How to Close an Inground Pool for Winter

When to close, how much chlorine to add, how far to lower the water, and how to protect pipes and equipment from freezing, in the order the work actually happens.

By PoolOwlPublished October 2, 20264 min read

Closing a pool is mostly a week of small jobs, not one long Saturday. The chemistry decides what you’ll find under the cover in spring. In places where winter brings hard freezes, the plumbing work decides whether spring also brings a repair bill for a cracked pump or pipe.

If you’d rather have the amounts worked out for your own pool, the closing planner turns your test results into a dated checklist.

Pool closing plannerGet a closing checklist with your shock and balancing doses filled in.

Pick your week

The usual advice is to close once the water stays below about 65 °F. Algae needs warmth and food. Close while the water is still warm, and the chlorine you leave behind gets used up early, which is the most common route to a green pool in April. Close too late, and you risk a hard freeze arriving before the lines are protected.

Closing day itself takes a few hours. The cleaning and shocking start about a week earlier.

A week out: clean everything

Skim, brush the walls and floor, vacuum, and empty the skimmer and pump baskets. Leaves left on the floor become algae food for five months. Then clean the filter: backwash a sand or DE filter, or take out a cartridge and hose it off.

Test free chlorine, pH, total alkalinity and CYA (stabilizer) now, so you have time to correct anything that’s far off. Every dose below depends on your pool’s volume; if you’ve never measured it, the pool volume calculator takes a few minutes.

A few days out: balance, then shock and hold

Fix alkalinity first if it’s low, then bring pH into roughly 7.4–7.6. The pH calculator gives the acid or soda ash dose, and the pH guide explains why alkalinity changes it.

Then raise free chlorine to the shock level for your CYA, about 40% of your CYA reading and never below 10 ppm. Here’s an example: a 20,000-gallon pool with CYA at 40 ppm has a shock level of 16 ppm. Starting from 3 ppm, that takes about 2.6 gallons of 10% liquid chlorine. That figure comes from our shock calculator; your pool size, CYA and product strength will change it.

One dose isn’t the end of it. Retest every few hours and top back up to the shock level until two things are true:

the water is clear, and
the pool passes an overnight chlorine loss test: test at dusk, test again at dawn, and free chlorine has dropped by 1 ppm or less.

A small overnight loss means nothing in the water is still eating chlorine. A big one means algae or debris is still in there, and closing now will likely give you a green pool in spring. Our guide to shocking a pool covers this step in more detail.

Closing day

Work through these in order:

Leave a chlorine reserve. Chlorine slowly runs down under the cover, so close with free chlorine at the top of your normal range or higher.
Take out floaters and chlorine tablets. Undissolved trichlor sitting under a closed cover can bleach liners and damage equipment.
Remove ladders, rails and accessories that your manufacturer says to store for winter.
Lower the water if you get hard freezes. In-ground pools are commonly lowered to 4–6 inches below the skimmer opening so the lines can be blown out. Some solid safety covers are designed to rest on water at a higher level, so your cover maker’s instructions come first.
Blow out the lines. Push air through each skimmer and return line with a shop vac or blower until air bubbles out the other end, then plug the line. Many owners also add pool antifreeze (propylene glycol), following its label.
Drain the equipment. Open the drain plugs on the pump, filter, heater and chlorinator, and keep the plugs in the pump basket so they’re there in spring. Bring a salt cell indoors. Follow the heater manual; some heaters need extra steps.
Switch off the breakers for the pump and heater.
Put the cover on according to its instructions.

If your winters are mild

Where freezes are rare and brief, many owners never fully close. Instead they run the pump a few hours a day (with the freeze-protection setting turned on, if the controller has one), keep chlorine in range, and brush now and then. The water stays usable for a warm weekend, and spring opening is just another week.

Over the winter

Every month or so, look for tears, sagging and standing water on the cover, and check the water level against where you left it. A sudden drop can mean a leak. On a mild day, if you can reach the water at the skimmer, a quick chlorine test tells you whether the reserve is holding.

Common questions

What water temperature should I wait for before closing?

A common rule of thumb is to wait until the water stays below about 65 °F. Algae grows slowly in cold water, so the chlorine you leave behind lasts longer under the cover. It's guidance, not a hard line; a warm spell after closing is the bigger risk.

Do I need a closing kit or winter algaecide?

Not necessarily. Clean, balanced water with a good chlorine reserve, closed once it's cold, usually opens clear. If you do use an algaecide, avoid copper-based products, which can stain plaster.

Can I use car antifreeze in my pool lines?

No. Use only antifreeze made for pools, which is propylene glycol. Automotive antifreeze is usually ethylene glycol, which is toxic.

Sources

Home pool and hot tub water treatment and testing, US Centers for Disease Control and PreventionRecommended pH (7.2–7.8) and minimum free chlorine for pools.
Pool School (closing a pool; overnight chlorine loss test), Trouble Free PoolShock level of about 40% of CYA, held until the pool passes an overnight chlorine loss test.
Pool Safely, US Consumer Product Safety CommissionDrowning-prevention guidance, including keeping covers clear of standing water.

This guide was drafted with AI tools and fact-checked against the sources above on October 2, 2026. Calculated figures come from our tested calculators. See our editorial policy. Spotted an error? Tell us.