Use this calculator as a starting point, not as a livestock guarantee. It estimates a circulation target from aquarium volume and bioload, then adds a practical cushion because real flow usually drops once media and hoses are in use.
Aquarium filter size calculator
Saved locally in this browser only. This tool estimates circulation. It cannot measure actual biological capacity, oxygenation, dead zones or species-specific flow tolerance.
How the calculator works
The basic math is tank gallons × turnover target × cushion. A 55-gallon moderate community aquarium at five turnovers per hour gives 275 GPH before the cushion. With a 25% planning cushion, the shopping target becomes about 344 GPH of nominal flow.
This is intentionally conservative because published flow is not the same as installed flow. Media, dirty prefilters, bends and vertical head reduce output.
Quick filter flow chart
| Tank | Light (4×) | Moderate (5×) | Heavy (7×) |
|---|---|---|---|
| 10 gal | 40 GPH | 50 GPH | 70 GPH |
| 20 gal | 80 | 100 | 140 |
| 40 gal | 160 | 200 | 280 |
| 55 gal | 220 | 275 | 385 |
| 75 gal | 300 | 375 | 525 |
| 125 gal | 500 | 625 | 875 |
The chart shows circulation before adding a shopping cushion. Low-flow livestock can require gentler return velocity even when total filtration capacity is high.
Why media capacity can beat GPH
A pump can move a lot of water through a tiny cartridge. That does not make it equivalent to a filter with several liters of mature bio-media. When comparing filters, look at the actual basket volume, prefilter design and how easy it is to clean mechanical media without disturbing the biological stage.
When two filters make more sense
For long 75–125 gallon tanks, two filters can provide better end-to-end circulation and redundancy than one unit with the same combined GPH. Split the load so one filter can keep operating while the other is serviced. Read one big filter vs two smaller filters.
Adjust for the fish, not the spreadsheet
Bettas, long-finned fish and some shrimp setups can be stressed by concentrated high-velocity flow. Large plecos, goldfish and big cichlids create much more waste. The solution is often to separate filtration capacity from return velocity: use ample media, then diffuse or aim the output so the livestock is comfortable.
Frequently asked questions
How many GPH should my aquarium filter have?
There is no universal number, but a freshwater planning range of roughly 4–6 tank turnovers per hour is a useful baseline for many community tanks. Heavy bioloads may need more circulation and more media; low-flow species may need less current.
Should I use the manufacturer's tank rating or GPH?
Use both as clues, then look at media capacity and the fish you keep. Tank ratings are marketing simplifications, while GPH does not tell you how much biological media the filter holds.
New to fishkeeping? Start with our complete beginner aquarium setup guide. Choosing filtration? Use the filter hub to compare options by tank size.