How much flow and head
does your job need?
Two numbers decide which pump fits: how much water it has to move (flow) and what it has to push against (head). Work these out first. Horsepower comes later.
1. Work out the flow
Flow is how much water the pump moves in a set time. Start with how much water there is and how quickly it has to go.
To empty 100 m³ of water in 2 hours: 100 m³ ÷ 2 hours = 50 m³/h.
Our model tables give flow in litres per minute. To convert, multiply m³/h by 16.67. So 50 m³/h is about 833 L/min.
| m³/h | L/min |
|---|---|
| 5 | 83 |
| 10 | 167 |
| 25 | 417 |
| 50 | 833 |
| 100 | 1 667 |
2. Work out the head
Head is not only how high the water has to be lifted. It is the height from the water surface to where the water comes out, plus the resistance of the pipe: its length, its diameter, and every bend and valve along the way.
So two sites with the same lift can need different heads. A long, narrow pipe with lots of bends needs more head than a short, straight, wide one. Measure the height, then note the pipe length, diameter and fittings. If you are not sure how to add these up, send them to us and we will work it out.
3. Check both numbers together
A pump gives less flow the higher it has to push. Its maximum head is reached at almost no flow, and its maximum flow at very low head. So a pump listed with a maximum head of 25 m and a maximum flow of 80 m³/h cannot deliver 80 m³/h at 25 m.
The question to ask is: what flow does this pump give at the head my job needs? The answer is on the pump's performance curve in the factory catalogue. If the curve shows it can deliver 50 m³/h at 12 m, and your job needs 50 m³/h at 12 m, it fits.
4. Then the rest
Once flow and head are settled, check the liquid (sand, solids, fibres, sewage or chemicals), the power supply on site, the material the pump needs to be made of, and how it will be installed. These decide the series. Horsepower is one of the specifications to compare, not the starting point: two pumps with the same horsepower can give quite different flow and head.
Send us your numbers
Tell us the flow, the head (or the lift and pipe details), the liquid and your power supply, and we will recommend a model.