Aquarium filter size & flow rate calculator
Enter your water volume and the kind of tank, and see how much flow your filter should really deliver, what rating to look for on the box, and whether the filter you already have is enough.
How filter size is calculated
Filters are sized by turnover: how many times an hour the whole tank passes through the filter. Multiply your water volume by the turnover you want to get the real flow, then allow for the flow a filter loses in use. A box rating is measured with clean, empty housings and no lift, so a working filter delivers less.
| Tank | Turnover per hour | Source |
|---|---|---|
| Shrimp, fry, bettas | 2–4× | gentle current matters more than flow |
| Lightly stocked or planted | 4–6× | Aqueon and OASE filter guides |
| Community tank | 6–8× | where most stocked tanks end up |
| Goldfish, cichlids, oscars | 8–10× | heavy waste output |
| Reef sump return | 3–5× real flow | Bulk Reef Supply |
| Reef powerheads (display) | 20–30× soft & LPS, 30–40× SPS | Bulk Reef Supply |
Filter size chart for common tanks
Minimum filter rating (box GPH) for each tank size, from the same formula as the calculator, with a 30% flow loss and rounded up to the nearest 5 GPH. Each range runs from the low to the high end of that tank type's turnover.
| Tank | Litres | Light or planted (4–6×) | Community (6–8×) | Messy fish (8–10×) |
|---|---|---|---|---|
| 10 gallon | 38 L | 60–90 GPH | 90–115 GPH | 115–145 GPH |
| 20 gallon | 76 L | 115–175 GPH | 175–230 GPH | 230–290 GPH |
| 29 gallon | 110 L | 170–250 GPH | 250–335 GPH | 335–415 GPH |
| 40 gallon | 151 L | 230–345 GPH | 345–460 GPH | 460–575 GPH |
| 55 gallon | 208 L | 315–475 GPH | 475–630 GPH | 630–790 GPH |
| 75 gallon | 284 L | 430–645 GPH | 645–860 GPH | 860–1,075 GPH |
| 125 gallon | 473 L | 715–1,075 GPH | 1,075–1,430 GPH | 1,430–1,790 GPH |
Rows use nominal gallons. Real water volume is often 10–15% lower after substrate and rock, which you can work out with the tank volume calculator.
Worked examples
A 120 L planted tank
At 4–6× the filter should really move 480–720 L/h. With 30% loss, look for a filter rated 686–1,029 L/h (about 180–270 GPH). Keep the outlet below the surface if you inject CO₂.
A 50 gallon reef with a sump
The return pump needs 150–250 GPH of real flow (3–5×). Bulk Reef Supply suggests expecting about half the advertised flow once it pushes through plumbing, so choose one rated about 300–500 GPH, ideally with a flow chart showing that much at your head height. The corals' water movement comes from powerheads: 1,000–1,500 GPH in total for soft corals and LPS, split across two pumps.
Is my 200 GPH filter enough for a 40 gallon?
200 × 0.7 = 140 GPH real flow, and 140 ÷ 40 = 3.5×. That's below the 4–6× minimum for even a lightly stocked tank, so add a second filter or step up to one rated about 230–345 GPH.
Limits
- Turnover is a rule of thumb, not a guarantee of clean water. Stocking, feeding, media volume and maintenance matter as much as flow.
- Real loss varies by filter and by how clogged the media is. If your filter's maker publishes flow with media installed, or a pump's flow at your head height, use that and set the loss to 0%.
- A sump return can't exceed what your overflow drains, and too much flow through a sump cuts the time water spends in the skimmer and media.
Common questions
What size filter do I need for a 55 gallon tank?
For a community tank, aim for 6–8 turnovers an hour of real flow: 330–440 GPH. Filters lose about 30% of their box rating to media, hoses and lift, so buy one rated 475–630 GPH. A lightly stocked or planted 55 gallon can get by with 315–475 GPH, while goldfish or cichlids want 630–790 GPH.
How many GPH filter for a 20 or 29 gallon tank?
For a community tank, a 20 gallon needs a filter rated about 175–230 GPH and a 29 gallon about 250–335 GPH. That gives 6–8 tank volumes an hour after a typical 30% loss. Lightly stocked or planted tanks can use the low end, or 115–175 GPH and 170–250 GPH.
What is filter turnover rate?
Turnover is how many times an hour the filter moves the whole tank volume through its media: flow per hour divided by water volume. A filter really delivering 200 GPH on a 40 gallon tank is 5× turnover. Aqueon and OASE both recommend 4–6× for a typical aquarium.
Why is my filter weaker than the box says?
The rated GPH is measured with no media and no height to lift water. Sponges, ceramic media, hoses and the lift back over the rim all slow it, and it drops further as the media clogs. Expect about 30% less from a hang-on-back or canister filter, and about half the rating from a sump return pump pushing through plumbing.
How much flow does a reef tank need?
Two separate numbers. The return pump only needs to move the water through the sump about 3–5× an hour after head loss: 150–250 GPH for a 50 gallon, so a pump rated 300–500 GPH at 50% loss. Powerheads in the display then add 20–30× for soft corals and LPS (1,000–1,500 GPH) or 30–40× for SPS (1,500–2,000 GPH), as Bulk Reef Supply recommends.
Can an aquarium filter be too strong?
Yes, for some tanks. Bettas, long-finned fish, shrimp and fry struggle in strong current, and a sponge filter or a turned-down filter around 2–4× suits them. In CO₂-injected planted tanks, heavy surface splashing drives off the CO₂ you are paying for. For most other freshwater fish, a bit more filter than the minimum does no harm.
Is a filter rated "up to 50 gallons" enough for my 50 gallon?
Not always. "Up to" ratings are based on the box flow and often assume light stocking. Check the rated GPH against the calculator instead, and size up a step if your tank is well stocked or holds messy fish.
References
- Aqueon. How to choose the best filter for you and your tank (flow rate 4–6 times the tank volume per hour)
- OASE. All about aquarium filters (flow rate at least 4–6 times the tank volume; 60 L tank needs 240–360 L/h)
- Fin & Filter. How to size an aquarium filter (6–8× for community tanks, 8×+ for messy fish, about 30% loss from rated flow)
- Fishiverse. Aquarium filter sizing guide (2–4× for shrimp, breeder and fry tanks; 8–10× for high bioload)
- Bulk Reef Supply. How to properly size a return pump (2–5× after head pressure is common; up to 10× maximum through a sump)
- Bulk Reef Supply. BRStv buyer's guide to return pumps (about 3× is enough; expect about 50% of advertised flow after plumbing)
- Bulk Reef Supply. FAQ #19: How much water flow do I need in my reef aquarium? (20–40× display volume; SPS at the high end)
- NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L)
Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.
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