Closing a pool properly in the fall is the best way to have an easy opening in spring. A pool that is closed with balanced water and plenty of chlorine usually opens clear or close to it. One closed in a hurry often opens as a green swamp, with stains and scale to deal with. This guide covers in-ground and above-ground pools in areas where the water freezes. Always follow your equipment manufacturer's instructions where they differ.
When to close
Wait until the water temperature stays below about 65°F (18°C). Algae grows slowly in cold water, so the colder the water when you close, the cleaner it will stay. Closing too early, while days are still warm, is the most common reason for a green pool in spring.
What you need
- A reliable test kit, and the products to adjust pH, alkalinity and calcium.
- Liquid chlorine (or cal-hypo) for the final shock.
- A winter cover that fits, plus water bags or straps.
- For in-ground pools: skimmer plugs or ice compensators (often called "Gizzmos"), line plugs and a shop vac or air blower.
- For above-ground pools: an air pillow and a skimmer cover or plug.
- Optional: non-toxic pool antifreeze (propylene glycol) for the lines.
Step 1: Clean the pool
About a week before closing, remove leaves with a net, brush the walls and floor and vacuum. Anything organic left in the pool will use up chlorine all winter and feed algae.
Step 2: Balance the water
Balanced water protects the surfaces while the pool sits for months. Aim for:
| Level | Target for closing |
|---|---|
| pH | 7.4–7.6 |
| Total alkalinity | 80–120 ppm |
| Calcium hardness | 200–400 ppm (plaster); 150–250 ppm (vinyl, fiberglass) |
| Stabilizer (CYA) | 30–50 ppm (60–80 ppm salt pools) |
Cold water tends to be aggressive toward plaster, so it is better to close with pH and calcium in the middle of their ranges rather than at the bottom. Use the pH, alkalinity and calcium calculators for the doses, and adjust one thing at a time, with the pump running.
Step 3: Shock the pool
A day or two before covering, raise free chlorine to shock level (about 40% of your CYA, at least 10 ppm) with the shock calculator, and let the pump run. You want to close with a high, stable chlorine level, not with chlorine that is already falling. Do not add algaecide or other winter chemicals at the same time as a big chlorine dose: add them afterwards, following the label, once chlorine has started to come down.
Step 4: Lower the water level
In-ground pools: many owners lower the water a few inches below the skimmer mouth, so that ice cannot crack the skimmer. Others keep the water at normal level and protect the skimmer with an ice compensator. Follow your pool builder's or cover manufacturer's advice. Never drain a pool completely: in-ground pools can float out of the ground and liners can shrink or tear.
Above-ground pools: lower the water just below the skimmer, or leave it at normal level and use a skimmer cover plate.
Step 5: Protect the plumbing from freezing
- Turn off the power to the pump, heater and salt cell at the breaker.
- Remove the drain plugs from the pump, filter, heater and chlorinator so they can drain. Keep the plugs in the pump basket so you can find them in spring.
- For in-ground pools, blow the water out of each line with a shop vac or blower until air bubbles out of the returns and skimmers, then plug each line. Pool antifreeze in the lines is extra protection in very cold areas.
- Remove the salt cell, clean it and store it indoors.
- Store the chemicals somewhere dry, ventilated and out of the reach of children. Never store acid and chlorine next to each other.
Step 6: Put the cover on
Remove ladders, rails and diving boards if possible. Place the air pillow in the center of above-ground pools, then put the cover on and secure it with water bags, straps or a cable. A tight, well-anchored cover keeps out leaves and sunlight, the two things that feed algae.
During the winter
- Pump rain water and melted snow off the top of the cover so it doesn't sag or tear.
- Keep the cover clear of leaves.
- In mild climates, open the cover once a month or so, test the chlorine and top it up if needed.
When spring comes, follow our guide to opening your pool.
Safety first: Always read the product label. Add chemicals to water, never water to chemicals. Add one chemical at a time with the pump running, never mix products (especially chlorine and acid), and keep everyone out of the water until levels are back in range. Wear gloves and eye protection when handling acid or shock.
Sources and methodology
Last reviewed: November 2026. Doses are calculated from the chemistry of each product (molecular weights and available chlorine) and cross-checked against manufacturer and industry dosing charts. Recommended ranges follow CDC guidance for home pools and hot tubs.
Frequently asked questions
What temperature should the water be when I close my pool?
Ideally below about 65°F (18°C). Algae grows slowly in cold water, so waiting until the water cools is the best protection against a green pool in spring.
Should I drain my pool for winter?
No. Only lower the level as your pool or cover manufacturer recommends. A fully drained in-ground pool can lift out of the ground, and vinyl liners can shrink and tear.
Do I need winterizing chemicals?
The essentials are balanced water and a strong chlorine dose before covering. Winter algaecides and enzyme products can help in warm climates or with long winters, but follow the label and do not add them together with a large chlorine dose.