Aquarium filter flow rate calculator
Use this aquarium filter GPH calculator to see the filter flow your tank needs in GPH and L/h. Enter your tank size and what lives in it. Add your filter's rating to check whether it's too weak, about right or too strong.
Your tank
Your filter (optional)
The number on the box or in the manual. Running two filters? Add their ratings together. Leave empty if you're choosing a new filter.
Result
Rated filter flow to look for
80–120 GPH
303–454 L/h · 4–6× your tank volume per hour
Between two filter sizes? Pick the bigger one. Many filters can be turned down, but none can be turned up past their rating.
Measure your filter's real flow
Hold a measuring jug under a hang-on-back filter's spillway and time how long it takes to collect a known amount of water.
Choose a filter rated for 4 to 6 times your tank volume per hour for a typical community tank: 80–120 GPH (about 300–450 L/h) for a 20 gallon. Use 6–10 times for goldfish, large cichlids and other messy fish, 3–5 times for lightly stocked or planted tanks, and a gentler 2–4 times for bettas and long-finned fish, or a standard filter with the flow turned down or baffled.
How it works
Turnover is how many times your filter could pump the whole tank volume in one hour, based on the flow rate printed on the box. The calculator multiplies your tank volume by the turnover range for your type of tank:
Rated flow needed (GPH) = tank volume (US gallons) × turnoverTurnover of your filter = rated flow (GPH) ÷ tank volume (gallons)L/h = GPH × 3.785 (1 US gallon = 3.78541 litres)
The same formulas work in metric: a 100 litre tank at 4–6× needs a filter rated 400–600 L/h.
Worked example
A 29 gallon (110 L) tank with two fancy goldfish counts as messy, so the target is 6–10× an hour.
- 29 × 6 = 174 GPH and 29 × 10 = 290 GPH, so look for a filter rated 174–290 GPH (659–1,098 L/h).
- The current filter is rated 150 GPH: 150 ÷ 29 = 5.2× an hour. That's a bit low; it needs about 24 GPH more.
- Once full of media it probably moves 90–120 GPH, or 3.1–4.1× an hour in practice.
Turnover targets by tank type
| Tank type | Rated turnover | Why |
|---|---|---|
| Gentle flow: bettas, long-finned fish, fry | 2–4× | Weak swimmers get pushed around, and bettas come from still and sluggish water. This is our own suggested range, not a published figure. A 4–5× filter can also work if you turn it down or baffle the outlet. |
| Lightly stocked or planted (no injected CO2) | 3–5× | Less waste to process, plants take up some of it, and less splashing keeps CO2 in the water. |
| Community | 4–6× | Published sizing guides use 4–5× or 5×. We allow up to 6× as a margin. |
| Heavily stocked or messy fish | 6–10× | Goldfish, large cichlids, crowded cichlid tanks and big eaters make much more waste, so the water needs to pass through the filter more often. One published calculator uses 8×. |
The community and messy ranges follow published filter sizing advice (see sources below). The lightly stocked range is our own adjustment: one step lower than community because there is less waste to remove. The gentle-flow range is also our own suggestion. Sources agree bettas want gentle water movement, but we found no agreed turnover figure for them, so watch your fish rather than the number.
Filter flow chart by tank size
Recommended rated filter flow in GPH, with L/h underneath. Tap a tank size for a full setup guide.
| Tank | Gentle flow fish (2–4×) | Lightly stocked or planted (3–5×) | Community tank (4–6×) | Heavily stocked or messy fish (6–10×) |
|---|---|---|---|---|
| 5 gal 19 L | 10–20 GPH 38–76 L/h | 15–25 GPH 57–95 L/h | 20–30 GPH 76–114 L/h | 30–50 GPH 114–189 L/h |
| 10 gal 38 L | 20–40 GPH 76–151 L/h | 30–50 GPH 114–189 L/h | 40–60 GPH 151–227 L/h | 60–100 GPH 227–379 L/h |
| 20 gal 76 L | 40–80 GPH 151–303 L/h | 60–100 GPH 227–379 L/h | 80–120 GPH 303–454 L/h | 120–200 GPH 454–757 L/h |
| 29 gal 110 L | 58–116 GPH 220–439 L/h | 87–145 GPH 329–549 L/h | 116–174 GPH 439–659 L/h | 174–290 GPH 659–1,098 L/h |
| 40 gal 151 L | 80–160 GPH 303–606 L/h | 120–200 GPH 454–757 L/h | 160–240 GPH 606–908 L/h | 240–400 GPH 908–1,514 L/h |
| 55 gal 208 L | 110–220 GPH 416–833 L/h | 165–275 GPH 625–1,041 L/h | 220–330 GPH 833–1,249 L/h | 330–550 GPH 1,249–2,082 L/h |
| 75 gal 284 L | 150–300 GPH 568–1,136 L/h | 225–375 GPH 852–1,420 L/h | 300–450 GPH 1,136–1,703 L/h | 450–750 GPH 1,703–2,839 L/h |
| 125 gal 473 L | 250–500 GPH 946–1,893 L/h | 375–625 GPH 1,420–2,366 L/h | 500–750 GPH 1,893–2,839 L/h | 750–1,250 GPH 2,839–4,732 L/h |
Rated flow vs real flow
The flow rate on the box is a best case. In your tank, water has to push through sponge, ceramic rings and floss, travel through hoses and bends, and be lifted from the filter up to the tank. Each of those slows it down. A pump's flow falls as the height it has to lift water rises, until at its maximum lift (the "shut-off head") no water moves at all. Flow drops further as the sponge clogs between cleanings.
We assume a filter moves about 60–80% of its rated flow once it is set up with media, a 20–40% loss. That is a planning estimate, not a measurement of any one model. The more media, hose and lift a filter has to work through, the bigger the loss.
The turnover targets above already apply to rated flow, which is how published filter guides size filters, so you don't need to add the loss on top. Use the real flow estimate to understand what's happening in the tank, and measure it if the flow looks weak.
How to measure real flow
- Hold a measuring jug under the spillway of a hang-on-back filter.
- Time how many seconds it takes to collect a known amount, such as half a gallon, 1 quart (0.25 gallons) or 1 litre.
- Real flow per hour = amount collected × 3,600 ÷ seconds. For example, 1 gallon in 45 seconds = 1 × 3,600 ÷ 45 = 80 GPH. You can also enter your numbers in the calculator above.
Don't run a canister filter's outlet hose into a bucket to measure it. If the bucket sits lower than the tank, the hose keeps siphoning water out even after you switch the filter off.
If you get less than half the rating, the media is probably clogged or the impeller is dirty. Rinse the sponge in old tank water (chlorinated tap water can kill the filter bacteria) and clean the impeller.
Practical tips for choosing a filter
- Between two sizes, pick the bigger one. Many filters have a flow control, so you can turn a strong filter down, but you can't turn a weak one up past its rating.
- Flow isn't everything. A filter also needs enough room for media and regular cleaning. A big pump with a clogged sponge filters less than a smaller, clean one.
- Big tanks: on tanks of 55 gallons or more, two filters at opposite ends spread the flow, reduce dead spots and give you a backup if one stops.
- Bettas and long fins: a sponge filter, or a small filter with adjustable flow or a baffled outlet pointed at the glass, works well. If a fish hides, clamps its fins or gets pushed around, the current is too strong. See our betta fish care guide.
- Goldfish: they need strong filtration and weekly water changes. Fancy goldfish are slow swimmers, so spread the outflow with a spray bar. See fancy goldfish care.
- Planted tanks: aim for a gently rippling surface. Heavy splashing drives off the carbon dioxide (CO2) plants use. Tanks with injected CO2 are different: growers usually run stronger circulation to spread CO2 and nutrients to every plant, so treat them as community or higher, while still keeping the surface from splashing.
- Fry and shrimp: cover the filter intake with a sponge so babies don't get pulled in.
- Flow doesn't make room for more fish. Our stocking calculator gives a small bonus for strong filtration (8× or more), but too many fish is still too many fish.
Related calculators and guides
- Aquarium volume calculator: not sure how many gallons your tank holds? Work it out from its dimensions.
- Fish tank stocking calculator: check how many fish your tank and filter can handle.
- Aquarium heater size calculator: the right wattage for your tank and room.
- Water change and nitrate calculator: how big and how often your water changes should be.
- Aquarium unit converter: gallons, litres, temperature and hardness.
- Tank size guides and fishkeeping guides, including setting up and cycling a new tank.
- All aquarium calculators.
Shop for this
- Aquarium filter (opens Amazon in a new tab)Pick one rated 4–6 times your tank volume per hour for a community tank. How to choose
- Sponge filter with air pump (opens Amazon in a new tab)Gentle flow for bettas, shrimp and fry, and very hard to get wrong. How to choose
- Replacement filter sponge (opens Amazon in a new tab)Rinse sponges in old tank water, never under the tap. How to choose
Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.
Frequently asked questions
What GPH filter do I need for a 20 gallon tank?
For a typical community tank, choose a filter rated 80–120 GPH (about 300–450 L/h), which is 4–6 times the tank volume per hour. For goldfish or other messy fish aim for 120–200 GPH. For a betta, a gentle filter around 40–80 GPH is a reasonable start, or a bigger one with the flow turned down or the outlet baffled.
How many times per hour should aquarium water go through the filter?
Most freshwater community tanks do well with a filter rated for 4–6 times the tank volume per hour. Go up to 6–10 times for goldfish, large cichlids and heavily stocked tanks, 3–5 times for lightly stocked or planted tanks without injected CO2, and about 2–4 times for bettas, long-finned fish and fry. There is no agreed figure for bettas. What matters is a gentle current, so many keepers use a sponge filter, or a standard filter of about 4–5 times with adjustable flow or a baffle on the outlet.
Can a filter be too strong for my fish?
Yes. Bettas, long-finned guppies, fancy goldfish and fry are weak swimmers and tire or get pushed around in a strong current. Signs include hiding, clamped fins and struggling to swim. Turn the flow down, cover the outlet with a sponge or point it at the glass. Most other fish are fine with extra flow as long as they have calm spots to rest.
Why is my filter's real flow lower than the rating?
The rating is a best case. Filter media, hoses, bends and the height the water has to be lifted all slow it down, and flow drops further as sponges clog. Plan on roughly 60–80% of the rated flow, and rinse the sponge in old tank water when the flow slows.
How do I convert GPH to L/h?
Multiply gallons per hour by 3.785 to get litres per hour, or divide L/h by 3.785 to get GPH. For example, 100 GPH is about 379 L/h and 500 L/h is about 132 GPH. These use US gallons; a UK gallon is 4.546 litres.
Sources
- Seriously Fish species profiles for neon tetra, bronze corydoras and yellow lab cichlid: filters sized at four to five times the tank volume an hour.
- Seriously Fish, Betta splendens profile: lives in still and sluggish waters, strong water movement should be avoided, and an air-powered sponge filter set to turn over gently is adequate.
- All Pond Solutions, aquarium filter size calculator: 5× for freshwater community tanks and 8× for heavily stocked tanks or messy fish such as goldfish and cichlids.
- Wikipedia, Goldfish: goldfish produce a large amount of waste, in their feces and through their gills.
- Wikipedia, Aquarium filter: the pump has to push water through the filter media and overcome drag in the pipes; power filters can cause too much flow in small tanks.
- The Engineering ToolBox, Centrifugal pumps: at a pump's shut-off head the flow rate is zero.
- RSPCA, Fish environment: keep filters clean, including the impeller, so they run efficiently.
- Wikipedia, Gallon: 1 US gallon = 3.785411784 litres; 1 imperial gallon = 4.54609 litres.
The 60–80% real flow figure and the lightly stocked and gentle-flow ranges are our own planning estimates, explained above. Reviewed October 2026.