How this calculator works
Chlorine dosing is a simple mass balance. One part per million in water is one milligram per litre, so a pool of 50,000 litres needs 50 grams of available chlorine for each ppm. The product you pour in is not pure chlorine: calcium hypochlorite granules are about two thirds available chlorine and liquid sodium hypochlorite is around 12 percent, so divide by that strength to find how much product to use. Liquid products are usually measured by volume, which is why density appears in the calculation. Breakpoint shocking uses ten times the combined chlorine level as the target, which is the amount found in pool operator guidance to clear chloramines.
Formula
available chlorine in grams = ppm increase x volume in litres / 1,000. granules = available chlorine / available chlorine fraction. liquid volume = (available chlorine / fraction) / density. breakpoint ppm = 10 x combined chlorine - free chlorine, where combined chlorine = total chlorine - free chlorine.
Worked example
A 50,000 litre pool at 1 ppm free chlorine raised to 5 ppm needs 4 ppm, which is 4 x 50,000 / 1,000 = 200 g of available chlorine. At 65 percent strength that is 200 / 0.65 = 308 g of granules. If total chlorine is 2 ppm, combined chlorine is 1 ppm and breakpoint needs 10 x 1 - 1 = 9 ppm, or 450 g of available chlorine.
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Frequently asked questions
What is combined chlorine?
It is the chlorine that has already reacted with sweat, urine and other nitrogen compounds to form chloramines. It smells like a strong pool and irritates eyes, and it does not disinfect well. Total chlorine minus free chlorine gives the combined figure.
When should I shock a pool?
When combined chlorine is above about 0.5 ppm, after heavy bather use, after a contamination event, or when the water is cloudy. Follow your local public health guidance for reopening after a shock.
Can I swim right after shocking?
Wait until free chlorine has fallen into the normal range for your jurisdiction, commonly 1 to 3 ppm, and the water is clear. That usually takes several hours and sometimes overnight.
Is calcium hypochlorite the same as stabilized chlorine?
No. Cal-hypo adds calcium and no cyanuric acid, while dichlor and trichlor add stabilizer. Repeated cal-hypo use raises calcium hardness, so check that level as well as chlorine.