Swimming Pool Chlorine Calculator

The CDC recommends maintaining free chlorine at 1 to 3 parts per million (ppm) for residential pools and 2 to 4 ppm for public pools. The ideal target for most residential pools is 2 to 3 ppm. Levels below 1 ppm may not effectively kill pathogens; levels above 5 ppm can cause eye irritation.

The amount of chlorine product to add equals: pool volume (gallons) * chlorine increase needed (ppm) / (product chlorine concentration * 10,000). For example, to raise a 10,000-gallon pool by 1 ppm using 12.5% liquid chlorine (sodium hypochlorite), you need about 10,000 * 1 / (0.125 * 10,000) = approximately 8 fluid ounces.

Free chlorine is the active sanitising chlorine available to kill bacteria and algae. Combined chlorine (chloramines) is chlorine that has already reacted with nitrogen compounds (from sweat, urine, sunscreen) and is no longer effective. Total chlorine = free + combined. Healthy pools have very low combined chlorine (below 0.2 ppm).

Chlorine effectiveness depends strongly on pH. At pH 7.2, about 67% of chlorine exists as the active hypochlorous acid (HOCl). At pH 7.8, only about 33% is active. The CDC recommends maintaining pool pH at 7.2 to 7.8, with 7.4 to 7.6 being optimal for both sanitation and swimmer comfort.

Shock treatment raises chlorine rapidly to 5 to 10 times the normal level to break down chloramines, kill algae, and reset the sanitiser baseline. The CDC recommends shocking when combined chlorine exceeds 0.5 ppm, after heavy swimmer use, after storms, or if the pool shows signs of algae growth. Always shock at dusk or night to prevent UV degradation.