Pool Filter Size Calculator

Filter sizing starts with flow, not the filter. The required flow is the gallons the system must move per minute to turn the pool over in your chosen time: required GPM = volume ÷ (turnover hours × 60). (In By pump GPM mode you skip this and use your pump's max flow directly.) The minimum filter area for each media is then area = required GPM ÷ the media's design rate.

required GPM = volumeGal / (turnoverHours × 60)     (or: your pump's GPM)
min area     = required GPM / design rate (GPM per sq ft)

design rates (GPM per sq ft):  sand 20 · DE 2 · cartridge 1
recommended  = min area × 1.25 (optional headroom)

dimensions mode: volumeGal = pool-volume engine (shape × avg depth × 7.48 gal/ft³)
                 avgDepth = (shallow + deep) / 2 (deep blank ⇒ shallow)

We publish the design rate we used rather than hiding it — sand 20, DE 2, cartridge 1 GPM/sq ft — so you can translate GPM into the square feet you actually shop for. These rates are a band, not a single magic number: cartridge especially is sold much larger than the 1 GPM/sq ft peak figure because sustained daily loading is closer to 0.3–0.35 GPM/sq ft. The pump-vs-filter check compares your pump's GPM against a filter's max throughput (sand 22, DE 1.75, cartridge 0.35 GPM/sq ft) — a pump must never exceed the filter's rated flow.

In From dimensions mode the gallons come from the same engine the Pool Volume Calculator uses (7.48 gal/ft³, primary-source validated shape constants), not a rule-of-thumb multiplier. Suggested stock sizes are illustrative shopping hints, not a product catalog. This is an advisory planning figure — verify against the filter's rated capacity and your pump's flow curve before buying.