Direct answer: Correct filter sizing is based on your required flow rate (from pool volume ÷ turnover time), matched against a specific filter model’s rated flow capacity — not the pool’s surface size or a generic “small/medium/large” label.
The Sizing Logic
- Calculate pool volume with the Pool Volume Calculator
- Determine required flow rate with the Pump Flow Calculator, using the correct turnover standard: 4 hours for residential pools, 3 hours (with 2/2.5/3-hour presets) for commercial pools
- Match that required flow rate against a filter’s rated capacity using the Filter Calculator
Sand vs Cartridge — Which Type First?
Filter type (sand, cartridge, or DE) is a separate decision from sizing — see our Sand vs Cartridge comparison for the trade-offs, then size whichever type you choose to your actual required flow rate.
Common Mistake
Selecting a filter based on the pool’s general size category rather than its calculated flow requirement is one of the most common sizing errors — two pools of similar surface area can have very different required flow rates depending on depth and turnover standard.
When to Call a Professional
If your required flow rate falls between two standard filter model sizes, or your setup has unusual circulation requirements, a professional review avoids under- or over-sizing.