Filtration rate is the key number
Every filter can only push a certain flow through each square foot of its media before it stops filtering well. That limit is the filtration rate, in gallons per minute per square foot. Divide your pump’s flow by it and you have the smallest filter that can cope.
| Filter type | Home pool rate | Public pool limit (CDC code) |
|---|---|---|
| Cartridge | 0.357 gpm/ft² | 0.3 gpm/ft² |
| Sand (high-rate) | 19.7 gpm/ft² | 15 gpm/ft² |
| DE (precoat) | 2 gpm/ft² | 2 gpm/ft² |
The home pool rates come from typical manufacturer ratings: Hayward rates its 24-inch ProSeries sand filter, with 3.14 ft² of filter area, at 62 gpm (19.7 gpm/ft²), and Pentair rates its 420 ft² Clean & Clear Plus cartridge at 150 gpm (0.357 gpm/ft²). The public limits come from the CDC’s Model Aquatic Health Code, which also drops sand to 12 gpm/ft² when the bed is less than 24 inches deep.
The formula
Minimum filter area (ft²) = flow (gpm) ÷ filtration rate (gpm per ft²)
Sand tank diameter (in) = √(4 × area ÷ π) × 12
Worked example
A pump delivering 50 gpm
- Cartridge: 50 ÷ 0.357 = 140 ft² minimum. A 150 ft² cartridge just covers it; 200 ft² or more gives headroom and longer cleaning intervals.
- Sand: 50 ÷ 19.7 = 2.54 ft², which a 21.6-inch tank provides. In practice that means a 22- or 24-inch filter.
- DE: 50 ÷ 2 = 25 ft² of grid area.
Minimum cartridge area by pump flow
Show the numbers
| Flow (gpm) | Home (ft²) | Public (ft²) |
|---|---|---|
| 30 | 84 | 100 |
| 40 | 112 | 133 |
| 50 | 140 | 167 |
| 60 | 168 | 200 |
| 70 | 196 | 233 |
| 80 | 224 | 267 |
Choosing between types
- Cartridge filters need no backwashing, so they waste no water and save chemicals. They filter finer than sand. Oversizing pays off: a larger cartridge runs at lower pressure and goes longer between cleanings.
- Sand filters are simple and cheap to run but filter the coarsest particles, and backwashing sends hundreds of gallons of treated water to waste each time.
- DE filters catch the finest particles. They need fresh DE powder after each backwash and careful handling of the powder.
Not sure of your flow? Work it out from your pool volume with the pump size calculator. If you have a variable-speed pump, size the filter for the highest speed you’ll run, such as when the cleaner or a water feature is on.
Frequently asked questions
What size cartridge filter do I need?
Divide your pump’s flow in gpm by about 0.36. For a 50 gpm pump that’s 140 ft², and many owners go larger for longer cleaning intervals.
What size sand filter for a 20,000 gallon pool?
With an 8-hour turnover you need about 42 gpm, which needs about 2.12 ft² of sand area: a 22- or 24-inch tank.
Can a pool filter be too big?
Not really. A larger filter runs at lower pressure and needs cleaning less often. The only downsides are cost and space.
What happens if my filter is too small?
Water goes through faster than the filter can handle, so it filters poorly, the pressure climbs quickly and you clean it more often. It can also void the warranty.
How often should I clean a cartridge filter?
When the pressure gauge reads about 8 to 10 psi above its clean reading, or as the manufacturer recommends.
Which filter is best for a salt pool?
Any type works. Cartridge filters are popular because they don’t send salt water to waste with every backwash.
Sources
- ProSeries 24 in. sand filter (S244T2) specifications. Hayward.
- Pentair Clean & Clear Plus 420 cartridge filter. Leslie’s product listing.
- Model Aquatic Health Code, 5th edition (2024). CDC.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.