Pool filters

Pool Filter Size Calculator

Enter your pool's volume and your target turnover time. The calculator gives you the required flow in GPM and GPH. That number is the starting point for matching a filter and pump, not the final answer by itself.

Calculate your required flow

Enter your pool volume, choose a turnover time, and see the flow your system needs to deliver.

Pool volume (gallons) Target turnover (hours)

These are calculation options, not universal recommendations. Choose a turnover target appropriate for your pool, equipment and operating conditions.

Pool volume helper (rectangular pool)

Length (ft) x Width (ft) x Average Depth (ft) x 7.48 = approximate gallons

Length (ft) Width (ft) Avg Depth (ft)

This is an estimate for a rectangular pool. Round, oval, and irregular shapes use different formulas. If your pool manufacturer provides a volume, use that instead.

Required flow 31.3 GPM

Equivalent 1,875 GPH

This is a calculated required flow based on your inputs, not a measurement of actual pump or system flow. Actual flow depends on plumbing, fittings, equipment, and system resistance.

Before you buy: four checks

Filter flow rating

The filter's manufacturer-rated maximum flow (GPM) must meet or exceed your calculated required flow.

Pump delivered flow

The pump must deliver the required GPM through your actual plumbing, not just on the box label. Check the pump's performance curve at your estimated system resistance.

Plumbing and equipment

Pipe, fittings, valves, heaters and cleaners add hydraulic resistance, which can reduce the flow the pump delivers at a given operating speed.

Manufacturer limits

Both filter and pump must operate within the manufacturer's published limits for the exact model. Do not exceed the filter's rated maximum flow.

Pool filter size calculator infographic: 6-step workflow from pool volume through turnover time, calculated required flow (formula: pool volume / turnover minutes = required GPM), checking filter rating, matching pump flow, and confirming manufacturer limits, with an example calculation and filter type reference.

How the calculation works

Required GPM = pool volume (gallons) / turnover time (minutes). For a 20,000 gallon pool at 8-hour turnover: 20,000 / 480 = 41.7 GPM, equivalent to 2,502 GPH. That is the flow your system needs to sustain to circulate the full volume within the chosen time.

What the result tells you

The number is the calculated flow required to complete one full turnover within the time you selected. Use it as the target when checking the manufacturer's specifications for the specific filter and pump models you are considering.

What the result does not tell you

It does not tell you what flow your pump actually delivers through your plumbing, because that depends on pipe diameter, pipe length, fittings, valves, elevation, and any equipment (heater, cleaner, salt cell) in the loop. A pump rated at 60 GPM on the box may deliver considerably less in a real installation. Check the pump manufacturer's performance curve at your estimated system head to see the actual operating flow.

GPM and GPH: do not mix them

1 GPM = 60 GPH. Some manufacturers rate filters and pumps in GPM, others in GPH. The calculator shows both so you can match either convention. If a filter is rated at 2,500 GPH, that is 2,500 / 60 = 41.7 GPM. Always convert before comparing specifications from different brands.

Reference examples (at 8-hour turnover)

Pool volume	Required GPM	Required GPH
10,000 gal	20.8	1,250
15,000 gal	31.3	1,875
20,000 gal	41.7	2,500
30,000 gal	62.5	3,750

These are calculated from the formula at an 8-hour turnover. A faster turnover raises the required flow; a slower turnover lowers it. The choice of turnover time depends on pool type, usage, and local conditions, not a universal rule.

Common calculation mistakes

Confusing GPH with GPM. A pump rated at 2,500 GPH is 41.7 GPM, not 2,500 GPM. Mixing the units leads to wildly oversized or undersized equipment.
Using calculated flow as actual pump flow. The calculator tells you what the system needs. The pump's actual delivered flow through your plumbing is a different, usually lower, number.
Sizing from pump horsepower. Two 1.5 HP pumps from different manufacturers can deliver very different GPM. Size from flow, not horsepower.
Ignoring manufacturer flow limits. Every filter has a rated maximum. Exceeding it pushes particles through the media and raises pressure.
Assuming one turnover chart fits all pools. A pool with heavy debris or heavy bather loads may benefit from faster turnover. A lightly used pool may be fine with slower. The chart is a reference, not a prescription.

Products on this site as sizing examples

For a worked example of how manufacturer-rated flow translates to a pool range: the Intex SX2800 (26647EG) is rated at 2,800 GPH pump flow and 2,150 GPH in-pool system flow for 5,500 to 17,200 gallons. The Intex C2500 (26633EG) is rated at 2,500 GPH for 4,000 to 15,200 gallons. Those are combo units where the pump and filter are pre-matched, so the sizing calculation confirms the manufacturer's range rather than replacing it.

For the full sizing methodology (why the calculator is only Step 3 of a five-step process), see How to Choose the Right Pool Filter Size. For help deciding between cartridge and sand before sizing, see Cartridge vs Sand Pool Filter.

Sources

The turnover formula (Required GPM = Volume / Turnover minutes) is standard pool industry practice documented in Pentair/Sta-Rite pool training material. Model-specific turnover capacities for cartridge systems are published on the Pentair Clean and Clear filter specifications page, and for sand systems on the Pentair Sand Dollar filter specifications page. Product specifications cited are from the manufacturer listings we have verified: Intex C2500 product page, Intex SX2800 product page.