Enter your gallons — get the cartridge size to buy, your cleaning trigger, and the replace-vs-clean call.
The sizing rule
cartridge sqft ≈ gallons ÷ 100 (round up)
clean at: baseline PSI + 8–10
Unlike pumps — where oversizing wastes electricity — a cartridge filter has no penalty for going big. More fabric means slower pressure rise, fewer cleanings, and finer filtering as a light dirt layer builds. The rule above is the floor; the next size up is the comfortable answer.
A worked example
A 20,000-gallon pool: 200 sqft minimum — a C-200 class element, or better, a 300 sqft unit that stretches cleanings from monthly to twice a season. If the gauge baseline is 12 PSI after a cleaning, the hose comes out again at 20–22 PSI, whenever that happens to be.
Clean it like it matters
Top-down with a garden hose nozzle, spreading each pleat — never a pressure washer. Once or twice a season, soak overnight in cartridge cleaner (body oils and sunscreen laugh at plain water), rinse, and reinstall. Keeping a second cartridge to swap means zero filter downtime. Your baseline PSI belongs on the cheat sheet — and if you're comparing filter types from scratch, the sand and DE calculators complete the picture.
Frequently asked questions
What size cartridge filter does my pool need?
The beginner-safe rule: about 1 square foot of cartridge area per 100 gallons of pool — a 20,000-gallon pool wants a 200 sqft cartridge, and rounding UP is free insurance. Oversizing a cartridge filter has no downside: bigger element, longer between cleanings, better water.
When should I clean my cartridge filter?
When the pressure gauge reads 8–10 PSI above your clean baseline — not on a calendar. A typical season needs 2–4 cleanings: hose off top-down (no pressure washer, it drives dirt deeper and tears pleats), and once or twice a year add an overnight soak in cartridge cleaner to cut the oils a hose can't.
How long do filter cartridges last?
2–3 seasons typically. Replace when the pleats look frayed or crushed, the bands have snapped, or the filter needs cleaning constantly even after a chemical soak — worn fabric passes fine debris no amount of cleaning fixes.
Cartridge vs sand vs DE filter?
Cartridge filters to ~10–15 microns (between sand's ~20–40 and DE's ~3–5), wastes no water to backwashing — you hose it off instead — and asks slightly more hands-on time. It's the default on newer pools and the water-restriction favorite. Our sand and DE calculators cover the other two.