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On the box of almost every garden pump there are two numbers: the maximum flow (for example 4,000 l/h) and the maximum head (for example 13 m). It is natural to read them as “this pump gives 4,000 litres an hour and reaches 13 metres”. That is not how it works, and this misunderstanding is behind many of the questions that have been coming up on Italian forums for years [5][7].

What head is

Head is the height to which the pump can push the water, expressed in metres of water column. It is a way of measuring pressure: 1 bar corresponds to about 10.2 m. A pump that delivers 20 m of head at a given operating point could lift the water by 20 m, or lift it by 10 m and deliver it at about 1 bar of pressure, or spend part of it overcoming friction in the hose.

The two numbers on the data sheet are two extremes

• The maximum head is reached with the outlet closed: the pump pushes as hard as it can, but the flow is zero.
• The maximum flow is reached with the outlet open, with no height difference and no hose: plenty of water, but zero head.

Neither is a real-use situation. Your pump will work at an intermediate point on its curve, which links flow to head: the more metres you ask of it, the less water it delivers.

Where your pump will work

The duty point is where the head the pump can give equals the head your system demands. The system demands the sum of three things:

1. the height difference between the water level and the highest point to be reached;
2. the hose losses, which grow a lot with flow and with narrow or long hoses;
3. the pressure required at the outlet, for example the one given on a sprinkler’s data sheet.

Hose losses are the most underestimated part: with 25 m of 13 mm hose and 20 litres a minute, about 15 m of head is lost, more than the height difference of many wells. We explain it with the numbers in the guide on pressure loss.

What to ask of a data sheet

• The full curve, or at least a few “flow at X metres” points. Some manufacturers publish it, many do not.
• The conditions under which the data applies: voltage, water temperature, tolerances.
• For surface pumps, the maximum suction depth.

If the data sheet gives only the two maximum values, any estimate of the real flow is only indicative and not a confirmation. Our calculator uses the curve when the manufacturer publishes it, and says so when it cannot.