What is a pool filter?
Updated August 2026
A pool filter is the tank downstream of the pump where sand, a pleated cartridge, or a coating of diatomaceous earth strains particles out of the water before it returns to the pool. It handles only the physical half of pool care. Chlorine kills what is living in the water; the filter catches everything else, from pollen and sunscreen oils to the fine dust that turns a pool hazy overnight.
Where it is
It is the tallest vessel on the equipment pad, a fiberglass or molded plastic tank between the pump and the heater, with a pressure gauge on top. Sand and DE tanks carry a multiport valve marked Filter, Backwash, Rinse, Waste, and Closed. Cartridge tanks have no multiport; they split at a stainless clamp band around the middle so the element lifts straight out. The label lists filter area in square feet, which is the number to match if you ever replace the tank.
What it does
Filtration is rated in microns, and the three media are not close. Sand catches down to roughly 20 to 40 microns, a cartridge to 10 to 15, and DE to 3 to 5, which is why DE pools look glassy. As dirt loads the media, resistance climbs, so the pressure gauge doubles as a service light: 8 to 10 PSI above the clean reading means it is time. Push past that and flow collapses, the heater trips on low flow, the pump seal runs hot, and the water clouds even with perfect chemistry. Read what skipping filter cleaning costs before letting a season slide.
Maintenance
Mark the clean-tank pressure on the housing, then service at 8 to 10 PSI over it, which usually lands every 4 to 8 weeks in swim season. Backwash sand and DE for 2 to 3 minutes, recharge DE grids after every backwash, and hose a cartridge fan by fan. Once a season, soak the cartridge overnight in filter cleaner to strip oils a hose cannot touch. Cartridges last 2 to 5 years at $30 to $100 each, sand needs swapping every 5 to 7 years (200 to 300 pounds for a mid-size tank), and DE grids run 5 to 10 years. Timing lives in how often to clean a pool filter and when to replace the cartridge.
What it costs to replace
A replacement pool filter runs $300–900 for a new filter tank installed, or $30–100 for a replacement cartridge.
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Frequently asked questions
Sand, cartridge, or DE: which pool filter is best?
Cartridge is the sensible default for most residential pools: no backwashing, no wasted water, and a hose-off takes about 20 minutes. Sand is cheapest to buy and the most forgiving, but it filters coarsest and dumps 200 to 500 gallons per backwash. DE delivers the clearest water and the most fuss, since every backwash means recharging the grids with fresh powder. Choose cartridge unless you deal with heavy debris or already own multiport plumbing.
What pressure should my pool filter gauge read?
There is no universal number. A clean filter settles somewhere between 10 and 20 PSI depending on pump size, plumbing, and filter area, and that first reading after a cleaning becomes your personal baseline. Service the filter once the needle sits 8 to 10 PSI above it. A gauge reading unusually low instead points the other direction: a clogged skimmer, a closed valve, or a pump that is barely moving water.
Why is my pool still cloudy after cleaning the filter?
Haze that survives a filter cleaning is almost always chemistry or particle size. Test free chlorine first, then pH and total alkalinity, since high pH clouds water by precipitating calcium. If the numbers look right, the particles are finer than your media can trap, a common story with sand. A clarifier clumps them large enough to catch, and a flocculant drops them to the floor to vacuum to waste. Run the pump continuously for 24 to 48 hours while it clears.