Pump power (BHP)
Hydraulic and brake horsepower from flow, head, fluid gravity, and pump efficiency — plus the next standard motor size up, so a replacement motor isn't undersized.
Pump power · hydraulic & brake horsepower
WHP = Q·H·SG / 3960 · BHP = WHP / η · input kW = BHP·0.746 / ηm
What this gives you
This computes the two horsepower numbers behind every pump job: the hydraulic (water) horsepower the fluid actually receives, and the brake horsepower the motor must deliver to the shaft once pump inefficiency is accounted for. Hydraulic power is WHP = Q × H × SG ÷ 3960, where Q is flow in gpm, H is total head in feet, and SG is the fluid's specific gravity. Divide by the pump efficiency to get brake horsepower: BHP = WHP ÷ η. The tool then picks the smallest standard motor at or above the BHP so a replacement isn't accidentally undersized.
Where 3960 comes from and why SG matters
The constant 3960 is just unit bookkeeping: one horsepower is 33,000 ft·lb per minute, and a gallon of water weighs 8.34 lb, so 33,000 ÷ 8.34 ≈ 3960 folds pounds, feet, and minutes into a single divisor for water. That is why specific gravity is a multiplier — pump a fluid heavier than water and it takes proportionally more power. A brine at SG 1.2 needs 20 percent more horsepower than water at the same flow and head, and skipping SG is the most common way these estimates come in low. Head must be total dynamic head — static lift plus friction plus any pressure difference — not just the vertical rise.
Field note — size the motor to the fluid, not the water
Efficiency and specific gravity are the two inputs people leave at their defaults, and both push the motor a size larger. A pump running well off its best-efficiency point can sit at 55–65 percent efficiency instead of the 70–80 percent you'd assume, and a heavy fluid adds its SG on top. Size the motor from the worst-case operating point — highest flow, coldest and heaviest fluid — and round up to the next standard frame. A motor that runs at 90 percent of its rating stays cool; one asked for 105 percent trips overloads on a hot afternoon.
Worked example
A pump moving 100 gpm of water against 150 ft of total head at 70 percent efficiency: hydraulic power is 100 × 150 × 1.0 ÷ 3960 = 3.79 hp, and brake power is 3.79 ÷ 0.70 = 5.41 hp. The next standard motor up the NEMA ladder is 7.5 hp — the 5 hp frame below it would be running over its rating from day one. Behind a 92 percent motor the electrical draw at the terminals is 5.41 × 0.746 ÷ 0.92 = 4.39 kW — the number that actually shows up on the energy bill and sizes the feeder and breaker.
Fluid presets and specific gravity
The Fluid selector loads the specific gravity for common liquids — water 1.00, diesel 0.85, gasoline 0.74, 30 percent glycol 1.04, seawater 1.025, crude oil 0.88 — or leave it on Custom to type your own SG. Because power scales directly with SG, the fluid choice moves the answer: run the same 100 gpm at 150 ft through 0.85 SG diesel and brake power drops to 100 × 150 × 0.85 ÷ 3960 ÷ 0.70 = 4.60 hp. Size the motor from the heaviest fluid the pump will ever see, not the lightest.