How the calculation works
Filter area is set by how fast water may pass through the medium without pushing dirt straight past it. Each type has its own design rate: sand about 15 GPM per sq ft, DE about 2 GPM per sq ft, and cartridge about 0.375 GPM per sq ft.
So area needed = flow rate ÷ design rate. The very different numbers explain why a cartridge filter is physically enormous compared with a sand tank handling the same pool — cartridge media is worked far more gently, which is why it catches finer particles.
Worked example
A 20,000 gallon pool on an 8-hour turnover moves about 42 GPM. As a cartridge filter that needs 42 ÷ 0.375 = about 111 sq ft. As a sand filter, 42 ÷ 15 = under 3 sq ft, which is a tank of roughly 22 inches diameter.
Assumptions
- Design rates are the commonly published residential figures, not the absolute maximum a filter will pass.
- Filter media is clean. A dirty filter passes less water at higher pressure.
Limitations
- Manufacturers rate filters differently, and some quote a maximum flow well above the design rate. Compare the square footage, not the marketing.
- An undersized filter is the single most common cause of water that never quite clears, and it cannot be fixed by running the pump longer.
Questions people ask
Can a filter be too big?
Not really — oversizing is the one place in pool equipment where bigger genuinely is better. A larger filter runs at lower pressure, catches more, and goes longer between cleanings. Aim to exceed the minimum.
Sand, cartridge or DE?
Sand is cheapest to run and simplest, but filters the coarsest. Cartridge catches finer particles and wastes no water backwashing, at the cost of hosing the elements periodically. DE filters finest of all and needs powder added after every backwash.
How often should I backwash?
When the pressure gauge reads 8 to 10 psi above its clean starting pressure — not on a schedule. Backwashing too often stops sand filtering properly, since a slightly dirty sand bed traps more than a spotless one.
My water stays cloudy. Is the filter too small?
Often, yes — particularly with a sand filter on a busy pool. Check chemistry first, but if the readings are in range and the water still will not clear, an undersized or worn-out filter is the usual culprit.