Pool filter size calculator
A filter must be big enough for your pump’s flow. Enter the flow rate and compare the minimum area for sand, cartridge and DE.
Don’t know your flow? For 15,000 gallons turned over in 8 hours you need about 31 gpm. Size the pump.
Minimum filter area
| Type | Design rate | Filters to |
|---|---|---|
| Sand (high-rate) Backwash when pressure rises 8–10 psi. Replace sand every 5–7 years. | 15 gpm/ft² | 20–40 µm |
| Cartridge Conservative commercial rate shown; many residential cartridges are rated up to ~1.0 gpm/ft². Oversize for longer cleaning intervals. | 0.375 gpm/ft² | 10–20 µm |
| DE (diatomaceous earth) Recharge DE after every backwash. Wear a dust mask when handling powder. | 2 gpm/ft² | 3–5 µm |
Reading a filter label
Filters are sold by area (square feet) and a maximum flow. Sand filters are often listed by tank diameter instead: a 24 inch tank has about 3.1 ft² of bed and handles roughly 47 gpm at 15 gpm/ft², a 30 inch tank about 4.9 ft² and 74 gpm. Design rates here follow the figures commonly used under NSF/ANSI 50, the standard filters are certified to; your filter’s own label always wins.
| Sand tank diameter | Bed area | Flow at 15 gpm/ft² |
|---|---|---|
| 19 inch | 2 ft² | 30 gpm |
| 22 inch | 2.6 ft² | 40 gpm |
| 24 inch | 3.1 ft² | 47 gpm |
| 26 inch | 3.7 ft² | 55 gpm |
| 30 inch | 4.9 ft² | 74 gpm |
| 36 inch | 7.1 ft² | 106 gpm |
Questions pool owners ask
How do I size a pool filter?
Divide your pump’s flow rate (gpm) by the filter type’s design rate (gpm per square foot) to get the minimum filter area. For 50 gpm: a sand filter at 15 gpm/ft² needs 3.3 ft², a DE filter at 2 gpm/ft² needs 25 ft², and a cartridge at 0.375 gpm/ft² needs about 133 ft². Bigger is better; oversizing lengthens time between cleanings.
Which pool filter is best: sand, cartridge or DE?
Sand is cheapest and simplest but filters the coarsest (about 20–40 microns). Cartridge filters catch finer dirt (10–20 microns), use no backwash water, and just need hosing off. DE filters are finest (down to 3–5 microns) but need DE powder recharged after every backwash, and spent DE must be disposed of properly.
When should I clean my pool filter?
When the pressure gauge reads 8–10 psi above its clean starting pressure, or when flow noticeably drops. Note the pressure right after a cleaning so you know your baseline.
What happens if my filter is too small?
Water passes through the media too fast, pushing dirt through, raising pressure quickly and meaning more frequent cleanings. Manufacturers list a maximum flow; stay under it.