Fire Water Storage Calculator

NFPA 13's Table 11.2.3.1.2 pairs a specific supply duration with a specific hose stream allowance for each of the five occupancy classes, so this calculator looks up both values together from the single hazardClass input rather than treating them as independent settings. Light Hazard uses 30 minutes and 100 GPM, Ordinary Hazard (both groups) uses 60 minutes and 250 GPM, and Extra Hazard uses 90 or 120 minutes with 500 GPM. If your project needs a duration that doesn't match its class, use the manual duration override instead of trying to reverse-engineer it from the hazard input.

Entering any value greater than 0 in that field replaces the NFPA-table duration for your chosen hazard class in the fire water storage calculation, while the hose stream allowance still comes from the hazard class regardless. Leaving it at 0 tells the calculator to use the NFPA default duration for whichever hazard classification you selected. This lets you match an AHJ-specified duration without having to also fake a different hazard class.

The calculator treats fire protection water as a dedicated volume that must always be available in full, so it computes the fire-only requirement first and then adds any domestic reserve gallons as a separate, additive amount rather than assuming the two uses can share the same gallons. This mirrors how combined fire/domestic tanks are actually plumbed — a dip tube or standpipe protects a reserved fire volume so domestic draw-down can never eat into it.

The calculator converts total system flow (GPM) to cubic feet per second, divides by a 10 ft/s velocity cap to get the minimum required pipe cross-sectional area, and solves for the diameter that area implies. It then rounds up to the next size in a fixed list of standard commercial pipe diameters (4, 6, 8, 10, 12, 14, 16 inches), since a suction line sized between standard sizes isn't something you can actually order.