Pool Filter Sizing Calculator
Filter Media Type Changes the Math Entirely
The same flow rate that a small sand filter handles easily would overwhelm an equally small cartridge filter, because filtration media differ enormously in how much water they can process per square foot of surface area. Sand filters run at roughly 15 GPM per square foot; cartridge filters, which trap finer particles, are rated closer to 0.375 GPM per square foot — a 40-fold difference that makes filter area, not filter "size" as a single number, the figure that actually matters.
The Formula
Filter Area (sq ft) = Required Flow ÷ Filtration Rate
Filtration rate is the maximum GPM each square foot of that media type can process without pushing debris through instead of trapping it — sand at ~15 GPM/sq ft, D.E. at ~2 GPM/sq ft, and cartridge at ~0.375 GPM/sq ft.
Filter Area Needed for the Same Flow Rate
| Filter type | Rated flow | Filter area needed |
|---|---|---|
| Sand Filter | 15.0 GPM/sq ft | 2.8 sq ft |
| D.E. (Diatomaceous Earth) Filter | 2.0 GPM/sq ft | 20.8 sq ft |
| Cartridge Filter | 0.375 GPM/sq ft | 111.1 sq ft |
Cartridge filters need far more total surface area for the same flow — manufacturers achieve this by pleating a large amount of material into a compact housing, not by making the tank itself larger.
Where This Calculation Matters
- Replacing a filter — matching the new unit's rated area (in square feet) to the pool's actual flow requirement, rather than assuming "bigger box equals better," avoids buying an undersized replacement.
- Switching filter media types — moving from a cartridge system to sand (or vice versa) requires a completely different tank size for the same performance.
- Diagnosing poor water clarity — an undersized filter relative to pump flow is a common, overlooked cause of persistently cloudy water even when chemistry is balanced.
How to Use This Calculator
- Enter your pool volume in gallons.
- Enter your desired turnover time in hours.
- Select your filter type — sand, cartridge, or D.E.
- Select Calculate to see the minimum filter area (or equivalent rated flow) needed.
Related Calculations
This calculation starts from the same flow rate as the Pool Pump Size Calculator — run both together to make sure pump and filter are matched, and check the Pool Volume Calculator for an accurate starting gallon figure.