Aquarium tools · Tank setupUpdated October 10, 2026

Aquarium Filter Flow Calculator

Enter your aquarium volume and your filter’s flow rate to see how many times per hour the filter moves the tank’s water. The calculator allows for real-world flow loss, checks the result against the right range for your fish, reef or pond, works out the flow you need, combines or compares several filters, and converts GPH to LPH.

Water circulation

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3.7× per hour: flow may be too low (guide 4–6× for a freshwater aquarium). Faster-moving dashes mean faster flow.

Your aquarium and filter

Not sure of your real water volume? Measure your tank and it will fill in here automatically.

1. Aquarium type

2. What do you want to calculate?

3. Aquarium water volume

4. Rated flow per filter

More options: filter type, real-world flow, priorities and decor volume

Filtration system

Estimated actual flow (% of rated flow)

User-selected estimate, not a universal correction factor

Filter condition

Filtration priority

Water-flow preference

Water volume after substrate and decor (optional)

Usually 10–20% less than the tank size

Filter model or manufacturer (optional)

Aquarium Filter Flow Result

Flow may be too low

3.7× per hour is slightly under the 4× guide minimum for a freshwater aquarium. A lightly stocked tank with clean test results may be fine; otherwise look for a total rating of about 1,189 L/h or more (at 70% actual flow), add a second filter, or clean the filter if its flow has dropped.

Turnover

3.7×per hour

Guide 4–6×

Water volume

208 L

54.9 US gal

Rated flow

1,100 L/h

291 GPH

Estimated actual flow

770 L/h

Estimate: 70% of rating

Actual flow = 1,100 L/h × 70% = 770 L/h. Turnover = 770 L/h ÷ 208 L = 3.7× per hour. For 4–6× you need about 1,189 L/h–1,783 L/h of rated flow at 70%.

About the 4–6× guide for a freshwater aquarium

About 4× per hour is the usual starting point; go higher for big or messy fish and lower for weak swimmers. Basis: LiveAquaria recommends turning the volume over about 4 times per hour on average.

Canister filter

Canisters push water up long hoses, through elbows and packed media, so actual flow is often well below the box figure (LiveAquaria). Check the manufacturer’s flow with media, or measure it.

Turnover is not biological filtration capacity

Turnover only says how often water passes the filter. How much waste the filter can process depends on its media volume, the bacteria on it and how well it is maintained. Test ammonia and nitrite to judge that.

Quick answer

Turnover = actual filter flow per hour ÷ water volume. A 55 US gallon tank with a 300 GPH filter running at about 70% of its rating gets 210 GPH, or 3.8× per hour. LiveAquaria recommends about 4× per hour on average; bettas and shrimp need less, goldfish and big cichlids more. Sump returns usually run 2–10× (Bulk Reef Supply) and fish ponds 0.5–1×.

How to calculate filter turnover

Turnover is how many times per hour the filter passes a volume of water equal to your tank. It is a quick way to compare filters, but it says nothing about how much media the filter holds or how well it is maintained.

Example: a 55 US gallon tank and a filter rated at 300 GPH. At an estimated 70% of the rating, actual flow is 300 × 0.70 = 210 GPH, and turnover is 210 ÷ 55 ≈ 3.8× per hour.

Actual flow = rated flow × estimated actual flow %

Turnover per hour = actual flow ÷ water volume (same volume unit)

How to work out the filter flow rate you need

Multiply the water volume by the turnover you want. For a 55 US gallon tank at 6× per hour you need 55 × 6 = 330 GPH of actual flow. If you expect 70% of the rating to reach the tank, look for about 330 ÷ 0.70 ≈ 471 GPH on the box.

The target is a maths result, not a universal recommendation: the right turnover depends on the fish, the filter design and how much current the animals tolerate.

Required actual flow = volume × target turnover

Rated flow to look for = required actual flow ÷ estimated actual flow %

Why actual flow is lower than the box rating

Pump ratings are measured under ideal conditions at a head height of zero, meaning the water is not lifted at all (LiveAquaria). Canisters and sump pumps must push water up to a tank rim that is often about four feet off the floor, and every elbow, narrow hose and clogged sponge slows the flow further. Bulk Reef Supply suggests assuming about 50% of a return pump’s advertised flow after plumbing.

The percentages in the calculator (100% down to 40%) are estimates you choose, not universal correction factors. For a real figure, time how long the return takes to fill a 1 litre jug: flow in L/h = 3,600 ÷ seconds. If it takes 6 seconds, the filter moves about 600 L/h. With a gallon jug, the same sum gives GPH.

Recommended turnover by aquarium type

Filter turnover guide (actual flow ÷ water volume per hour)
Aquarium type	Turnover per hour	Basis
Freshwater, tropical or community	4–6×	LiveAquaria: about 4× on average
Coldwater (not goldfish)	4–6×	LiveAquaria: about 4× on average
Betta	2–4×	LiveAquaria: very slow flow; OATA: gentle current
Shrimp	3–5×	Slow flow for small animals; protect the intake
Planted	4–8×	LiveAquaria: calm surface to hold CO₂, plants out of the flow stream
Goldfish	6–10×	OATA: goldfish can be very dirty
Cichlid (large species)	6–10×	LiveAquaria: large fish need higher flow
Brackish	4–6×	Community-level flow
Saltwater / marine (filter or sump)	5–10×	Bulk Reef Supply: 5–10× common, ~10× practical maximum through a sump
Reef (sump return)	3–10×	Bulk Reef Supply: 2–10× after head loss; plus 20–40× from powerheads
Freshwater fish pond	0.5–1×	All Pond Solutions: at least once every two hours
Koi pond	1–2×	All Pond Solutions: at least once per hour

These are hobby and trade guidelines, not laboratory standards; ranges without a named figure are our guideline. Watch your animals: if they struggle to hold position, reduce or spread the flow.

Two filters, sumps and filters in series

Filters in parallel (each with its own intake and outlet) add together: two 200 GPH filters move 400 GPH.
Filters in series, or several stages on one pump, do not add: the same water passes each stage, so turnover is set by the one flow path.
Sumps: count the return pump’s actual flow at your head height. Keep it below the overflow’s maximum drain rating.
Powerheads and wavemakers move water around the tank but do not filter it, so they are circulation, not filter turnover.

How filter types differ

Hang-on-back and internal filters lift water very little, so they stay close to their rating while the sponge is clean.
Canister filters lose more flow to long hoses, elbows and packed media; check the maker’s flow with media.
Sponge and undergravel filters are driven by air or a powerhead. Their flow is rarely published, so follow the tank-size rating.
Reactors, fluidized beds and moving bed filters run on a set side flow; follow the maker’s range rather than tank turnover.
Pond pressure and gravity filters list a maximum flow; keep the pump within it so water is not forced through too fast.

GPH to LPH conversion

1 US gallon = 3.785411784 litres (NIST), so LPH = GPH × 3.785 and GPH = LPH ÷ 3.785. 1 GPM = 60 GPH, 1 LPM = 60 LPH and 1 m³/h = 1,000 LPH.

Common filter flow rates in GPH and LPH
GPH	= LPH	LPH	= GPH
50	189	500	132
100	379	1,000	264
200	757	1,500	396
300	1,136	2,000	528
500	1,893	3,000	793

Worked example

A tank holds 120 L of real water and the filter box says 600 L/h. Allowing 30% for media and lift, the real flow is 600 × 0.7 = 420 L/h, so turnover is 420 ÷ 120 = 3.5× per hour.

That is slightly under the 4–6× range for a community tank. To reach 4×, buy a filter rated for at least 120 × 4 ÷ 0.7 ≈ 690 L/h, or add a second small filter. You could also accept 3.5× if the tank is lightly stocked and the water tests clean.

Filter rating to look for (box rating, assuming 30% flow loss)
Real water volume	Real flow at 4×	Box rating for 4×	Box rating for 6×
10 US gal (38 L)	40 GPH	57 GPH	86 GPH
20 US gal (76 L)	80 GPH	114 GPH	171 GPH
29 US gal (110 L)	116 GPH	166 GPH	249 GPH
55 US gal (208 L)	220 GPH	314 GPH	471 GPH
60 L	240 L/h	343 L/h	514 L/h
120 L	480 L/h	686 L/h	1,029 L/h
200 L	800 L/h	1,143 L/h	1,714 L/h

Flow is not the whole story

Biological media needs time and oxygen. LiveAquaria notes that biological and chemical media often work best at about 4× per hour or less, while flow below 1× can starve the bacteria of oxygen. Merck notes that a new biofilter can take up to 8 weeks to establish in a tropical tank.
Media volume matters. For heavy waste producers such as goldfish, choose a filter with plenty of media space, not only a high flow figure.
Clean gently. Rinse sponges in water taken out of the tank, not under the tap, so chlorine does not kill the bacteria (OATA).
Spread strong flow. A spray bar or a second outlet spreads the current so fish are not blasted from one point.

Frequently asked questions

What size filter do I need for a 20 gallon tank?

For about 4× turnover you need roughly 80 GPH of real flow. Allowing 30% for media and lift, look for a filter rated around 115 GPH. Goldfish or a planted tank call for more; a betta for less.

Can a filter be too strong?

Yes. Bettas, fancy goldfish, fry and other weak swimmers get exhausted in strong current. Signs include fish hiding behind decor, struggling to swim, or being pushed around. Baffle the outlet or turn the flow down.

Is 10× turnover too much?

Not for goldfish, large cichlids or many planted tanks, as long as the fish swim comfortably. For bettas and shrimp it is usually far too much.

Should I count powerheads in turnover?

Count them for circulation, not filtration. A powerhead moves water around the tank but does not trap waste or hold beneficial bacteria.

How do I measure my filter’s real flow?

Time how long the outlet takes to fill a 1 litre container. Flow in L/h = 3,600 ÷ seconds. Repeat after cleaning the filter to see how much it loses as it clogs.

Do canister filters lose flow?

Yes. Canisters push water through long hoses, packed media and up to the tank, so real flow is often well below the rating. Check the manufacturer’s figures for flow with media, or measure it.

Do two filters double the turnover?

Only if each filter has its own intake and outlet (in parallel). Filters plumbed in series, or several stages on one pump, share one flow path, so their flows do not add up.

What turnover does a reef tank need?

Through the sump, Bulk Reef Supply says 2–10× per hour after head loss is generally acceptable, with many reefers running 2–5×. Most water movement in a reef comes from powerheads: about 20–40× the display volume per hour, more for SPS corals.

What flow rate does a pond or koi pond need?

All Pond Solutions advises passing the whole pond through the filter at least once every two hours for decorative ponds and at least once per hour for heavily stocked koi ponds. A 9,000 litre koi pond therefore needs about 9,000 L/h at your real head height.

How do I convert GPH to LPH?

Multiply GPH by 3.785 (1 US gallon = 3.785411784 litres). 300 GPH is about 1,136 LPH. To go back, divide LPH by 3.785.

What to calculate next

Sources

Disclaimer: Turnover ranges are general hobby and trade guidance. Every tank differs: water test results and fish behaviour matter more than any single number, and turnover does not measure biological filtration capacity. See our disclaimer.

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