Pool & spa
Pool CYA Calculator
Cyanuric acid to raise stabiliser, which protects chlorine from sunlight.
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. CYA cannot be removed chemically; the only way down is dilution. It also accumulates on its own if you use stabilised tablets, which is why pools drift high over a season. High CYA does not stop chlorine working, but it slows it enough that the required chlorine level rises with it.
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About this calculator
What CYA level should an outdoor pool hold?
Roughly 30 to 50 ppm for a chlorine pool, higher for a salt pool. Zero means chlorine burns off within hours in summer sun.
Why does high CYA cause trouble?
It binds chlorine into a reserve that releases slowly. At 100 ppm you need several times the free chlorine to get the same sanitising effect, and test kits do not make that obvious.
How does it get high without adding any?
Stabilised tablets and granules contain it. Every tablet adds a little, none of it leaves except with water, and a season of tablets can double it.
The formula
about 0.85 lb of cyanuric acid per 10,000 gallons raises CYA 10 ppm
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