# Aquarium filter flow calculator

> Enter your tank volume in US gallons, filter type (mechanical, biological, or chemical), and water type (freshwater or saltwater). A 29 US-gallon freshwater tank with a mechanical filter needs about 116–174 GPH — the worked example below uses the same inputs so the numbers shown here always match the calculator.

Use the interactive calculator on the page: https://petguidecalc.com/aquarium/filter-flow/. Enter your tank size in US gallons, pick the filter type (mechanical, biological, or chemical), and choose freshwater or saltwater. The recommendation is a GPH range based on 4–6× tank turnover, with a 1.2× boost for saltwater.

## How much filter flow do I need (GPH)?

The hobby rule of thumb is to turn over the tank volume 4–6× per hour in US gallons per hour (GPH). Mechanical filters can run at the high end (4–6×); biological and chemical filters sit at the low end (about 4×) because they rely on contact time with the media. Saltwater tanks need about 20% more flow (× 1.2) to handle higher oxygen demand and waste load.

## Common tank sizes at a glance

| Tank | US gal | Water type | Min GPH | Max GPH |
|---|---|---|---|---|
| 24 × 12 × 16 in (20 US gal high) | 20 | freshwater | 80 | 120 |
| 24 × 12 × 16 in (20 US gal high) | 20 | saltwater | 96 | 144 |
| 48 × 13 × 21 in (55 US gal) | 55 | freshwater | 220 | 330 |
| 48 × 13 × 21 in (55 US gal) | 55 | saltwater | 264 | 396 |
| 48 × 18 × 21 in (75 US gal) | 75 | freshwater | 300 | 450 |
| 48 × 18 × 21 in (75 US gal) | 75 | saltwater | 360 | 540 |
| 72 × 18 × 27 in (150 US gal) | 150 | freshwater | 600 | 900 |
| 72 × 18 × 27 in (150 US gal) | 150 | saltwater | 720 | 1080 |

All values use a mechanical filter (4–6× turnover). Saltwater rows apply the 1.2× boost. Real-world GPH is typically ~70% of nameplate after head loss.

## How the GPH range is calculated

Tank turnover: 4–6× the tank volume per hour, expressed in US gallons per hour (GPH). For a 29 gallon freshwater mechanical filter that is 116–174 GPH (29 × 4 to 29 × 6).

Biological and chemical filters rely on contact time, so we hold them at the low end (about 4×) to give the media time to do its work. Mechanical filters can run at 4–6× without losing efficiency.

Saltwater tanks need about 20% more flow (× 1.2) to keep up with the higher oxygen demand and waste load of marine life. The same 29 gallon saltwater mechanical filter ranges 139–209 GPH.

Worked example: a 29 gallon freshwater tank with a mechanical filter returns 116–174 GPH. Change any input and the result updates from the same shared formula.

Head loss: real-world GPH is roughly 70% of the manufacturer's nameplate once you add hose runs, fittings, and a full media tray — buy a filter with nameplate GPH above the upper end of this range so you still hit the recommended turnover after derate.

## Filter flow questions

**Q: How many times per hour should an aquarium filter turn over the water?**

4–6× per hour is the mainstream hobby range. Mechanical filters can run at 4–6×; biological and chemical filters stay around 4× so the media has time to work. Saltwater tanks need about 20% more flow on top of those ranges.

**Q: What is GPH?**

GPH stands for US gallons per hour. It is the manufacturer's nameplate flow rate for a filter, measured with no head pressure. Real-world GPH typically lands around 70% of the nameplate figure once you add hose runs, fittings, and a loaded media tray.

**Q: Do I need a bigger filter for a saltwater tank?**

Yes — saltwater tanks need about 20% more flow than a freshwater tank of the same size, so size up by one filter class. Higher oxygen demand and waste load from marine life are the main reasons.

**Q: Is more flow always better?**

Not for biological and chemical filters. Too much flow shortens the contact time with the media and the bacteria or resin can't do their work. For those, stay near the low end (about 4×). Mechanical filters can run higher (4–6×) without losing efficiency.

**Q: What about a sponge filter or air-driven filter?**

Sponge filters are typically rated well below the 4× mark and work best as supplemental biological filtration. Pair them with a power filter or canister that handles the main turnover, and use this calculator to size the main filter.

## References

Turnover ranges, head-loss derating, and the saltwater boost all come from mainstream aquarium hobby sources; the calculator applies the same multipliers so the recommendation matches what those sources recommend.

- [How to choose an aquarium filter](https://www.aquariumcoop.com/) — Aquarium Co-Op, Aquarium Co-Op. Hobby reference for the 4–6× turnover rule of thumb, the saltwater 20% boost, and the case for two-stage filtration on heavily stocked tanks.
- [Aquarium filter flow rate guide](https://www.liveaquaria.com/) — LiveAquaria, LiveAquaria. Retail reference confirming the GPH ranges, the head-loss derate to ~70% of nameplate, and the higher flow needed for saltwater and planted tanks.
- [Filter turnover and GPH basics](https://www.thesprucepets.com/aquarium-supplies-5082682) — The Spruce Pets, The Spruce Pets. Pet-care reference for the 4–6× turnover ranges across mechanical, biological, and chemical filter types, and the contact-time reasoning behind the lower turnover for biological filters.
- [Review of Flow through Porous Media as Applied to Head Loss in Water Filters](https://doi.org/10.1061/(ASCE)0733-9372(1999)125:11(998)) — Trussell RR, Chang M, Journal of Environmental Engineering, 1999. Peer-reviewed background for the relationship between filter flow and head loss; aquarium-specific turnover ranges remain anchored to the hobby sources above.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_