What is a fire flow formula?

A fire flow formula estimates how much water a building needs to control a fire, from its size and the area or volume involved. It gives a target flow during pre-planning or size-up, before crews commit to a supply plan.

What is the difference between the national-academy and Iowa methods?

The national-academy method scales the flow from the involved floor area (length × width, divided by 3, times the fraction involved). The Iowa method keys off the room volume (length × width × height, divided by 100). Both are rough planning estimates, not precise requirements — follow your AHJ's pre-plan method.

Formulas

Fire Flow Formula

NFF

A fire flow formula estimates how much water a building needs to control a fire, from its size and the area involved. Pre-planning methods like the national-academy and Iowa approaches give a target flow before crews arrive.

These are planning estimates, not precise numbers. One common method scales the flow from the building's floor area and the fraction involved; another keys off the volume of the space. Both translate a structure into an approximate gallons-per-minute goal.

Knowing the needed fire flow shapes the whole supply plan: how big a line, how many lines, and whether a single hydrant can carry it or the operation needs a relay or a shuttle. It is where size-up meets water supply.

Formula

NFF = (L × W) ÷ 3 × (fraction involved)

National-academy method: L × W = the involved floor area, divided by 3, scaled by the fraction of the structure involved. The Iowa method instead divides the room volume (L × W × H) by 100. Both are planning estimates only.

Common questions

What is a fire flow formula?: A fire flow formula estimates how much water a building needs to control a fire, from its size and the area or volume involved. It gives a target flow during pre-planning or size-up, before crews commit to a supply plan.
What is the difference between the national-academy and Iowa methods?: The national-academy method scales the flow from the involved floor area (length × width, divided by 3, times the fraction involved). The Iowa method keys off the room volume (length × width × height, divided by 100). Both are rough planning estimates, not precise requirements — follow your AHJ's pre-plan method.

Where this shows up

Calculator
Fire Flow (NFA & Iowa)