Pool & spa
Pool Shock Calculator
Shock dose to raise free chlorine sharply and clear combined chloramines.
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. Shock at dusk or after dark, because sunlight destroys a large share of it within hours. Cal hypo also adds calcium, which matters in hard water; liquid chlorine adds none. Swimmers stay out until free chlorine falls back to normal.
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About this calculator
When does a pool need shocking?
After heavy use or rain, when combined chlorine rises above about 0.5 ppm, or when the water turns dull. Chloramines, not chlorine, cause the smell and the stinging eyes people blame on chlorine.
How high should the shock level be?
Breakpoint chlorination needs roughly ten times the combined chlorine reading. Going part way is worse than not shocking at all, because it creates more chloramines than it destroys.
Cal hypo or liquid?
Liquid for pools already high in calcium, cal hypo where hardness needs raising anyway. Cal hypo is also easier to store and transport.
The formula
about 0.114 lb of 73% cal hypo per 10,000 gallons raises free chlorine 1 ppm
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