Pump head calculator
Enter each component in feet, the outlet pressure in psi, and the flow. Friction loss comes from the table below or the pipe manufacturer's data. Efficiency for a small centrifugal pump is 50 to 70 percent; use 0.65 if the curve is not to hand.
- Total dynamic head
- 78.1 ft
- Brake horsepower
- 1.52 hp
Suction lift plus discharge height plus friction loss, plus outlet pressure at 2.31 feet per psi.
Hydraulic horsepower is gpm times head over 3,960; brake horsepower divides that by pump efficiency.
What makes up total dynamic head
Static suction lift is the vertical distance from the water surface up to the pump centreline. If the source is above the pump it becomes negative and helps.
Static discharge head is the vertical distance from the pump up to the highest point of discharge. Horizontal runs do not count here; they count under friction.
Friction head is the loss through every foot of pipe and every fitting, and it rises with the square of flow. Doubling the flow through the same pipe roughly quadruples the friction loss.
Pressure head is any pressure required at the outlet, a sprinkler or a pressure tank for instance, converted at 2.31 feet of water per psi.
Friction loss in pipe
Loss in feet of head per 100 feet of new schedule 40 steel pipe. Plastic pipe is smoother and loses 15 to 20 percent less; old rough pipe loses considerably more.
| gpm | 1 in | 1 1/4 in | 1 1/2 in | 2 in | 3 in |
|---|---|---|---|---|---|
| 10 | 6.9 | 1.8 | 0.8 | 0.3 | - |
| 20 | 25.0 | 6.3 | 3.0 | 0.9 | 0.1 |
| 30 | 53.0 | 13.7 | 6.4 | 1.9 | 0.3 |
| 50 | - | 35.5 | 16.6 | 4.9 | 0.7 |
| 75 | - | - | 35.5 | 10.4 | 1.5 |
| 100 | - | - | - | 17.8 | 2.5 |
| 150 | - | - | - | 38.0 | 5.4 |
Pump power formula
Water horsepower, the useful work done on the fluid, is gpm x head in feet x specific gravity / 3,960. That 3,960 comes from 33,000 foot-pounds per minute per horsepower divided by 8.33 pounds per gallon of water.
Brake horsepower, what the motor has to deliver, is water horsepower divided by pump efficiency. A pump moving 100 gpm against 100 feet of head does 2.5 water horsepower; at 65 percent efficiency it needs a 3.9 hp motor, so you fit a 5 hp.
In metric, kW = flow in litres per second x head in metres x 9.81 / (1,000 x efficiency).
