Go with a 24-inch sand filter for a 20,000-gallon pool.

That’s the standard size and it works. A 24-inch filter handles the flow and turnover fine for that volume.

You could go up to a 30-inch if you want slightly longer cycles between backwashes and a bit more tolerance for heavy bather loads or debris. But it’s not necessary. The 24-inch will keep your water clear with normal use.

The real mistake is going too small. A 20-inch filter on a 20k pool means you’re backwashing constantly and the pump fights the head loss. That’s where you waste time and electricity. Match the filter to the pump’s flow rate too — a big pump on a small filter is a bad combo.