Size any clean-water pump: water horsepower (WHP), brake horsepower (BHP), total dynamic head (TDH), standard NEMA motor size, wire-to-water efficiency, and annual energy cost — imperial, metric, and SI units.
Planning estimate, not an engineering stamp. Verify critical selections with a licensed engineer.
WHP = (GPM × TDHft) / 3,960 | BHP = WHP / pump efficiency | 1 psi = 2.31 ft head | 1 HP = 0.746 kW
Pump Horsepower, Motor Size & Energy Cost
Water horsepower (WHP)
–
Brake horsepower (BHP)
–
Shaft power
–
Recommended NEMA motor
–
Wire-to-water efficiency
–
Electrical input
–
Annual energy
–
Annual cost
–
Total Dynamic Head Builder
Use this total dynamic head calculator to build each part, then send the total to the horsepower tab. TDH = static lift + suction lift + pressure head + friction loss + minor losses.
Fittings (minor losses)
Static + suction head
–
Pressure head
–
Friction loss (Hazen-Williams)
–
Minor losses
–
Pipe velocity
–
Total dynamic head
–
How pump horsepower works
Think of this tool as two calculators in one: a water horsepower calculator for the hydraulic power the water actually receives, and a brake horsepower calculator for the shaft power your motor must deliver. Total dynamic head (TDH) is the total resistance the pump works against, expressed in feet of water: lift + pressure + friction + fittings.
Size the motor to the next standard NEMA size at or above the computed BHP. Jet pumps (30–50% efficient) need roughly twice the shaft power of a centrifugal (60–80%) for the same water duty — that is the BHP vs WHP gap in action.
Water horsepower equals flow times head divided by 3,960: WHP = (GPM x TDH in feet) / 3,960. Example: a pump moving 20 GPM against 100 feet of total dynamic head produces 20 x 100 / 3,960 = 0.51 water horsepower. Shaft power is higher: at 70 percent pump efficiency the brake horsepower is 0.51 / 0.70 = 0.72 HP, so you would pick a 0.75 HP motor.
What is the difference between water horsepower and brake horsepower?
The brake horsepower vs water horsepower question comes down to efficiency. Water horsepower (WHP) is the power actually delivered to the water. Brake horsepower (BHP) is the power the motor must supply to the pump shaft, and it is always higher because no pump is 100 percent efficient: BHP = WHP / pump efficiency. Example: 50 GPM at 150 feet TDH gives 50 x 150 / 3,960 = 1.89 WHP. With a 65 percent efficient pump the brake horsepower is 1.89 / 0.65 = 2.91 HP, so the next standard motor is 3 HP.
How do I calculate total dynamic head (TDH)?
Add four parts: static lift, pressure head, friction loss, and minor losses. Worked example: 25 ft of lift, 40 psi discharge pressure (40 x 2.31 = 92.3 ft), 120 ft of 1-inch PVC at 10 GPM with C = 150 (friction 8.5 ft via Hazen-Williams), and fittings K = 4.1 at 4.08 ft/s (1.1 ft minor). TDH = 25 + 92.3 + 8.5 + 1.1 = 126.8 ft. At 10 GPM that is 10 x 126.8 / 3,960 = 0.32 WHP, or 0.49 BHP at 65 percent pump efficiency.
What size pump motor do I need?
Compute brake horsepower, then choose the next standard NEMA motor size at or above it. Common NEMA sizes are 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 75, 100, 125, and 150 HP. Example: a duty needing 2.91 BHP takes a 3 HP motor. Example: 10 GPM at 120 ft TDH with 60 percent pump efficiency needs 10 x 120 / 3,960 / 0.60 = 0.51 BHP, so a 0.75 HP motor. Never size below the computed BHP; motors should not run continuously above nameplate.
What is wire-to-water efficiency?
Wire-to-water efficiency is the fraction of electrical input that reaches the water: pump efficiency times motor efficiency. Example: a 65 percent pump on a 90 percent motor gives 0.65 x 0.90 = 58.5 percent wire-to-water. Electrical input power is WHP x 0.7457 / wire-to-water efficiency. For 1.89 WHP that is 1.89 x 0.7457 / 0.585 = 2.41 kW drawn from the supply.
How much does it cost per year to run a pump?
Multiply input kW by run hours per year by your electricity rate. Example: 10 GPM at 120 ft TDH with 60 percent pump and 90 percent motor efficiency draws 0.303 x 0.7457 / 0.54 = 0.42 kW. Running 8 hours a day for 365 days uses 0.42 x 2,920 = 1,221.9 kWh per year, which costs about 183.29 dollars at 0.15 dollars per kWh. Higher efficiency or fewer hours cut this directly.
How do I estimate pipe friction loss with Hazen-Williams?
Use hf = 10.67 x L x Q^1.852 / (C^1.852 x d^4.8704), with L in feet, Q in GPM, d in inches, and C the pipe roughness coefficient. Example: 100 ft of 1-inch pipe at 10 GPM with C = 140 gives hf = 8.05 ft. New PVC uses C = 150, new steel about 140, and old iron about 100. Keep velocity under about 5 ft/s in this example it is 4.08 ft/s; undersized pipe makes friction explode because it scales with Q^1.852.
Does pump type change how much horsepower I need?
Yes. The water horsepower is the same for a given flow and head, but shaft power depends on efficiency. Example: 25 GPM at 60 ft TDH is 25 x 60 / 3,960 = 0.38 WHP. A jet pump at 40 percent efficiency needs 0.38 / 0.40 = 0.95 BHP, while a centrifugal at 70 percent needs only 0.38 / 0.70 = 0.54 BHP. Typical efficiencies: centrifugal 60 to 80 percent, submersible 55 to 75 percent, multistage booster 65 to 80 percent, jet 30 to 50 percent.
How do I size a pump in metric units?
Water power in kW equals 9.81 x flow in cubic meters per second x head in meters, divided by pump efficiency. Example: 80 L/min is 0.001333 m3/s, so at 25 m head the water power is 9.81 x 0.001333 x 25 = 0.327 kW. At 65 percent pump efficiency the shaft power is 0.327 / 0.65 = 0.503 kW, which is 0.67 HP, so pick a 0.75 HP motor. This calculator handles GPM, L/min, feet, meters, psi, and bar.
Is this calculator for pool pumps or well pumps?
This is the general pump horsepower calculator for any clean-water duty: irrigation, boosting, transfer, and dewatering. If you are sizing a swimming pool circulation pump with turnover rate and filter matching, use the pool pump size calculator instead. For drilling cost plus well pump sizing by depth and pressure, use the well drilling cost lab. For a deep dive into friction with the Darcy-Weisbach equation, use the Darcy-Weisbach calculator.