Balance the water first
Before anything else, get the chemistry right, because the water sits for months and you want it stable and non-corrosive. Set total alkalinity with the alkalinity calculator, pH to 7.4 to 7.6 with the pH calculator, and confirm calcium hardness is in range so the closed water is not aggressive. Run it through the saturation index calculator so it is balanced going into winter.
Clean the pool thoroughly too: skim, brush, and vacuum, so it goes under the cover clean.
Lower the water and clear the lines
Freezing water expanding in pipes is what cracks equipment, so the plumbing has to be protected. Lower the water level as your method requires, often below the skimmer and returns for an inground pool, then blow out the lines and plug the returns, or use a skimmer cover and antifreeze rated for pools. Above-ground pools follow their own method, usually lowering below the skimmer and disconnecting the hoses.
Drain the pump, filter, heater, and any other equipment fully, since trapped water in them freezes and cracks housings. If you are unsure of the exact steps for your setup, this is the part worth getting right or having done professionally.
Add winterizing chemicals
Give the water a final balanced-and-sanitized state for the long sit. Many people add a winter algaecide and bring chlorine up before closing so it coasts through the cold months. Avoid leaving chlorine extremely high against a cover, and follow your winter kit's instructions.
The goal is water that stays clear and algae-free under the cover, not a heavy dose that degrades or bleaches the cover over winter.
Cover it well
A snug, intact cover keeps out leaves, debris, and sunlight, which is what keeps the closed water from going green. Make sure it is secured against wind and that water and debris cannot pool on top and sink in. Check it through the winter after storms.
Done properly, winterizing is what makes opening the pool in spring a quick job instead of a battle with green water.
Frequently asked questions
How do I winterize a pool?
Balance the water, clean the pool, lower the water level as your method requires, blow out and plug the plumbing and fully drain the equipment so nothing freezes, add winterizing chemicals such as a winter algaecide, and put on a secure cover. The key is protecting the pipes from freezing and leaving the water balanced and clean.
When should I close my pool?
Once the water stays consistently cool, often when temperatures drop into the 50s or 60s Fahrenheit and you are done swimming. Closing while the water is cold slows algae growth under the cover. Closing too early in still-warm water risks a green pool over winter.
Do I need to lower the water to winterize a pool?
Usually yes, to protect the plumbing from freezing. Inground pools often lower the level below the skimmer and returns, then blow out and plug the lines or add pool-rated antifreeze. Above-ground pools lower below the skimmer and disconnect hoses. Always fully drain the pump, filter, and heater.
What chemicals do I add to winterize a pool?
Balance alkalinity, pH, and calcium hardness first, then typically add a winter algaecide and bring chlorine up before covering, following your winter kit. The aim is clean, balanced, sanitized water that stays clear for months, not an extreme dose that degrades the cover.
Will my pool turn green over winter?
Not if you close it correctly: balanced and sanitized water, a winter algaecide, and a snug cover that blocks light and debris. Pools go green over winter mainly when they are closed too warm, under-sanitized, or under a cover that lets in light and leaves.