Max Sprinkler Design Area for a Water Supply (NFPA 13)
The largest hydraulic design area a water supply can serve at a density, once the hose allowance is taken off the top.
Example
You enter
- Available water supply (gpm) 550
- Design density (gpm/ft²) 0.2
- Hose-stream allowance (gpm) 250
You get
- Max hydraulic design area 1,500 ft^2
- Sprinkler flow available 300 gpm
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeMax area = (supply - hose) / density. A 550-gpm supply with 250 gpm of hose at 0.20 gpm/ft^2 covers up to 1,500 ft^2; a lighter hazard (lower density) covers more. The area/density screen; a most-remote-area hydraulic calculation governs. A design aid; a fire-protection engineer and the AHJ govern.
Formula or tablesprinkler_gpm = available_supply_gpm - hose_gpm; max_design_area_ft2 = sprinkler_gpm / density. The NFPA 13 area/density demand solved for the area.
Source and editionNFPA 13 (Standard for the Installation of Sprinkler Systems), 2022, by name, solved for the design area.
Free accessNFPA 13 is viewable free of charge through NFPA's online free-access reader; the arithmetic is public.
What governsEstimate. AHJ and licensed professional govern.
API field namesavailable_supply_gpm, density, hose_gpm, max_design_area_ft2, sprinkler_gpm
- Hose taken off the top sprinkler flow = supply - hose; the remainder / density is the coverable areaNFPA 13 (2022)
- Screening demand the area/density product is the area/density screen (not the NFPA 13 pipe-schedule method); the governing area comes from a full most-remote-area hydraulic calculation at the supply's flowing pressureNFPA 13 (2022)
- Density from hazard the design density comes from the applicable NFPA 13 density-area curve for the actual commodity and arrangementNFPA 13 (2022)