What this calculator works out
This calculator multiplies pond volume by the number of turnovers you want each hour, then adds an allowance for head loss, giving a required pump flow rate in litres per hour.
Head loss is the part people miss. A pump rated at 2,400 litres per hour delivers considerably less once it is pushing water up to a filter and along a pipe.
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Before you buy a pump
- Volume in litres is length × width × average depth in metres, multiplied by 1,000.
- One turnover per hour is the usual guidance for a filtered fish pond. A wildlife pond may need no pump at all.
- The head loss allowance covers vertical lift and pipe friction. A pump lifting water two metres to a filter loses a substantial share of its rated output.
- Pump ratings are quoted at zero head. Check the manufacturer's flow curve at your actual lift height.
A balanced planted pond with no fish maintains itself through plants and natural processes. Pumps and filters exist to cope with the waste load that fish create. Adding a pump to a wildlife pond consumes electricity year-round and can disturb the still water that many invertebrates and amphibians prefer.
How this calculator works
Volume times turnovers, plus head loss:
Required flow = pond volume × turnovers per hour × (1 + head loss ÷ 100)Treating head loss as a flat percentage is a simplification. Real losses depend on the vertical lift, the pipe diameter and length, and every bend and fitting. Manufacturers publish flow curves showing output against head, and reading yours off the curve is more reliable than any percentage.
The result is a minimum. Pumps are more efficient running below their maximum, so choosing one comfortably above the calculated figure usually costs less to run than a smaller one working flat out.
Worked example: a 2,000 litre pond
Using the default figures — 2,000 litres, one turnover an hour, 20% head loss:
- Base flow: 2,000 litres per hour
- With head loss: 2,400 litres per hour
A pump advertised at 2,400 l/h will not deliver that at two metres of head — many deliver half their rated flow at that height. The honest reading is that a 2,000 litre pond with a raised filter needs a pump rated well above 2,400 l/h, and the flow curve is the only way to know how far above.
Common mistakes
- Buying to the rated flow. That figure is at zero head, not at your filter height.
- Using maximum rather than average depth for volume. It overstates the pond considerably.
- Fitting a pump to a wildlife pond. It is usually unnecessary and disturbs still water.
- Undersizing the pipework. Narrow pipe adds friction loss and wastes pump capacity.
- Running a pump intermittently on a filtered pond. Filter bacteria need continuous flow to survive.
Frequently asked questions
How many turnovers per hour do I need?
Roughly one for a general filtered pond, and up to twice that for a heavily stocked koi pond. Below one turnover per hour the filter cannot process the waste load; far above it, water passes through too fast for the biological filter to work efficiently.
What is head and why does it matter?
Head is the vertical height a pump must lift water, plus the equivalent resistance of the pipework. Every pump's output falls as head increases, following a curve published by the manufacturer. A pump rated 3,000 l/h at zero head might manage 1,500 at two metres, which is why the rating alone is not enough to choose one.
Should the pump run all the time?
On a filtered pond with fish, yes. The biological filter depends on bacteria that need a continuous supply of oxygenated water; switching the pump off for a day can kill enough of them to cause an ammonia spike when it restarts. A pump running only a fountain for effect can be switched off freely.
How much does a pond pump cost to run?
A small pond pump might draw 20 to 50 watts, which running continuously is roughly 175 to 440 kWh a year — a meaningful sum at current electricity prices. Larger pumps draw considerably more. It is a recurring cost worth calculating before installing a filtered system.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
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References
- Royal Horticultural Society — pond maintenance, filtration and water quality
- Health and Safety Executive — electrical safety for outdoor equipment and RCD protection
Sources are checked at publication and can change — how I choose and check references.