Quick answer: Required (“needed”) fire flow is the water rate a building's fire system must deliver, set by floor area, construction type and occupancy. Two common routes: the ISO/IFC area formula NFF = 18 × C × √A (US gpm), or the fixed pump capacities in NBC 2016 Part 4 for India. Once you know the demand, confirm your hydrants can meet it with a fire flow test and the fire hydrant flow calculator.
Demand first, supply second
Sizing a fire-water system is a two-part problem. First work out how much water the building needs (the required fire flow and its duration). Then prove the source can supply it — town main, tank and pump, or a combination. This article covers the demand side; the hydrant flow test guide covers the supply side.
Method 1 — ISO / IFC area formula
NFF (gpm) = 18 × C × √A A = effective floor area (sq ft) C = construction coefficient
| Construction | Coefficient C |
|---|---|
| Wood frame | 1.5 |
| Ordinary (masonry) | 1.0 |
| Non-combustible | 0.8 |
| Fire-resistive | 0.6 |
The result is rounded to the nearest 250 gpm and adjusted for occupancy hazard and exposures. The IFC also caps and floors the value (typically 1,000–8,000 gpm for buildings).
Method 2 — NBC 2016 fixed pump capacities (India)
Indian practice does not use the area formula; NBC 2016 Part 4 prescribes pump capacities by occupancy and height. Indicative hydrant / wet-riser pump duties:
| Occupancy | Indicative hydrant pump |
|---|---|
| Residential 15–24 m | 1,620 L/min |
| Residential > 24 m | 2,850 L/min |
| Business / mercantile | 2,850 L/min |
| Industrial (high hazard) | 2,850 L/min + hazard uplift |
Sprinkler and terrace-tank pumps are added on top, and static storage is sized for the required run time.
Duration — flow is only half the answer
A fire flow of 2,000 gpm for 5 minutes is useless; the standards fix a minimum duration. The IFC ties duration to flow (commonly 2 hours at higher flows); NBC sets minimum fire-water storage (often 30–90 minutes of pump duty depending on occupancy). Water required = flow × duration.
Worked example
Non-combustible warehouse, effective area 20,000 sq ft, C = 0.8:
NFF = 18 × 0.8 × √20,000 ≈ 2,036 → 2,000 gpm Duration 2 h → water = 2,000 × 120 = 240,000 gal (≈ 900 m³)
Now compare with supply: if a hydrant flow test shows only 1,540 gpm available at 20 psi, the site is short by ~460 gpm — you need a fire pump and stored water to make up the difference.
FAQ
What is required fire flow? The minimum water rate, at a usable residual pressure, that the fire-protection system must deliver for a set duration to control a fire in that building.
How do I calculate fire flow for a building? Use NFF = 18 × C × √A for the international method, or NBC 2016 pump capacities in India, then check the source with the fire hydrant flow calculator.
Does required fire flow include sprinklers? A sprinklered building earns a large fire-flow reduction (often up to 75% under IFC), because the sprinklers control the fire and cut the hose demand.