Setup & Sizing
Filter Flow Rate Calculator
Work out the filter flow rate (litres per hour) your tank or pond really needs, and the rated spec you should shop for given real-world derating.
Filter Flow Rate Calculator
5-6x turnover suits a typical mixed community tank.
Target turnover
1,200L/h
- Turnover per hour
- 6x
- Rated spec to buy
- 1,600 L/h
- Expected real-world flow
- 900 L/h
Accounts for ~25% derating from head height and media.
Why turnover matters
Turnover - how often your filter cycles the whole tank volume - is the most-quoted and most-misunderstood filter metric. A higher turnover keeps waste suspended long enough for the filter to pick it up, prevents dead spots where mulm and debris settle, and distributes oxygen and heat evenly.
Formula: target flow (L/h) = tank volume (L) x turnover factor
Head height and mulm
Published filter ratings assume the filter is brand new, empty of media, and sitting at the same height as the outlet. In practice: add sponges and biomedia, place the canister below the tank, and a few weeks of mulm buildup, and a filter rated 1000 L/h delivers 700-800 L/h on a good day and noticeably less when it's due for a clean.
We already apply a 25% derating in the "Rated spec to buy" figure. That's the number you take shopping - the real-world flow figure is what you should expect to measure if you ever time a bucket fill at the outlet.
Pond-specific notes
For ponds, always cross-check the filter's maximum head height against your real lift. A 6000 L/h pond pump that has to push water 2 m up to a gravity filter will deliver nearer 3000 L/h at the top. Pond pump and filter manufacturers publish flow-vs-head charts; use them.
Frequently asked questions
How many times per hour should a filter turn over my tank?
A community tropical tank wants 5-6x per hour. A low-tech planted tank is fine at 3-4x. Messy fish like plecos, goldfish or big cichlids want 8-10x. For ponds, the minimum is one full turnover per hour, and heavily-stocked koi ponds should aim for 2x.
Why does my filter actually flow less than its rating?
External filter ratings are quoted on an empty canister with no lift and no media. In a real aquarium you lose output to: the lift from floor to tank (head height), the restriction from sponges, biomedia and floss, and dirt gradually clogging everything. Expect about 70-80% of the rated figure on day one, less as it fills with mulm.
What is head height and why does it matter?
Head height is the vertical distance from the pump to the highest point the water has to reach. Every filter has a maximum head height on its spec sheet. If your cabinet + tank combo needs 1.5 m of lift and the filter is rated to 1.5 m max, it's already at its limit - you'll barely get any flow.
Should I oversize the filter?
Yes - within reason. A filter rated 20-30% above your target delivers close to spec after derating, and lasts longer between cleans because the media isn't constantly at its flow ceiling. If you find the output too strong for shy fish or plants, fit a spray bar or divert the outflow.
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