Enter your pool’s volume and how many hours you want to circulate all the water once (turnover). The calculator finds the flow rate and the minimum cartridge area.
Pool filter size calculator
Enter your pool size and how fast you want to turn the water over. We’ll estimate flow rate and filter area.
Minimum cartridge filter area
0 sq ft
Flow rate needed0 gpm
Gallons per hour0
Suggested size -
Flow = volume ÷ (turnover hours × 60). Area = flow ÷ design flow rate. Size up rather than down: a larger cartridge filter runs at lower pressure and needs cleaning less often. Your filter must also handle your pump’s actual flow: check the pump curve and the filter’s rated maximum flow.
The math
- Flow rate (gpm) = pool gallons ÷ (turnover hours × 60).
- Minimum cartridge area (sq ft) = flow rate ÷ design flow rate. Cartridge filters are commonly rated at a maximum of about 0.375 gpm per sq ft for residential pools.
Example: 20,000 gallons, 8-hour turnover → 41.7 gpm → at 0.375 gpm/sq ft, at least 111 sq ft, so a 150-200 sq ft cartridge filter is a comfortable choice.
Bigger is better
An oversized cartridge filter runs at lower pressure, filters better between cleanings and needs cleaning less often. Make sure the filter’s rated flow also exceeds your pump’s real-world flow.
Pool volume quick formulas
- Rectangle: length × width × average depth × 7.5
- Round: diameter × diameter × average depth × 5.9
- Oval: length × width × average depth × 5.9
Frequently asked questions
Can a filter be too big?
Not really for cartridge filters: larger is generally better, within space and budget.