Aquarium Filter Size Calculator
Enter your tank volume and stocking level to get your target turnover in gallons per hour, the rated GPH you should actually buy once media and clogging are accounted for, and the right filter type for your tank.
Not sure? Use the aquarium volume calculator first.
Higher bioload needs more turnover. Messy eaters push toward the top of the range.
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target GPH
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buy rated GPH
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turnover / hr
Buy a filter rated about . Rated GPH drops roughly 25 to 50 percent once you add media and as it clogs, so sizing up about 1.5 times keeps real flow near your target. A healthy turnover band for your tank is . Suggested filter type: .
Right-sized filters for a -gallon tank
Auto-matched to your rated GPH. Sizes are starting points; check our reviews for specific picks.
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How to Size an Aquarium Filter
Picking a filter is not about matching the box label to your tank label. It is about turnover, the number of times your filter cycles your entire water volume through its media in one hour. The widely used healthy range is four to ten times per hour. To find your target, multiply your tank volume in gallons by a number that reflects how much waste your fish produce: about four for a lightly stocked, planted tank, around six for a normal community tank, and eight to ten for a heavily stocked or messy setup like goldfish or cichlids. A 40-gallon community tank, for example, lands near 240 gallons per hour of real flow.
Rated GPH Is Not Real GPH
Here is the mistake that sinks most beginner filter choices. The gallons-per-hour number printed on a filter is a best-case figure, measured with an empty unit and no head height to fight against. The moment you load it with sponges, ceramic rings, and carbon, flow drops. As that media collects debris over the weeks between cleanings, flow drops further. Lifting water up to the tank rim and pushing it through plumbing costs even more. In practice, real flow commonly runs 25 to 50 percent below the rated number. That is why this calculator does not tell you to buy a filter rated exactly at your target. It tells you to buy one rated about 1.5 times your target, so that even a loaded, slightly dirty filter still delivers the turnover your fish need.
Why Oversizing Filtration Is a Good Thing
In most tanks, generous filtration is one of the cheapest forms of insurance you can buy. A larger filter holds more media, which means a larger colony of beneficial bacteria and a bigger buffer against an ammonia or nitrite spike when you add fish, overfeed, or miss a water change. Extra mechanical capacity also means longer stretches between cleanings and clearer water. If the current ends up stronger than your fish like, you can tame it easily with a spray bar, a flow deflector, or by aiming the return at the glass. It is far easier to reduce flow than to conjure more of it from an undersized unit.
Sponge, HOB, or Canister
The right filter type depends mostly on tank size. For tanks up to about 20 gallons, a sponge filter or a hang-on-back (HOB) filter is inexpensive, gentle, and easy to maintain. Sponge filters are especially good for fry, shrimp, and bettas because they cannot trap small creatures and they produce soft flow. From roughly 20 to 55 gallons, an HOB or a canister filter both work well. Canisters hold more media and run quietly, while HOBs are simpler and cheaper. Above 55 gallons, a canister filter is usually the best choice because it offers large media capacity, strong flow, and the ability to hide all the hardware below the tank. Many experienced keepers run two filters at once, splitting the turnover so the tank stays filtered even while one unit is being cleaned.
Match Filtration to Stocking, Not Just Tank Size
Two 40-gallon tanks can need very different filters. One holding a small school of tetras in a planted layout sits happily near four times turnover. Another packed with goldfish, which are heavy waste producers, wants eight to ten times turnover or more. Bioload, the total amount of waste your livestock creates, is what actually loads the filter, so always size to your real stock plan rather than the number on the tank. Pair this calculator with the aquarium volume calculator to confirm your true water volume, and with the heater size calculator to round out the core equipment for a stable, healthy tank.
Keep going: size the rest of your build.
Frequently Asked Questions
How many gallons per hour (GPH) does my filter need?
A healthy filter turns your entire water volume over about 4 to 10 times per hour, so multiply your tank size by 4 for a lightly stocked tank and up to 10 for a heavily stocked or messy one. A 40-gallon community tank wants roughly 240 GPH of real flow. Because rated GPH drops once you add media, aim to buy a filter rated about 1.5 times your target so it still performs after the media and a little gunk slow it down.
What is filter turnover and why does it matter?
Turnover is how many times the filter cycles your whole tank volume through its media in one hour. More turnover means waste, uneaten food, and dissolved ammonia reach the beneficial bacteria faster, which keeps the water clearer and more stable. Too little turnover leaves dead spots and lets debris settle, while extremely high turnover can stress calm fish like bettas. The 4 to 10 times per hour band covers almost every freshwater setup.
Why is rated GPH higher than the real flow I get?
Manufacturers measure rated GPH with an empty filter and no head height, which is the best case the unit will ever see. Once you load sponges, ceramic media, and carbon, and once that media starts collecting debris, real flow commonly drops 25 to 50 percent. Lift height to the tank rim and plumbing also rob flow. That is why we size your filter up: a unit rated about 1.5 times your target still hits the number after media and clogging.
Is it bad to have a filter that is too powerful?
For most tanks, slightly oversized filtration is a feature, not a flaw. Extra media volume means more beneficial bacteria and more buffer against a spike, and you can always reduce the current with a spray bar, a flow baffle, or by aiming the output at the glass. The main exceptions are gentle, long-finned fish like bettas and fancy goldfish, which dislike strong current, so dial the flow down for them rather than choosing a weak filter.
What type of filter should I use, sponge, HOB, or canister?
For tanks up to about 20 gallons, a sponge filter or a hang-on-back (HOB) filter is simple, cheap, and gentle. From roughly 20 to 55 gallons, an HOB or a canister both work well, with canisters holding more media. Above 55 gallons, a canister filter is usually the best choice because it provides large media capacity and strong, quiet flow. Many keepers run two filters so the tank keeps cycling if one is being cleaned.
Can I run two filters on one tank?
Yes, and many experienced keepers do. Splitting your target GPH across two filters gives you redundancy, since the tank stays filtered while you clean or service one unit, and it spreads flow more evenly to reduce dead spots. Combining a sponge filter with an HOB or canister is a popular setup. Just add the rated GPH of both units together when checking that you are inside the 4 to 10 times per hour turnover band.
How does stocking level change the filter size I need?
More fish, bigger fish, and messier eaters all add bioload, which is the amount of waste the filter and bacteria must process. A lightly stocked planted tank can sit near 4 times turnover, a normal community tank wants about 6 times, and heavy or messy stockings like goldfish and cichlids do best at 8 to 10 times or more. Sizing filtration to your real stocking, not just the tank label, is what keeps ammonia and nitrite at zero.