Aquarium Forge

Aquarium Filter Calculator

Enter your tank volume and setup type to get the filter flow you should target: a minimum, a recommended value based on turnover rate, and a high-flow option for demanding setups.

Recommended filter flow
80 GPH (303 LPH)
Minimum flow
60 GPH
High-flow option
120 GPH
Assumed turnover rate
4× per hour
  • Manufacturer rated flow differs from actual operating flow under media load and head height — size up when in doubt.
  • Use the result as a selection range, not an absolute guarantee.
  • Reef tanks typically need much higher random flow from powerheads in addition to filter turnover.

How It Works

Filter sizing starts with turnover rate: how many times per hour the filter circulates the full tank volume. Freshwater community tanks are comfortable around 4× per hour, planted tanks with gentle-flow species can drop to 3×, heavily stocked or large-fish tanks benefit from 5×, and reef systems typically need 8× or more (plus random powerhead flow). The calculator multiplies your volume by the appropriate turnover and rounds out a practical selection range.

Formula

Recommended Flow = Tank Volume × Turnover Rate

Minimum Flow = Tank Volume × 3

High-Flow Option = Tank Volume × Turnover × 1.5

Example

A 55 gallon freshwater tank at 4× turnover wants about 220 GPH of actual flow. Because manufacturer ratings overstate real-world performance, a filter (or pair of filters) rated 250–30 GPH is a sensible shortlist.

Important Limitations

  • Manufacturer rated flow differs from actual operating flow under media load and head height.
  • Results are for canister/HOB filtration —sumps and sponge filters follow different sizing logic.
  • Reef tanks need additional random flow from powerheads; filter turnover alone is not enough.

FAQ

What is filter turnover rate?

Turnover is how many times the filter processes the tank's full volume each hour. A 4× turnover on a 20 gallon tank means the filter moves 80 gallons per hour (GPH) through the media. Most freshwater community tanks do well at 4—×; reef systems typically want much more.

How many GPH does my filter need?

Multiply your tank volume by the turnover target. A 20 gallon freshwater tank needs about 80 GPH of actual flow; a 55 gallon needs about 220 GPH. The calculator above handles any size and tank type.

Why does the box rating differ from the calculator result?

Manufacturer flow ratings are measured on an empty, media-free filter. With sponges, biomedia, and head height, real flow is often 20–0% lower than the box claim —which is why the recommended range matters more than a single number.

Is more filtration always better?

More media capacity is almost always better, but excessive outflow current can stress fish. Betta, gourami, and other long-finned or slow-water species prefer gentler flow; planted tanks and reef tanks are different stories entirely.

Can I use two smaller filters instead of one large one?

Yes —two filters sharing the load give redundancy if one fails and distribute flow more evenly. Combined rated flow should still meet the recommended GPH for your tank.

Sources

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