How much chlorine do I need?
Cyanuric acid (CYA, stabilizer) protects chlorine from sunlight but also binds most of it, so the right free-chlorine level depends on how much CYA you have. A widely used rule of thumb keeps FC at least 7.5% of CYA and aims for about 11.5%: with 40 ppm CYA that is a minimum of 3 ppm and a target of about 4.5 ppm. Salt-water pools can usually run a little lower because the cell adds chlorine all day. To clear algae, pool-care communities raise FC to about 40% of CYA and hold it there until the water clears and overnight chlorine loss is under 1 ppm.
Product strengths
- Liquid pool chlorine is sold by “trade percent”: 12.5% means 12.5 g of available chlorine per 100 mL. About 10.2 fl oz raises 10,000 gallons by 1 ppm. It loses strength in heat and with age, so old jugs dose weaker.
- Household bleach labels give sodium hypochlorite by weight (often 6–8.25%). Use plain, unscented bleach without additives.
- Cal-hypo (65–73%) adds about 0.7 ppm calcium hardness for every 1 ppm of chlorine.
- Dichlor adds about 0.9 ppm CYA and trichlor about 0.6 ppm CYA per 1 ppm chlorine, so stabilizer builds up when they are your main chlorine source. Trichlor is also acidic and lowers pH and alkalinity.
Never mix different chlorine products, or chlorine and acid, in the same container.
Not professional advice. Results are estimates for residential pools and hot tubs. Read and follow product labels: add chemicals separately, to water (never water to chemicals), with the pump running, and retest before adding more. Commercial and public pools must follow local health codes.
FAQ
Can I swim right after adding liquid chlorine?
Once it has circulated and FC is back in your normal range, generally yes; follow the product label. Wait longer after shocking to high levels.
Why does my chlorine disappear overnight?
Losing more than about 1 ppm overnight (no sun) suggests something is consuming it, most often algae or organic debris. Low CYA causes fast loss in sunlight.