Pump Head & Power Calculator
Calculates the brake horsepower a pump motor must deliver to move a fluid at a given flow rate against static head and pipe friction losses. Use it when selecting pumps for irrigation, HVAC, chemical processing, or water supply systems.
About this calculator
Brake horsepower (BHP) is the actual shaft power a pump motor must supply, accounting for the pump's internal inefficiency. The standard formula is: BHP = (flow rate × total head × specific gravity) / (3960 × pump efficiency). Total head combines static head—the vertical elevation difference the fluid must overcome—and friction loss, which represents energy lost to pipe wall resistance, fittings, and valves. The constant 3960 converts gallons per minute and feet of head into horsepower for water (SG = 1.0). Specific gravity adjusts for fluids denser or lighter than water; pumping crude oil (SG ≈ 0.85) requires less power than pumping brine (SG ≈ 1.2). Pump efficiency, typically 60–85%, reflects how much input shaft power is converted to useful fluid energy.
How to use
A pump must deliver 150 GPM of water (SG = 1.0) against 80 ft of static head with 20 ft of friction loss, using a pump with 75% efficiency (0.75). Total head = 80 + 20 = 100 ft. BHP = (150 × 100 × 1.0) / (3960 × 0.75) = 15,000 / 2,970 = 5.05 HP. You would select a 5.5 HP or 7.5 HP motor to provide a safety margin above the calculated 5.05 BHP requirement.
Frequently asked questions
What is the difference between static head and friction loss in a pump system?
Static head is the height the pump must lift the fluid, determined purely by the elevation difference between the source and destination. It does not change with flow rate. Friction loss, by contrast, depends strongly on flow rate, pipe diameter, pipe roughness, and the number of fittings and valves. As flow increases, friction loss rises roughly with the square of velocity. Total system head is the sum of both, and the pump must overcome it entirely to maintain the desired flow rate.
How does fluid specific gravity affect pump power requirements?
Specific gravity is the ratio of a fluid's density to water's density. Since pumping power is proportional to the weight of fluid moved per unit time, denser fluids (SG > 1) require proportionally more power for the same flow rate and head. Pumping brine at SG = 1.2 uses 20% more power than pumping plain water. Lighter fluids like gasoline (SG ≈ 0.74) require less power. The pump's hydraulic performance curve (head vs. flow) is unaffected by fluid density, but the motor power draw scales directly with SG.
Why does pump efficiency matter so much when selecting a motor size?
A pump operating at 60% efficiency wastes 40% of the shaft power as heat in the fluid and mechanical losses, whereas one at 80% efficiency wastes only 20%. For the same flow and head, the 60% efficient pump needs 33% more motor power than the 80% efficient one. Over a pump's typical 20-year service life, this difference can cost thousands of dollars in electricity. Always select a pump whose best efficiency point (BEP) falls near your intended operating flow, and oversize the motor slightly—10–15%—to handle variations without overloading.