Fire officers use quick fire-flow formulas on scene and in pre-incident planning to decide how many hose lines and how much water supply a fire needs. The NFA formula assumes 1 gpm for every 3 ft² of fire area; the Iowa formula, developed for enclosed spaces, divides the volume by 100. Both are estimates for tactics, not for designing a water system.
How to use the Fire Flow Calculator
- Length
- Width
- Ceiling height
- Floors involved
- Fire involvement
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the building or fire-area length and width, ceiling height, the number of floors involved and the percentage of the area on fire.
Tips for an accurate estimate
- Measure inside dimensions of tanks and pools – wall thickness can change the volume noticeably.
- Use the average depth for pools with a sloping floor.
- Remember the weight: water weighs about 8.34 lb per US gallon (1 kg per litre), so large volumes need solid support.
- Check whether a figure is in US gallons, imperial gallons or litres before ordering chemicals or equipment.
Common mistakes to avoid
- Using wall height instead of water depth for above-ground pools.
- Assuming a horizontal tank’s volume is proportional to the liquid level – it is not.
- Mixing up flow units such as GPM, GPH and litres per minute.
Inputs explained
| Field | What to enter |
|---|---|
| Length | Units: ft, m, in, yd, cm |
| Width | Units: ft, m, in, yd, cm |
| Ceiling height | Units: ft, m, in, yd, cm |
| Floors involved | A number |
| Fire involvement | Choose one: 25%, 50%, 75%, 100% |
What each result means
- NFA needed fire flow = ([Length]×[Width]÷0.09290304)÷3×[Fire involvement]÷100×[Floors involved]×6.30901964e-5
- Iowa formula flow = [Length]×[Width]×[Ceiling height]×[Floors involved]÷0.028316846592÷100×6.30901964e-5
- 1¾ in hand lines at 150 gpm (NFA) = ceil([NFA needed fire flow]÷(150×6.30901964e-5) – 0.000001)
Example values
How the main result changes when Length changes and everything else stays at the example values above:
| Length (ft) | NFA needed fire flow (gpm) |
|---|---|
| 30 | 400 |
| 45 | 600 |
| 60 | 800 |
| 75 | 1,000 |
| 90 | 1,200 |
| 120 | 1,600 |
Formula
NFA: needed fire flow (gpm) = length × width ÷ 3 × % involvement × floors involvedIowa: flow (gpm) = room volume (ft³) ÷ 100Worked example
Inputs — Length: 60 ft; Width: 40 ft; Ceiling height: 10 ft; Floors involved: 1; Fire involvement: 100%.
Results — NFA needed fire flow = 800 gpm; Iowa formula flow = 240 gpm; 1¾ in hand lines at 150 gpm (NFA) = 6.
Frequently asked questions
What is the NFA fire flow formula?
Needed fire flow (gpm) = (length × width) ÷ 3, multiplied by the percentage of involvement.
What is the Iowa formula?
Gallons per minute = volume of the space (ft³) ÷ 100, applied over 30 seconds.
How much does a hand line flow?
A 1¾ in attack line typically flows 150–185 gpm; a 2½ in line about 250–300 gpm.
What formula does the Fire Flow Calculator use?
NFA: needed fire flow (gpm) = length × width ÷ 3 × % involvement × floors involved.