Filter Turnover (GPH) Calculator
Enter your pond volume and pick a population (or your own turnover time) to get the pump flow rate, in gallons and litres per hour, your filter needs to keep up with.
Required pump flow rate
This is the flow the pump must deliver after lifting the water and pushing it through your pipe and filter. A pump's headline GPH is measured with no lift at all, so read its flow chart at your own head height and choose a pump that still meets this figure there.
How the calculation works
Required flow rate is simply pond volume divided by turnover time: a 2,000-gallon pond on a 2-hour turnover needs a pump rated at roughly 1,000 gallons per hour. The calculator defaults that turnover time from your pond's population — a faster 1-hour turnover for koi, a 2-hour turnover for goldfish, and a slower 4-hour turnover for a lightly stocked water garden — or you can enter your own target turnover time directly.
The result is a sizing target for choosing a pump and filter box, not an exact biological requirement; a pump's actual delivered flow also depends on the vertical lift and plumbing between the pump and the pond.
Frequently Asked Questions
What is a filter "turnover rate"?
It's how long it takes your pump and filter to cycle a volume of water equal to the whole pond. A 2-hour turnover on a 1,000-gallon pond means the filter needs to move roughly 500 gallons per hour (1000 ÷ 2).
Why do koi ponds need a faster turnover than goldfish ponds?
Koi grow larger, eat more, and produce more waste per fish than goldfish, so a koi pond's bioload is typically heavier for the same water volume. A faster turnover — commonly around once per hour — helps the filter keep up with that extra waste load.
Can I use a pump that's rated higher than the GPH I calculated?
Yes — a pump rated above your requirement gives you headroom, and you can often throttle flow with a valve or by choosing return fittings. Just remember that a pump's rated GPH drops as you add vertical lift (head height) and plumbing length, so check the manufacturer's flow-at-head chart, not just the top-line number.
Does a bigger filter mean I can stock more fish?
A stronger filter and faster turnover help a pond handle a somewhat heavier bioload, but filtration alone doesn't replace a sensible stocking density — check the fish stocking density calculator alongside this one.
Sizing guideline only. Check a pump's flow-at-head chart against your plumbing before buying, and confirm your filter's biological capacity matches your actual stocking level.
Safety: Outdoor pond pumps, lights and UV units need a supply protected by an RCD (UK) or GFCI (US), and any new outdoor socket or cable installed by a qualified electrician. Unplug the pump before your hands go in the water.