How to do a sprinkler density/area flow calculation
Before running a full hydraulic calculation, most designers do a quick density/area estimate: how much water the design area needs, how many sprinklers flow, and the minimum pressure at the most remote head. It is a fast sanity check on a water supply or a fire pump.
The method
- Take the design density (gpm/ft²) and design area (ft²) for your hazard from the design criteria in the code edition your authority having jurisdiction has adopted. This page does not supply those values.
- Sprinkler flow in the design area = density × area.
- Number of flowing sprinklers = design area ÷ coverage area per sprinkler, rounded up.
- Minimum flow per sprinkler = density × coverage area per sprinkler (and never less than any minimum pressure your criteria require).
- Minimum pressure at that sprinkler: P = (Q ÷ K)², where K is the sprinkler's K-factor from its data sheet.
- Add the hose stream allowance your criteria require to get the total demand at the source, before friction and elevation losses.
Free calculator
All default values above are example inputs only. Use the criteria from your adopted code edition and the sprinkler's listing.
Worked example (example inputs)
0.20 gpm/ft² over 1,500 ft² gives 300 gpm of sprinkler flow. At 130 ft² per sprinkler, 1,500 ÷ 130 = 11.5, so 12 sprinklers flow. Each needs at least 0.20 × 130 = 26.0 gpm, so with K = 5.6 the remote sprinkler needs (26.0 ÷ 5.6)² = 21.6 psi, above a 7 psi minimum. With a 250 gpm hose allowance the total is 550 gpm. A full hydraulic calculation will come out higher, because sprinklers closer to the riser see more pressure and flow more than the minimum.
Next step: size the pipe losses with the free friction loss calculator, or check the whole system with the Sprinkler Friction Loss Calculator workbook and the Fire Pump and Tank Sizing Workbook.