What an open-outlet measurement pins down
It fixes one point on a pump curve: the flow through an open outlet with no resistance downstream. That is a reproducible measurement, and two makers measuring it honestly will arrive at comparable numbers.
It also fixes, indirectly, the upper bound of everything else. No installation will ever exceed it, so it tells you where the curve starts.
Why it is not a turnover
It does not promise turnover. Turning a pool over means moving its whole volume through the filter, and the time that takes depends on installed flow rather than bench flow.
It does not promise that two pumps quoting 19 200 l/h are equivalent. One may reach a maximum head of 11 m and the other 6 m, in which case they behave nothing alike once a sand filter, three metres of hose and a metre of lift are in the way.
And it does not promise filtration. Water that passes through a filter faster than its medium can catch anything has been moved, not cleaned. This is the reason pages here print flow next to tank diameter or cartridge area rather than on its own.
The other end of the same curve
Maximum head, in metres, which is the other end of the same curve and the single most useful companion figure. Connection size, because a 38 mm pump feeding 32 mm hose is throttled by the hose. Self-priming, which says whether a pump sited above the water line can start at all. And power draw in watts, which is what the flow costs to produce and the figure a season’s running is actually measured in.
Every family that moves water
Circulation pumps, where it is the headline. Cartridge and sand filter systems, where the pump figure is quoted for the whole assembly and the useful comparison is turnover against a pool’s volume rather than litres per hour. And drain pumps, where the same convention applies with one addition — residual depth, the height of water a pump can no longer pick up, which decides whether a pool empties or stops two centimetres short.
What readers keep writing in to ask
What is head, in plain terms?
The resistance a pump works against, expressed as the height of a water column it could lift. Every metre of vertical lift, every metre of hose, every bend and the filter itself all add to it. A pump quoted at 19 200 l/h and 11 m maximum head delivers the 19 200 only at zero, and nothing at all at 11 m.
Why do makers not just quote installed flow?
Because there is no such thing to quote: installed flow depends on the installation, which differs for every pool. Zero head is the only condition that can be measured repeatably on a bench. The problem is not the figure, it is printing it without the head figure beside it.
How does this site compare two pumps then?
By printing the flow and the maximum head together, and by saying when a maker published only one of the two. Where a maker publishes a curve or a flow at a stated head, that is the figure the page leads with, because it is the more useful one.
Does a bigger pump clean better?
It moves more water, which is not the same thing. Past a point, extra flow pushes water through a filter faster than the medium can catch anything, and on a cartridge system it simply shortens the interval between cleanings. Pages here report the turnover a flow gives against the pool’s own volume rather than treating flow as a score.
Last reviewed 21 September 2026