Pool Closing Chemical Calculator
Closing your pool for the season? Pool Balance's pool closing chemical calculator turns your last water test of the year into an exact dosing plan. Enter your pool volume and your current free chlorine, pH, total alkalinity and cyanuric acid (CYA) readings, and the calculator shows which levels are off and exactly how much shock and balancing chemicals to add before the cover goes on.
Never mix different pool chemicals directly with each other — especially chlorine products with acid, which can produce toxic gas. Always add chemicals to the water, never water to chemicals, wear gloves and eye protection, and handle chemicals in a well-ventilated area without breathing fumes.
Whatever state your water is in when you cover it is roughly the state it stays in all winter — minus the chlorine, which slowly depletes. Low pH and low alkalinity make water aggressive, which can etch plaster and corrode metal parts over months of sitting. High pH and alkalinity push calcium out of solution as scale. And water that goes under the cover with low chlorine is an open invitation for algae, so you open in spring to a green pool instead of a quick startup.
The usual closing order is: clean the pool, balance pH (7.2–7.6) and total alkalinity (80–120 ppm), check calcium hardness, shock and let it circulate, then add a winter algaecide per its label once chlorine has come back down. Adjust one chemical at a time with the pump running and retest before adding the next — pH also controls how well chlorine works (at pH 7.5 only about half is active, and at pH 8.0 roughly a quarter), so balance pH before you shock.
Because the calculator works from your actual pool size, a 12,000-gallon above-ground pool and a 30,000-gallon in-ground pool get different, sensible amounts. Always check the concentration on your product labels — doses vary by product strength. Your first three pool tests are free.
Pool closing calculator FAQ
How much shock should I add before closing my pool?
It depends on your pool volume, current free chlorine and CYA. As a rule of thumb, about 13 fl oz of 10% liquid chlorine per 10,000 gallons raises free chlorine by about 1 ppm. Enter your readings and the calculator works out the exact shock amount for your pool, then let it circulate before closing.
Do I need algaecide if I'm already shocking?
Usually, yes. Shock kills what's in the water now, but chlorine depletes over winter. A winter algaecide added per its label after chlorine levels drop helps keep algae from growing under the cover. Don't add algaecide and shock at the same time.
What happens if I close my pool without balancing the chemistry first?
The water sits that way for months. Low pH and alkalinity can etch surfaces and corrode metal; high levels can leave scale; and low chlorine often means opening to a green, cloudy pool that takes more chemicals to clear.
What chemical levels should a pool have before closing?
Aim for pH 7.2–7.6 and total alkalinity 80–120 ppm, with calcium hardness in range for your surface and free chlorine raised by shocking. The calculator flags each reading as low, ideal or high.
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