up you actually did.

Disclaimer. A training and pre-planning aid — not a substitute for your department's SOGs, your water supply test, or the incident commander's size-up. Sources: National Fire Academy, Preparing for Incident Command · Royer & Nelson, Iowa State University · ISO Guide for the Determination of Required Fire Flow · Firehouse and Fire Engineering treatments of the three methods. Last reviewed: 2026-09-21.