How do I calculate the right pump flow rate for my pool?

Your pump should cycle all the water through the filter in roughly 8 hours. Divide your pool volume (in gallons) by 480 (8 hours × 60 minutes) to get the flow rate — for a typical 20,000-gallon pool that’s about 42 GPM. Don’t just buy the biggest pump; your filter has a maximum flow rate, and exceeding it damages the filter or wastes energy. Check the filter’s spec sheet and plumbing.