Pool Filter Size Calculator

Getting Your Filter Size Right (It Matters More Than You Think)

What Does Filter Sizing Actually Mean?

Here's the deal: filters work by forcing water through media. The smaller the media area, the more pressure you need to push the same amount of water through. We measure this as GPM/sqft—basically, how many gallons per minute can push through each square foot of filter area without choking it.

The math is simple. Take your pump flow (say 50 GPM) and divide it by the filter's GPM/sqft rating. For sand at 15 GPM/sqft, you're looking at 50 ÷ 15 = 3.3 sqft minimum. That's it.

Sand Filters: The Workhorse (15–20 GPM/sqft)

Most residential pools run sand filters. You're dumping silica sand in a tank, water comes in dirty, leaves clean. They're relatively affordable, durable, and easy to backwash. As a planning rule, sand filters usually tolerate higher loading rates than cartridge or DE systems, which is why their required filter area tends to be smaller for the same pump flow.

Cartridge Filters: The Premium Option (2–3 GPM/sqft)

Cartridge filters use pleated synthetic cartridges. You get finer filtration, less wasted water from backwashing, and quieter operation. The tradeoff is that they need a lot more media area than sand filters for the same flow, so the square-foot ratings can look much larger on the label.

DE Filters: Overkill for Most People (3–5 GPM/sqft)

DE (diatomaceous earth) gives very fine filtration and usually falls between sand and cartridge in required area for a given flow rate. It can keep water exceptionally clear, but it also comes with more maintenance steps and stricter handling requirements than a basic sand setup.

Undersizing Kills Filters; Oversizing Saves You Money

Here's what happens when your filter's too small: pressure rises faster, cleaning intervals shrink, and the whole system works harder. A modestly oversized filter usually gives you more margin and longer service intervals. The tradeoff is simple: more upfront cost for a little more breathing room.

Pressure Tells You When Your Filter's Working Too Hard

Every filter has a pressure gauge, but the actual "clean" and "dirty" numbers depend on the system. Record your clean starting pressure, then service the filter when pressure rises meaningfully above that baseline according to the manufacturer guidance. A single universal PSI rule is too blunt for every setup.

Replacing Your Filter Media (It's Not a Forever Thing)

Sand lasts 5–7 years, then it gets too compacted and needs replacing. Cost is maybe $30–40 for a 50-pounder. Cartridges last 3–5 years depending on how much you swim, and a replacement runs $120–250. DE powder you're buying every backwash, so that adds up. Budget accordingly.

Filter Sizing Reference Table

Costs are for purchase. Maintenance assumes 8-hour daily operation during summer season. DE filters best for commercial; sand/cartridge for residential.