Sprinkler Water Demand Calculator (NFPA 13 Density/Area)

The density/area method

1. Sprinkler demand = design density × design area. The calculator uses the bottom point of each NFPA 13 density/area curve: light hazard 0.10 gpm/sq ft over 1,500 sq ft, ordinary group 1 0.15 over 1,500, ordinary group 2 0.20 over 1,500, extra hazard group 1 0.30 over 2,500 and extra hazard group 2 0.40 over 2,500. Any point on the curve is allowed, so a designer may trade a larger area for a lower density.
2. Sprinklers in the design area = design area ÷ coverage per sprinkler, rounded up. Coverage is the actual spacing on the drawings; 130 sq ft (light and ordinary) and 100 sq ft (extra) are typical maximums for standard spray sprinklers.
3. Minimum flow per sprinkler = density × coverage, and the pressure to produce it is P = (Q ÷ K)², never less than the 7 psi listing minimum of a standard spray sprinkler.
4. Total demand = sprinkler demand + the inside and outside hose allowance: 100 gpm for light hazard, 250 gpm for ordinary and 500 gpm for extra hazard, for 30, 60 to 90 and 90 to 120 minutes respectively (the calculator uses 30, 90 and 120). Supply volume = total gpm × minutes.

This is the starting point of a hydraulic calculation, not the whole thing. The flow at the most remote sprinkler is the minimum; each sprinkler closer to the supply flows more because it sees more pressure, and pipe friction, fittings and elevation are added back to the source. Expect the calculated system demand to be above the simple density × area figure.

Worked example

Ordinary hazard group 1: 0.15 gpm/sq ft over 1,500 sq ft is a sprinkler demand of 225 gpm. At 130 sq ft per head, 12 sprinklers fall in the design area, each flowing at least 0.15 × 130 = 19.5 gpm, which a K 5.6 sprinkler needs (19.5 ÷ 5.6)² = 12.1 psi to deliver. With the 250 gpm hose allowance the total demand is 475 gpm, and 90 minutes of duration calls for a supply of 42,750 gallons.