How to size a pool filter
filter sq ft = flow in gpm ÷ rate per sq ft (sand 20 max, 15 comfortable · DE 2 and 1.5 · cartridge 1 and 0.375)
Work out the flow first: gallons ÷ (turnover hours × 60). A 20,000-gallon pool turned over in 8 hours needs 41.7 gpm. At the rated maximum that is 2.1 sq ft of sand, 20.8 sq ft of DE or 41.7 sq ft of cartridge; at the comfortable rates it is 2.8 sq ft of sand, 27.8 of DE or 111 of cartridge. Home cartridges are mostly rated near 1 gpm per sq ft (a 50 sq ft cartridge for 50 gpm), while 0.375 is the commercial rate, so going up a size or two is cheap insurance.
| Pool | Flow | Sand sq ft | DE sq ft | Cartridge sq ft |
|---|---|---|---|---|
| 10,000 gal | 20.8 gpm | 1 / 1.4 | 10 / 14 | 21 / 56 |
| 15,000 gal | 31.3 gpm | 1.6 / 2.1 | 16 / 21 | 31 / 83 |
| 20,000 gal | 41.7 gpm | 2.1 / 2.8 | 21 / 28 | 42 / 111 |
| 25,000 gal | 52.1 gpm | 2.6 / 3.5 | 26 / 35 | 52 / 139 |
| 30,000 gal | 62.5 gpm | 3.1 / 4.2 | 31 / 42 | 63 / 167 |
| 40,000 gal | 83.3 gpm | 4.2 / 5.6 | 42 / 56 | 83 / 222 |
Sand filters: the tank diameter is the size
A sand filter's area is the circle of its tank: 0.785 × diameter² in square feet, with the diameter in feet. Read the tank diameter off the label and check its flow below.
| Tank | Area | Max flow (20 gpm/sq ft) | Comfortable (15 gpm/sq ft) |
|---|---|---|---|
| 19 in | 2 sq ft | 39 gpm | 30 gpm |
| 22 in | 2.6 sq ft | 53 gpm | 40 gpm |
| 24 in | 3.1 sq ft | 63 gpm | 47 gpm |
| 26 in | 3.7 sq ft | 74 gpm | 55 gpm |
| 30 in | 4.9 sq ft | 98 gpm | 74 gpm |
| 36 in | 7.1 sq ft | 141 gpm | 106 gpm |
A filter can be too small but not too big: a larger filter only means slower water through it, finer filtering and longer between cleanings. The pump is what you must not oversize, so start with the pool pump size calculator; for the gallons, use the pool volume calculator, and if a heater sits after the filter, the pool heater calculator sizes it for the same pool.
Rates: sand 20 gpm/sq ft maximum (NSF/ANSI 50, as stated in NSF ballot 50i169); DE 2 (Waterway Crystal Water 60 sq ft DE filter, rated 120 gpm); cartridge about 1 for home models (Pentair Clean & Clear CC50: 50 sq ft, 50 gpm) and 0.375 as the commercial design rate. Our comfortable size is 75 % of the rated maximum for sand and DE (15 and 1.5) and 0.375 for cartridge. Check your filter's label for its own rated flow. Read October 23, 2026.
Frequently asked questions
How do I know what size filter to get for my pool?
Find the flow first: gallons ÷ (turnover hours × 60), or your pump flow at its head. Divide by the filter rate: sand 20 gpm per sq ft at most, DE 2, home cartridges about 1. A 20,000-gallon pool at 8 hours needs 41.7 gpm, so at least 2.1 sq ft of sand or 42 sq ft of cartridge.
What size sand filter for a 30000 gallon pool?
At an 8-hour turnover a 30,000-gallon pool needs 62.5 gpm, which is 3.1 sq ft of sand at the 20 gpm maximum, a 24 in tank. For the comfortable 15 gpm per sq ft you want 4.2 sq ft, a 30 in tank, which also filters finer and needs fewer backwashes.
What size cartridge filter do I need for a 15,000 gallon pool?
An 8-hour turnover is 31 gpm. Home cartridges are rated near 1 gpm per sq ft, so 31 sq ft is the minimum. At the 0.375 commercial rate it is 83 sq ft, and a cartridge that size or larger runs slower, catches more and goes longer between cleanings.
Can a pool filter be too big?
No. A larger filter only means slower water through the media, finer filtering and longer between cleanings. It costs more to buy and to replace cartridges or sand. What must not be too big is the pump: flow above the filter rating pushes dirt through it.
Which filters best: sand, cartridge or DE?
DE filters finest, down to a few microns, cartridge sits in the middle and sand is the coarsest but the simplest to look after. Sizing differs because each handles a different flow per square foot: sand about 20, DE about 2 and a home cartridge about 1.