Aim for 40 to 50 GPM for a 20,000-gallon pool.

Somewhere around 40 to 50 gallons per minute.

That’s based on the standard rule of turning over the entire pool volume once every 8 hours. 20,000 gallons ÷ 8 hours = 2,500 gallons per hour, which is about 42 GPM. If you want a faster 6-hour turnover (common for heavy-use pools or hotter climates), you’d need about 56 GPM. But most residential pools are fine in the 40–50 GPM range.

Don’t buy a pump that’s way oversized — it wastes electricity and can stress your filter. A variable-speed pump is worth the extra cost because you can dial in the exact flow you need.