Pool & spa
Chlorine Calculator
How much chlorine to add to bring a pool from its current reading to the level you want.
Result
Dose against a test result, never against a schedule. Add chemicals in stages with the pump running, wait for a full turnover, then retest before adding more. Never mix chemicals, and never add water to acid. Where a product label differs from the figure here, the label wins. Free chlorine is consumed by sunlight and by whatever is in the water, so a pool needs topping up continually rather than dosing once. Cyanuric acid protects chlorine from UV but also slows it down, which is why the right chlorine level depends on the stabiliser level.
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About this calculator
What free chlorine level should I hold?
Between 1 and 3 ppm for most pools, higher where cyanuric acid is high. The two are linked: at 80 ppm stabiliser, 2 ppm of chlorine behaves like far less.
Liquid, tablet or granular?
Liquid adds nothing but chlorine and salt. Tablets add cyanuric acid with every dose, which accumulates and eventually locks the chlorine up. Granular types vary; read what else is in them.
Why does my chlorine vanish overnight?
Either there is no stabiliser to protect it from UV, or there is a real demand in the water such as algae or organic load. A chlorine demand test tells you which.
The formula
ounces of 12.5% liquid = ppm increase x 10.7 per 10,000 gallons
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