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Pool Filter Size Calculator

Sand, cartridge or diatomaceous earth: two answers give the filter area and, for sand, the pounds to buy.

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Which kind of filter?

Your filter size appears here.

20 GPM
per sq ft for sand
0.375 GPM
per sq ft for cartridge
2 GPM
per sq ft for diatomaceous earth

This pool filter size calculator starts with two things: how many gallons your pool holds and which kind of filter you have. The calculator then tells you the filter area needed to push all that water through in about 8 hours.

  • Sand filters do their best work at roughly 20 gallons per minute for every square foot of filter area.
  • Cartridge filters run at a gentler pace, around 0.375 gallons per minute per square foot.
  • Diatomaceous earth filters can handle about 2 gallons per minute for each square foot.
  • Plan on backwashing once the gauge rises 8 to 10 psi over its clean baseline.

GPM stands for gallons per minute. It's simply how fast the pump moves water through the filter. Filter area is the surface the water flows across, measured in square feet (sq ft). Going bigger than the calculator says is never a problem. A larger filter just goes longer between cleanings.

Sand filter size: how many bags of sand?

Check the tank label for how much sand goes inside. Hayward's usual tanks take 100 lb of sand at 16 inches, 200 lb at 21 inches, and 300 lb at 24 inches. Since sand ships in 50 lb bags, filling a 200 lb tank calls for 4 of them.

Tank sizeFilter areaSand capacityPools up to (8 hour turnover)
16 in1.4 square feet100 lb13,440 gallons
21 in2.4 square feet200 lb23,040 gallons
24 in3.1 square feet300 lb29,760 gallons

Pool filter sizing chart

Here's how much filter area each type needs so the whole pool turns over in 8 hours. One turnover simply means every gallon gets a pass through the filter.

Pool volumeFlow needed for a full 8 hour turnoverSandCartridgeDiatomaceous earth
5,000 gallons10 GPM0.5 square feet50 square feet6 square feet
10,000 gallons21 GPM1 square foot75 square feet11 square feet
15,000 gallons31 GPM1.6 square feet100 square feet16 square feet
20,000 gallons42 GPM2.1 square feet125 square feet21 square feet
25,000 gallons52 GPM2.6 square feet150 square feet27 square feet
30,000 gallons63 GPMnot listed175 square feet32 square feet
40,000 gallons83 GPM4.2 square feet225 square feet42 square feet

For a pool that lands between two rows, size up to the bigger one.

How often should you backwash a pool filter?

It's time to backwash when the pressure gauge shows a rise of 8 to 10 psi over the clean reading. Write down the gauge number after a fresh cleaning, and you'll always know where "clean" sits for your filter. Cartridge filters don't get backwashed. Just rinse them off with a hose instead.