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The maximum flow printed on the box applies at zero head: water coming out at the same level as the pump, with no hose. As soon as the water has to rise or pass through a hose, the flow drops, sometimes by a lot. It is not us saying this: the manufacturers themselves say it, when they publish full data.

What manufacturers say, when they say it

Gardena 4000/1, tank pump: maximum flow 4,000 l/h [26]. In the 2019 German manual the same manufacturer gives [28]:

Gardena 4700/2 inox, which has two stages: 3,700 l/h at 5 m, 2,850 at 10 m, 1,900 at 15 m [28]. It loses less with height: that is exactly the advantage of a multi-stage pump.

Einhell GE-SP 3546 RB, barrel pump: the manual gives 4,600 l/h with a 1-inch hose and 1,750 l/h with the 16 mm hose intended for the barrel [29]. Same pump, same manufacturer, less than half the water: that is the effect of the hose, explained in the guide on pressure loss.

The numbers cannot be compared across brands

• Pedrollo JSWm 2A: the official table goes up to 4,200 l/h, but that value corresponds to 28 m of head, not zero [10].
• Kärcher BP 1 Barrel: the flow is written as “< 3,800 l/h”, that is an upper limit and not a guaranteed value [33].
• Many other data sheets in our sample give a maximum flow without saying at what head or with what hose it was measured.

Comparing “4,200” with “3,800” without knowing the conditions they apply to makes no sense.

Who publishes the curve

In our sample of 24 pumps whose data sheets we have collected, 12 have a curve with at least three points published by the manufacturer and 12 have maximum values only:

• With the table: manufacturers of pumps for building services (Pedrollo, DAB, Lowara, Ebara) publish it in their data sheets.
• Among garden brands: only Gardena, and only in the 2019 German edition of the manual; the 2024 international edition we read no longer gives those points.

The calculator keeps the two groups apart: where there is a curve it uses it, where there is none it gives only an indicative estimate and does not rank it with the others.

What to do in practice

1. Look for the curve, or at least one “flow at X metres” point, in the manual or data sheet, not on the box.
2. Work out the head you need: height difference, hose losses, pressure at the outlet.
3. Choose a pump that gives the flow you need at that head, with a margin.