Pool Chlorine Dosage Calculator (FC Raise)

The free-chlorine target depends on your CYA. This is the part most pool stores skip. Free chlorine alone does not tell you whether the pool is sanitized, because cyanuric acid binds most of it. What matters is FC as a fraction of CYA, and the working figure is about 11-13%.

Enter your CYA below and the calculator will tell you where your target should sit. If you leave it blank it simply skips that part and doses whatever raise you asked for.

Each chlorine product has a different active-chlorine percentage. What this calculator handles:

• Liquid chlorine (sodium hypochlorite) at 10% or 12.5%: most common for daily dosing
• Calcium hypochlorite (cal-hypo) at 65-73%: most common for shock
• Dichlor at 56%: granular, neutral pH, adds CYA
• Trichlor at 90%: slow-dissolve tabs, very acidic, adds CYA

Reference doses (per 10,000 gallons, per 1 ppm FC raise):

• Liquid chlorine 12.5%: 10.7 fluid ounces
• Liquid chlorine 10%: 13.4 fluid ounces
• Cal-hypo 65%: 2.05 ounces by weight
• Dichlor 56%: 2.4 ounces by weight
• Trichlor 90%: 1.5 ounces by weight

All five come from one number. Raising 10,000 gallons by 1 ppm means putting 1 milligram of chlorine into every litre of 37,854 litres, which is 37.9 grams, or 1.34 ounces of pure chlorine. Divide that by the strength on the label and you have the dose. Cal-hypo at 65% needs 1.34 ÷ 0.65 = 2.05 oz. The liquids are quoted by volume rather than weight, which is why their numbers look so much larger for the same amount of actual chlorine.

Things that quietly trip people up:

1. Trichlor and dichlor both add CYA, and dichlor adds more, not less. Trichlor contributes about 6 ppm of CYA for every 10 ppm of FC; dichlor contributes about 9. Dichlor is the weaker sanitiser of the two by weight, so you use more of it, and the stabilizer comes along for the ride. This is why CYA creeps up over a season on either product, and high CYA neuters chlorine.

2. Cal-hypo adds calcium. Soft-water pools love it; hard-water pools end up scaling.

3. Liquid chlorine has no side effects on CYA or calcium, but it raises pH temporarily until the hypochlorite consumes itself.

Worked example. A 20,000-gallon pool needs FC raised from 2 ppm to 6 ppm using 12.5% liquid chlorine.

• Raise = 4 ppm
• Per 10,000 gal per 1 ppm = 10.7 fl oz
• Total = 10.7 × 4 × (20,000 / 10,000) = 85.6 fl oz, near enough two thirds of a gallon

Pour it in front of a return jet with the pump running. Wait 15 minutes before swimming.

A note on bottle strength. Liquid chlorine is the one product on this list that does not keep. It loses roughly half its strength over a few months at room temperature, faster in a hot garage, and faster still in sunlight. A jug bought in April and opened in July may be closer to 8% than the 12.5% printed on it, which is a large part of why people find their doses stop working mid-season. Buy it fresh from somewhere with turnover, store it cool and dark, and if a jug has been sitting a while, expect to add more than the calculator says.