Pool Chlorine Dosage Calculator (FC Raise)
How much liquid chlorine, cal-hypo, dichlor, or trichlor to raise free chlorine by N ppm.
Calibrated for common pool product strengths.
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:
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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.
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Cal-hypo adds calcium. Soft-water pools love it; hard-water pools end up scaling.
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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.