Water temperature is the trigger; your first hard frost is the deadline. Measure the water itself rather than reading air temperature — a pool lags the air by days, which is exactly the margin this decision turns on.
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The rule
close when water holds below 60°F · but before the first hard frost autumn cooling ≈ 4°F per week (above-ground pools faster, deep inground slower)
Sixty degrees is not folklore — it's roughly where algae stops reproducing fast enough to matter. Above it, a covered pool keeps growing things in the dark with no chlorine being replenished. Below it, the water essentially pauses until spring.
A worked example
Water at 68°F in a freeze–thaw climate: you're about two weeks early. Closing this weekend means eight degrees of algae-friendly water sitting under a cover for six months. Use the two weeks — balance the water, buy the cover and chemicals, book the blowout — then close at 60°F with everything already staged.
The two failure modes
Too early means a green opening: a spring that costs three gallons of chlorine and a week of brushing, which the green pool calculator will happily price out for you. Too late means ice in a return line — a repair that involves digging. Between those sits a window that's usually three or four weeks wide, and this page tells you where in it you are.
When the water holds below 60°F, not when the calendar says October. Algae goes dormant near 60°F, so water you lock in below that stays the way you left it. Closing a 75°F pool because the long weekend was convenient is the single most common reason pools open green in April.
Can I close my pool too early?
Yes, and it's the expensive mistake. Warm water under a cover stays biologically active for weeks — chlorine burns off in the dark with nothing replenishing it, and by November you have an algae bloom nobody will see until spring. Every week you wait for cooler water is a week of algae growth you don't pay for later.
Can I close my pool too late?
In freeze country, yes. Once air temperatures drop below freezing overnight, water sitting in skimmer lines and returns can freeze and crack them — and you can't blow out lines through ice. The safe window is after the water holds below 60°F but before your first hard frost.
Do I have to close my pool at all?
Not if your ground never freezes. Plenty of pools in the Sun Belt run year-round on reduced circulation and lower chlorine, which costs some electricity but skips both closing and opening entirely. Closing exists to protect plumbing from ice — no ice, no reason.
How fast does pool water cool in the fall?
Roughly 3–5°F per week once nights turn cold, faster for shallow above-ground pools and slower for deep inground ones. That's the basis for the estimate on this page — it's a planning number, not a forecast. Check the actual water temperature before you commit to a closing weekend.