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Before choosing a pump you need one number: how many litres a minute have to come out at the same time, and at what pressure. On Italian forums many discussions start right here, often with a measurement taken at home [7][8][9].

The flow you need

Add up the flows of the outlets that run at the same time, that is, those of the largest zone:

• Sprinklers: each model has a table with flow and pressure for each nozzle. Take the value at the pressure you will run it at.
• Dripline: flow of one emitter times the number of emitters, or the flow per metre given by the manufacturer times the total length.
• Margin: it is best not to size the pump right at the limit; the real flow drops with wear and with dirty filters.

Dividing the system into zones that open one at a time is the simplest way to reduce the flow required, and so the size of the pump you need.

The pressure you need

The data sheet of every sprinkler gives its operating pressure range. That pressure has to be added to the height difference and the hose losses: the result is the total head the pump has to give at that flow. The guide on head explains why looking at the maximum head is not enough.

Measuring with a bucket and a stopwatch

To find out how much water comes from a tap or an existing pump today:

1. fully open the tap or outlet you will use;
2. time how long it takes to fill a bucket of known volume, for example 10 litres;
3. flow in l/min = litres ÷ seconds × 60.

Example: 10 litres in 22 seconds gives about 27 l/min. Repeat the measurement two or three times and use the average.

This is the flow with an open outlet: with sprinklers connected it will be lower, because sprinklers need pressure. The calculator accepts “litres in seconds” directly and compares the flow you need with the flow estimated for each pump.