Skip to main content
Pool Filter Guide

Free Tool

Pool Filter Size Calculator

Enter your pool size and filter type to get the right filter dimensions, how much media it takes, what that media costs, and the pump flow rate you need. Sizing figures follow manufacturer specifications for Hayward, Pentair, and Intex tanks.

1

Your pool volume

2

Filter type

3

Your result

How Pool Filter Sizing Actually Works

Filter size is driven by one number: the flow rate needed to push your entire pool volume through the filter in about 8 hours. That is the turnover rate. Divide your pool gallons by 480 minutes and you have the minimum GPM the system must move. A 20,000 gallon pool needs roughly 42 GPM, and every filter is rated for a maximum flow it can handle without letting debris pass through.

Pool size does not set the media quantity, though. Sand quantity follows the tank diameter, which is why a 24 inch tank takes 300 lbs of sand whether it sits beside a 15,000 or a 30,000 gallon pool. Our guide on how much sand a pool filter needs has the full chart by diameter and by model.

Sand Filter Sizing by Pool Volume

Pool VolumeTank DiameterSand Needed50 lb Bags
Up to 10,000 gal16 inch100 lbs2 bags
10,000 to 15,000 gal19 inch175 lbs4 bags
15,000 to 20,000 gal21 inch200 lbs4 bags
20,000 to 25,000 gal24 inch300 lbs6 bags
25,000 to 35,000 gal27 inch350 lbs7 bags
35,000+ gal30 inch600 lbs12 bags

Sand figures follow manufacturer specifications. Hayward rates the 16 inch S166T at 100 lbs and the 24 inch S244T at 300 lbs, and Pentair rates the 30 inch Triton II TR-100 at 600 lbs. Always confirm against the label on your own tank, because it overrides any chart.

Cartridge and DE Filter Sizing

Cartridge filters are sized in square feet of filter area, and the working rule is at least 100 sq ft per 10,000 gallons. Going larger is the single cheapest upgrade you can make, because doubling the area roughly doubles the time between cleanings. DE filters are also rated in square feet, and they need about 1 lb of DE powder per 10 sq ft for a full charge, or roughly 80 percent of that after each backwash.

Why Oversizing Beats Undersizing

An undersized filter is the more expensive mistake. Pressure climbs within days rather than weeks, so you backwash constantly and send 200 to 500 gallons down the waste line each time. The water often stays faintly cloudy because the media never runs at a sensible flow rate, and the media itself wears out years early. A filter one size up costs a little more once and saves effort every week after that. The only real ceiling is your pump: the filter must be rated for at least the pump's flow rate. If you are choosing both together, our pool pump and filter combo guide covers matched systems.

Common Sizing Questions

Frequently Asked Questions

What size pool filter do I need?
Filter size follows pool volume and the flow rate needed to turn that volume over in 8 hours. A 20,000 gallon pool needs about 42 GPM, which suits a 21 inch sand filter, a 200 sq ft cartridge filter, or a 48 sq ft DE filter. Oversizing a filter is safe and extends the time between cleanings. Undersizing forces constant backwashing and never fully clears the water.
Is it better to oversize a pool filter?
Yes, within reason. A filter one size larger than the minimum holds more debris, runs at lower pressure, and stretches the interval between cleanings. The limit is the pump: the filter must be rated for at least the pump flow rate, and a pump that overpowers a small filter channels the media and pushes debris through. Matching the filter up rather than down is the safer error.
How do I calculate my pool volume in gallons?
For a rectangular pool multiply length by width by average depth by 7.5. For a round pool multiply diameter by diameter by average depth by 5.9. For an oval pool multiply length by width by average depth by 6.7. Average depth is the shallow end depth plus the deep end depth divided by two. The calculator above does this for you.
How much sand does my filter need?
Sand quantity follows tank diameter, not pool size. A 16 inch tank takes about 100 lbs, a 24 inch tank takes about 300 lbs, and a 30 inch tank takes about 600 lbs. Sand ships in 50 lb bags, so divide by 50 and round up. The exact figure is printed on a label on the side of your filter tank and it overrides every chart.
What happens if my pool filter is too small?
An undersized filter clogs quickly, so pressure climbs within days instead of weeks and you backwash constantly, wasting 200 to 500 gallons each time. Water often stays slightly cloudy because the media never gets a chance to work at a sensible flow rate, and the filter media wears out years early.

Related Guides