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Aquarium Filter Size Calculator
Calculate the filter flow rate your aquarium needs from usable water volume and turnover rate. See the calculated minimum flow and a practical rated-flow range in GPH and LPH.
Recommended filter flow
How is aquarium filter size calculated?
A filter's advertised flow can be higher than its installed flow after media, tubing, head height, and maintenance are considered. Match flow with biological capacity and the needs of your livestock.
How aquarium filter size is calculated
- The aquarium volume is converted to usable US gallons.
- Required flow is calculated as gallons multiplied by the selected turnover rate.
- A practical rated-flow margin is applied for aquarium type and rounded to useful filter sizes.
- The result is shown in both gallons per hour and liters per hour.
Understanding GPH and turnover rate
GPH means gallons per hour. A 20-gallon aquarium at 5x turnover has a calculated flow of 20 x 5 = 100 GPH. A practical recommendation may be about 100-150 GPH because the advertised rating is not always the flow delivered through an installed filter.
Choosing a filter type
Hang-on-back filters are convenient, canister filters offer media capacity, internal filters suit compact setups, and sponge filters provide gentle flow with useful biological surface area. Flow rate is only one part of filter performance.
Why advertised filter flow can differ
Manufacturers may measure an empty filter under ideal conditions. Filter media, tubing, lift height, clogging, and maintenance can reduce the flow that reaches the aquarium. Treat the rated range as a planning guide, not a guaranteed delivered flow.
Frequently asked questions
What size filter do I need for my aquarium?
Multiply usable gallons by the selected turnover rate, then choose a filter with a practical rated range that accounts for real-world flow loss.
How many times per hour should an aquarium filter turn over?
This tool offers 3x through 10x presets because the useful rate depends on aquarium type, livestock, flow tolerance, and maintenance routine. It is not a universal rule.
What filter size do I need for a 20-gallon tank?
At 5x turnover, the calculated flow is 100 GPH. A practical rated range may be about 100-150 GPH before considering the filter type and actual installed flow.
What is the difference between GPH and turnover rate?
GPH is a flow amount. Turnover rate expresses that flow relative to tank volume, so 100 GPH on a 20-gallon tank is 5x per hour.
Is a higher aquarium filter flow always better?
No. More flow can help some systems but may stress fish, disturb plants, or create dead spots elsewhere if circulation is poorly arranged. Biological capacity and livestock needs matter too.
Should I use one large filter or multiple filters?
Large aquariums may benefit from multiple filters for distribution and redundancy, but the calculator presents this as an option rather than forcing a split.
Does the filter type affect the recommendation?
The flow number is a starting point. HOB, canister, internal, and sponge filters differ in media capacity, maintenance, outlet design, and delivered flow.
Can this calculator choose a filter for my fish?
The optional fish selector is a local lookup convenience. This calculator does not claim species-specific filtration guidance when that data is not available.