The arithmetic. Foam solution flow = area in square feet × application rate in gpm/ft². Concentrate per minute = solution flow × the induction percentage. Total concentrate = that multiplied by the run time. The induction percentage is the volume of concentrate in the finished solution, not a dilution ratio — a 3 % solution is 3 gallons of concentrate in every 100 gallons of solution, and 97 gallons of water.

The rates. NFPA 11 gives 0.10 gpm/ft² for hydrocarbon spills and 0.20 gpm/ft² for polar solvents, which is why alcohol-type foams are rated at twice the hydrocarbon rate — polar solvents destroy an ordinary AFFF blanket and pull the water out of it. Fixed-roof storage tanks are sized by diameter: 0.16 gpm/ft² under 150 feet, 0.18 under 200, 0.20 under 250. Minimum run time is 15 minutes — the supply has to last that long, not just the nozzle.

Class A is a different calculation. Class A foam is used at 0.1 % to 1.0 %, typically 0.1–0.5 % for interior and exterior attack and up to 1 % for exposure protection and deep-seated fires. Its application rate is generally taken as the required fire flow for the compartment — (length × width) ÷ 3 — rather than a per-square-foot figure, because you are wetting fuel rather than blanketing a liquid surface.

Worked example. A 50 × 40 ft hydrocarbon spill — 2,000 ft² — at 0.10 gpm/ft² with a 3 % concentrate over 15 minutes. Solution flow = 2,000 × 0.10 = 200 gpm. Concentrate = 200 × 0.03 = 6 gal/min. Total = 6 × 15 = 90 gallons of concentrate, and 200 × 15 = 3,000 gallons of solution. If the apparatus carries 20 gallons, it will be dry in 3⅓ minutes — which is the calculation that actually decides the operation.

The three things that ruin it. First, proportioning accuracy: an inline eductor set to 3 % with a 1 % concentrate gives 1 %, not 3 %, and a mismatched nozzle flow will starve the eductor entirely — eductors only work at their rated nozzle flow and within their rated lift. Second, application: foam poured onto a burning liquid surface from a height, or plunged into it, mixes with the fuel instead of floating on it. Roll it on from the near edge, bank it off a wall, or rain it down through a fog pattern. Third, the concentrate itself — a fluorine-free (F3) product is not a drop-in replacement for a legacy AFFF. Many F3 foams require a higher application rate, a different nozzle, and a different proportioner setting, and using one at the other's rate will fail. Check the manufacturer's data sheet for the specific product in your tank, not the table above.

What this cannot see. Three-dimensional fires and running fuel, foam breakdown on hot metal surfaces, wind stripping a blanket off a tank farm, subsurface injection losses, water from attack lines diluting a blanket that took twenty minutes to build, and a supply line that cannot hold the calculated flow. A foam plan is only as good as the water supply behind it and the nozzle in front of it.

Disclaimer. A pre-planning and training aid. Foam operations must follow NFPA 11, your department's SOPs and the concentrate manufacturer's data sheet for the specific product carried. Sources: NFPA 11, Standard for Low-, Medium-, and High-Expansion Foam · IFSTA Foam Operations · manufacturer product data sheets. Last reviewed: 2026-09-21.