Pool Filter Size Calculator
Find out what size pool filter you need — cartridge, sand, or DE — based on your pool volume, flow rate, and turnover.
Safety
Always size your filter to handle your pump's maximum flow rate. An undersized filter causes high pressure, poor filtration, and premature equipment failure. Sand filters need backwashing; cartridge filters need cleaning; DE filters need recharging after backwash.
Work out the flow your pool needs — gallons ÷ (turnover hours × 60) — then divide by the media's design rate: 15 GPM per sq ft for sand, 2 for DE, 1 for cartridge. A 20,000-gallon pool on an 8-hour turnover needs 41.7 GPM, which is about 2.8 sq ft of sand, 21 sq ft of DE or 42 sq ft of cartridge. A filter cannot be too big, so buy above the minimum.
Pool Filter Size Chart
Filter size follows flow, and flow follows turnover. Work out the GPM your pool needs to turn over once in eight hours — gallons ÷ (hours × 60) — then divide that GPM by the design flow rate of the media: 15 GPM per sq ft for sand, 2 for DE, 1 for cartridge (NSF/ANSI 50 and manufacturer design guides). The three media land on wildly different square-footage numbers for the same pool, which is why comparing filters on sq ft alone is meaningless.
| Pool volume | Flow needed | Sand | DE | Cartridge |
|---|---|---|---|---|
| 10,000 gal | 20.8 GPM | 1.4 sq ft | 10.4 sq ft | 20.8 sq ft |
| 15,000 gal | 31.3 GPM | 2.1 sq ft | 15.6 sq ft | 31.3 sq ft |
| 20,000 gal | 41.7 GPM | 2.8 sq ft | 20.8 sq ft | 41.7 sq ft |
| 25,000 gal | 52.1 GPM | 3.5 sq ft | 26.0 sq ft | 52.1 sq ft |
| 30,000 gal | 62.5 GPM | 4.2 sq ft | 31.3 sq ft | 62.5 sq ft |
| 40,000 gal | 83.3 GPM | 5.6 sq ft | 41.7 sq ft | 83.3 sq ft |
Minimum filter area for an 8-hour turnover. Change the turnover time in the calculator above for 6- or 10-hour numbers.
These are minimums — buy above them
The chart is the smallest filter that can pass the flow without exceeding the media's design rate. It is not the filter to buy. Unlike a pump, a filter cannot be too big — going up in size buys longer intervals between cleanings, more dirt held before pressure climbs, and lower resistance for the pump. Cartridge is where the gap is widest: a 20,000-gallon pool needs about 42 sq ft on paper, and owners who are happy with their filter are usually running several times that. Sizing to the minimum is how people end up cleaning cartridges monthly.
The filter is part of your pump's workload
Every filter adds resistance, and that resistance is counted in total dynamic head — the total pressure the pump has to overcome. As the media loads up, head rises and actual flow falls, so a pool sized for an 8-hour turnover quietly slips to nine or ten hours without anything looking wrong. An undersized filter starts closer to that ceiling and reaches it sooner. Size the filter first, then size the pool pump against the system you actually built, and start from a real volume figure using the pool volume calculator rather than the number on the build contract.