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Filter Flow Rate (GPH) Calculator

How much filter flow (gallons per hour) your tank needs based on volume and stocking.

Your numbers
Your tank

Rated GPH isn't actual GPH

A filter's box rating is measured with no media or plumbing resistance -- real-world flow is typically lower, so treat the rated GPH as a ceiling, not a guarantee.

How this calculator works

Turnover rate -- how many times per hour your filter cycles the tank's total volume -- is a common way to size filtration. Lightly stocked freshwater tanks can work well with 4-6x turnover, heavily stocked freshwater tanks benefit from more (6-10x), and reef/marine tanks typically want the most flow (8-12x) both for filtration and to keep corals and inverts happy with water movement.

Where these numbers come from

Turnover rate guidelines reflect common aquarium hobby and filtration-industry recommendations. Actual needs vary by specific stocking, filter media, and tank shape -- treat this as a starting point, not an exact requirement.

Frequently asked questions

Does a higher turnover rate always mean better filtration?

Generally more flow helps with mechanical filtration and oxygenation, but too much flow can stress some fish or disturb substrate -- match turnover to what your specific livestock is comfortable with, not just the maximum your filter can provide.

Why is actual flow lower than a filter's rated GPH?

Filter ratings are typically measured with no media, no lift height, and no plumbing restriction -- in real use, filter media, hoses, and lift all reduce actual flow below the box number.

Do reef tanks really need more flow than freshwater tanks?

Generally yes -- corals and many reef invertebrates rely on water movement for feeding and gas exchange in ways most freshwater fish don't need, so reef tanks commonly run higher turnover rates.

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