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 — you need FC compared to cyanuric acid (CYA). The Trouble Free Pool community keeps a chart: at CYA 30, target FC is 4-6. At CYA 50, target FC is 6-8. At CYA 80, target FC is 9-11.

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

Things that quietly trip people up:

1. Trichlor and dichlor add CYA over time. About 6 ppm of CYA per 10 ppm of FC for trichlor, slightly less for dichlor. This is why pool stores often see CYA creep up over a season, 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 = 8 fl oz
• Total = 8 × 4 × (20,000 / 10,000) = 64 fl oz = half a gallon

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