Fire Flow Calculator

Base Fire Flow scales with the square root of Building Area, so even though the relationship isn't one-to-one, area still dominates because it directly drives the core ISO formula term, while Number of Stories only applies a secondary, smaller adjustment on top of that base figure. A large single-story building and a smaller multi-story building with the same total floor area will land in a similar flow range, with stories nudging the number rather than reshaping it.

No -- it implements the construction-and-area term of ISO's published Needed Fire Flow formula (Ci = 18 x F x sqrt(A)), but a full ISO rating also applies a separate occupancy factor and an exposure/communication charge for buildings close to their neighbors, neither of which this calculator computes. Treat the result as a base engineering estimate for planning purposes, not a substitute for an official ISO or fire marshal determination.

Selecting a sprinklered building applies a flat 50% reduction to the needed flow before rounding, reflecting that a functioning automatic sprinkler system is expected to control most fires well before they reach the size the unsprinklered flow figure assumes. This is a simplified credit -- real ISO sprinkler credits can vary by system type and design standard -- so treat it as a representative, not exact, discount.

This calculator uses the NFPA 220 building-construction-type naming (Types I through V) that most building professionals recognize day to day, mapped internally to the closest matching ISO construction class and its corresponding F coefficient. The mapping is a reasonable equivalence rather than an exact one-to-one match, since ISO's own six-class system and NFPA's five-type system aren't defined identically.

Hydrants Needed simply divides the Adjusted Flow w/ Sprinklers figure by a flat 1,000 GPM per hydrant -- a round planning assumption, not a measured figure. A real hydrant's available flow depends on the diameter and pressure of the municipal water main feeding it, and can be well above or well below 1,000 GPM at any given site. Treat this output as a ballpark figure to open a conversation with your local water authority or fire marshal, not as a substitute for an actual fire-flow-availability test at the hydrants you plan to rely on.