Fire Pump Sizing Calculator

Pump Pressure Needed is whatever remains after subtracting City Static Pressure from Total Required Pressure -- if the city water supply's static pressure alone already meets or exceeds what the sprinkler system requires at its most remote head (after accounting for friction loss and elevation), no pump adds pressure the system doesn't already have. A fire pump only becomes necessary when the supply falls short of the total requirement. In that case Selected Pump Size, Brake HP Required, and Selected Motor all report 0 rather than recommending real equipment, since there's nothing to size.

NFPA 20 (Standard for the Installation of Stationary Pumps for Fire Protection), Section 4.9, defines a fixed list of standard rated pump capacities (25, 50, 100 GPM and so on up through 5,000 GPM) rather than allowing arbitrary sizes, since fire pumps are manufactured and tested at those standard increments. This calculator rounds your Total Flow Demand up to the next standard size on that list, so the selected pump always has at least your calculated flow demand covered, generally with some margin.

Elevation is converted to pressure using water's standard weight -- 0.433 PSI per foot of vertical lift -- and added into Total Required Pressure alongside Required System Pressure and Pipe Friction Loss. A taller building or a pump located well below the highest sprinkler head needs more pump pressure simply to overcome gravity, before accounting for friction or the pressure the sprinkler head itself needs to operate.

Brake HP Required is calculated from the actual equipment being sized -- the rounded-up Selected Pump Size -- rather than the raw demand figure, since horsepower needs to match what the selected pump will actually be asked to move. This keeps the motor sizing consistent with the standard-capacity pump the calculator recommends, not an intermediate number that no real pump is rated at.

No -- it calculates flow demand, required pressure, and estimated brake horsepower to identify a starting standard pump and motor size, but a complete NFPA 20 specification also covers driver type (electric vs. diesel), pump curve verification, controller requirements, and acceptance testing that this calculator does not address. Use the results as an initial sizing estimate to bring to a fire protection engineer, not a final specification.