Four pool filter cartridges of different sizes on a daylight equipment pad

Choosing the Right Size Pool Filter

An undersized pool filter clogs constantly, drops flow, and shortens its own life by being run at maximum load continuously. An oversized one wastes money and runs at higher backpressure than it needs to. The good news: filter sizing is mostly arithmetic. Get two numbers right and you’ll buy a filter you keep for 15–20 years.

The two numbers that matter

  1. Pool volume in gallons. Your filter needs to turn over the full pool volume at least once every 8 hours.
  2. Pump GPM (gallons per minute). The filter’s rated maximum flow must be equal to or greater than your pump’s peak output. Mismatch this and you’ll exceed the filter’s design pressure on every cycle.

Sizing rule of thumb by filter type

Each filter type has a different square-footage-per-gallon target:

  • Cartridge filters: aim for 1 square foot of filter area per 1,000 gallons of pool. A 20,000 gal pool wants a 200–250 sq ft cartridge filter (CS200, CS250, or equivalent).
  • Sand filters: rate by filter diameter, but the practical target is 1 GPM per square foot of sand area. A 24-inch sand filter handles ~50 GPM — the right size for most 1.5 HP pumps.
  • DE filters: aim for 0.75–1 sq ft of grid area per 1,000 gallons. DE filters filter finer than cartridge or sand, but need more frequent maintenance.

When to size UP

Going bigger than the minimum almost always pays off:

  • Pools with heavy tree drop.
  • Saltwater pools (cell scale benefits from cleaner water).
  • Pools running variable-speed pumps at low RPM — larger filter means lower head loss at low flow.
  • Households with many swimmers or pets.

The cost difference between a 200 sq ft and a 250 sq ft cartridge filter is usually under 15%. The cost difference in time saved cleaning over a decade is several times that.

Most common residential cartridge filter sizes

Jandy CS100 SF replacement cartridge for 100 sq ft filter

Jandy CS100 SF Cartridge (small pools)

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Jandy CS150 SF replacement cartridge for 150 sq ft filter

Jandy CS150 SF Cartridge (mid-size)

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Jandy CS250 SF replacement cartridge for 250 sq ft filter

Jandy CS250 SF Cartridge (large pools)

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Quick rule: if you’re between two sizes, take the larger one. The bigger filter has more capacity, runs at lower pressure, and needs cleaning half as often. It will outlive its undersized counterpart by years.

What about filter type (cartridge vs sand vs DE)?

That’s a separate decision from sizing. See our guide on sand vs cartridge vs DE filters for the trade-offs. Whatever type you pick, the sizing math above still applies.

Send PST Pool Supplies your pool gallons, pump nameplate horsepower, and filter type, and we’ll match the right size on the first try.

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