How to size a fire protection water supply
US practice does not use a fixed volume per hose valve. A sprinkler supply is sized from the NFPA 13 density and area method: the design density in gpm/ft² is multiplied by the remote design area, a hose stream allowance is added, and the total is held for the required duration to give gallons. A standpipe supply follows NFPA 14, which calls for 500 gpm for the first riser plus 250 gpm for each additional riser, capped at 1,000 gpm in a fully sprinklered building and 1,250 gpm where it is not.
The hazard selector prefills representative density, area, hose allowance and duration pairs: light hazard 0.10 gpm/ft² over 1,500 ft² for 30 minutes with 100 gpm of hose, ordinary hazard 0.15 or 0.20 over 1,500 ft² for 60 minutes with 250 gpm, and extra hazard 0.30 or 0.40 over 2,500 ft² for 90 minutes with 500 gpm. Every value stays editable because the governing density/area curve, duration table and hose allowance in the edition adopted by your jurisdiction take precedence.
Site fire flow for the incoming main is a separate calculation under IFC Appendix B, based on building area and construction type, and may exceed the sprinkler demand. Storage tanks also need the usable volume above the suction intake, not the gross tank volume. Use this tool for early sizing and have a fire protection engineer confirm the final demand with the AHJ.
Frequently asked questions
Some countries set the supply as a fixed cubic meter figure per hose station or sprinkler head. US codes instead derive flow from density and area or from standpipe riser count, so the structure of the calculation is different.
In a combined system the required supply depends on how the risers and sprinkler zones are arranged. Calculate each basis, compare them, and confirm the combined demand with the fire protection engineer of record and the AHJ.