Filter boxes carry a number in litres per hour, and tank labels carry one in litres. The ratio of the two is the filter’s turnover: how many times an hour the filter could, in theory, pass the whole volume of the tank through itself. It is a useful number for comparing filters and tanks, but it is easy to misread. This guide shows the sum, a table for common tank sizes and flow rates, why the figure on the box is a ceiling rather than a promise and how to think about flow for different set-ups. It does not tell you how many turnovers your fish need; that depends on the species, the stocking and the plants, and a specialist shop or club is the right source.
The formulas
The Aquarium Filter Turnover Calculator uses:
turnovers per hour = flow rate (L/h) ÷ tank volume (L) minutes per full cycle = 60 ÷ turnovers per hour
A 500 L/h filter on a 100 litre tank: 500 ÷ 100 = 5 turnovers per hour. A full cycle takes 60 ÷ 5 = 12 minutes.
A table of turnovers
The table shows turnovers per hour and minutes per cycle for several flow rates and tank volumes.
| Tank volume | 300 L/h | 500 L/h | 800 L/h | 1,200 L/h |
|---|---|---|---|---|
| 60 L | 5.0 (12.0 min) | 8.3 (7.2 min) | 13.3 (4.5 min) | 20.0 (3.0 min) |
| 100 L | 3.0 (20.0 min) | 5.0 (12.0 min) | 8.0 (7.5 min) | 12.0 (5.0 min) |
| 200 L | 1.5 (40.0 min) | 2.5 (24.0 min) | 4.0 (15.0 min) | 6.0 (10.0 min) |
The same filter gives very different turnovers on different tanks: a 500 L/h filter turns a 60 litre tank over 8.3 times an hour and a 200 litre tank 2.5 times. Compare filters by turnover, not by flow alone.
Why the rated flow is a ceiling
Makers typically state the flow of a filter measured with little or no resistance, such as a short hose and a clean, empty media basket. In a real tank, flow is reduced by:
- Filter media, which resists flow, especially fine sponge and carbon.
- Hose length and bends, and the height the water must be lifted.
- Clogging, as dirt builds up in the media and the intake.
- Fittings and spray bars, which add restriction.
- Age and wear of the pump.
Real flow may be well below the rated figure and falls as the filter gets dirty. For a planning calculation, use the maker’s figure as the upper limit, and check whether they give a flow at working height or with media. If you want to know your actual flow, you can time how long the filter takes to fill a container of known volume from its outlet, with the media in place, before cleaning.
Use the real water volume
The tank volume in the sum should be the real water volume, not the figure on the box. A tank sold as 142 litres that holds 109 litres of water has 4.6 turnovers per hour at 500 L/h (500 ÷ 109), not 3.5 (500 ÷ 142). The difference matters when you compare against a guideline. Our guide to how to calculate aquarium volume shows how to find it, and the substrate guide shows what substrate does to the volume.
Flow is not only a number
Turnover tells you how much water passes through the filter, but not how the water moves in the tank. A filter’s outlet position, a spray bar or a second pump affects how well water circulates, whether dead spots form and how the surface moves. Some animals and plants like a strong current, and some do not: a fish from still water may be stressed by a strong flow, while a fish from rivers may enjoy it. Your stock’s needs should shape the choice.
| Consideration | What to ask |
|---|---|
| Fish species | What flow does it prefer? |
| Plants | Do they tolerate strong flow? |
| Stocking level | A heavily stocked tank loads the filter more |
| Filter type | Internal, hang-on, canister and sump differ in flow and media volume |
| Noise and size | Practical limits |
| Maintenance | How easy is it to clean without losing flow? |
Filter media volume matters too
A filter does two jobs: moving water and giving beneficial organisms somewhere to live and process waste. The volume of media and the time the water spends in contact with it matter as much as the flow. A high-flow filter with little media may be less effective than a lower-flow filter with more. When you compare filters, compare media volume as well as flow, and ask the shop what suits your tank.
Pump and sump systems
For a sump system, the return pump’s flow reduces with head height, so the rated figure may be much higher than the real one. Check the maker’s flow curve for the height your pump needs to lift the water. If you use a separate circulation pump, count its flow only if it passes the water through a filter; otherwise it moves water around without filtering it. The turnover figure for filtration counts only the water that goes through the filter.
A worked comparison
You are choosing a filter for a tank with a real volume of 109 litres. Three filters, with example figures:
| Filter | Rated flow | Estimated real flow (assume 70%) | Turnovers per hour (real) | Minutes per cycle |
|---|---|---|---|---|
| A | 400 L/h | 280 L/h | 2.6 | 23.4 |
| B | 600 L/h | 420 L/h | 3.9 | 15.6 |
| C | 1,000 L/h | 700 L/h | 6.4 | 9.3 |
The 70% real-flow figure is an assumption for illustration; your filter’s real flow will differ. The example shows the range, and how rated flow overstates what the tank will see. If a specialist suggests a target turnover for your stock, you can pick the filter whose real flow meets it with some headroom for clogging.
Common mistakes
- Dividing by the box volume instead of the real volume.
- Treating the rated flow as the real flow.
- Ignoring media volume and filter type.
- Choosing very strong flow for gentle species.
- Counting a circulation pump as filtration.
- Not cleaning the filter, so flow drops.
- Forgetting to review flow when the stock changes.
Questions keepers ask
Is a filter with a higher turnover always better? Not always. A strong current can stress fish and uproot plants that prefer still water, and a filter that is too strong may blow substrate around. A better filter is the one that suits your stock, with enough media and a flow you can maintain.
How do I measure my filter’s real flow? Time how long the outlet takes to fill a container of known volume with the media in place, before cleaning. Ten litres in 60 seconds is 600 L/h.
Do I need to count the heater and powerheads? Powerheads and circulation pumps move water but do not filter it unless they pass it through a filter. They help to mix the water and reduce dead spots.
Will a bigger filter let me keep more fish? Filtration is one limit among several, including oxygen, space and the behaviour of the species. Ask a specialist about your stocking.
How often should I change filter media? Follow the maker’s advice. Replacing everything at once removes the organisms that live in it, so many keepers change media in stages.
What if the flow drops suddenly? Check for a blocked intake, a kinked hose or a dirty impeller, and clean as the manual instructs. If the filter stops, check that the fish have enough oxygen while you fix it, and ask a specialist if you are unsure.
A maintenance log
| Date | Action | Flow before (L/h) | Flow after (L/h) | Notes |
|---|---|---|---|---|
Recording real flow before and after cleaning shows how fast the filter clogs, and helps you set a cleaning interval that keeps turnover near your target.
Planning for the future
Tanks change. You may add fish, plants or decor, and each change affects the load on the filter and the volume of water. Revisit the turnover sum whenever you change the set-up, and again once a year. Note the real water volume, the filter’s flow measured at the outlet and the media in the filter, and keep the figures in a notebook alongside your water-test results. A short record makes it much easier to ask a specialist for advice, because you can give real numbers rather than descriptions.
Quick summary
- Turnovers per hour = flow (L/h) ÷ tank volume (L); minutes per cycle = 60 ÷ turnovers.
- A 500 L/h filter on a 100 L tank gives 5 turnovers an hour, one every 12 minutes.
- Rated flow is a ceiling; media, hoses and clogging reduce real flow.
- Use the real water volume, and compare media volume as well as flow.
- Choose flow to suit your fish and plants; ask a specialist.
- Use the Aquarium Filter Turnover Calculator.
Frequently asked questions
How do I calculate filter turnover?
Divide the filter's flow rate in litres per hour by the tank's water volume in litres. 500 L/h on a 100 L tank is 5 turnovers per hour.
How many turnovers per hour should a tank have?
It depends on the fish, plants and filter type. Ask a specialist shop or club; this guide shows how to calculate what you have.
Why is real flow lower than the rated flow?
Ratings are typically given for ideal conditions. Filter media, hoses, bends and clogging add resistance and reduce flow.
Should I use the box volume or real volume?
The real water volume. Substrate and decor reduce it.
Is more flow always better?
Not necessarily. Some fish and plants prefer gentle flow. Check what your stock needs.
How often should I clean the filter?
Follow the maker's guidance and a specialist's advice, since cleaning affects flow and the media.
Sources
- Pure arithmetic: turnovers per hour = flow (L/h) ÷ volume (L); minutes per cycle = 60 ÷ turnovers per hour
- Manufacturer guidance on rated flow and media (varies by product)
Related calculators
- Aquarium Filter Turnover CalculatorSee how many times per hour your pump or filter moves the whole tank volume.
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Change CalculatorWork out how much water to replace for the percentage change you choose.
Keep reading
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- Aquarium Water Change Percentage ExplainedHow a water change percentage works: litres to replace, how much old water is left after repeated changes, buckets to carry and how to plan a weekly routine.
- How Much Substrate Do You Need for an Aquarium?How much gravel or sand an aquarium needs: volume = length × width × depth, weight = litres × density, with a table for 60, 90 and 120 cm tanks at 3, 5 and 7 cm.
- US Gallons vs UK Gallons for AquariumsA US gallon is 3.785 litres and a UK gallon 4.546, a 20% gap. Convert tank sizes both ways, see why 55 gallons can mean 208 or 250 litres, and fix water sums.