Calcium Hypochlorite (HTH 65–70%) Granular Dosing Equation
Calcium hypochlorite (HTH) is the dry alternative to liquid bleach — 65–70% available chlorine vs 12% for sodium hypochlorite. It is sold as granules, tablets or briquettes and dissolved on-site to a 1–5% solution before dosing. Best for plants where storage volume matters or where 12% NaOCl loses too much strength in transit.
Where:
- Flow = Main flow rate in m³/hr
- Dose_PPM = Target concentration in mg/L or ppm
- Strength_% = Percentage active ingredient of the stock chemical
- SG = Specific Gravity (density relative to water) of the stock
Related dosing calculators
Other chemical-specific dosing calculators in the same series — same formula, different defaults:
- Chemical Dosing Calculator (generic) — the universal seed page
- Chlorine Dosing Calculator — Drinking Water · Cooling Towers
- Chlorine Dioxide (ClO₂) Dosing Calculator — ClO₂ · Legionella · Cooling
- RO Antiscalant Dosing Calculator — RO · Desalination · Membrane Protection
- All Chemical Engineering Calculators →
Frequently Asked Questions
For a 50,000 L pool to maintain 1–3 mg/L free chlorine: daily makeup ~5–15 g of 65% HTH per day (depends on bather load and sun). Shock dose: 50 g per 10,000 L of 65% HTH = 32.5 mg/L FAC, then wait 2–4 hours before re-entry.
AWWA C652 calls for 50 mg/L FAC for 24 hours or 200 mg/L for 3 hours. For a 100,000 L tank at 50 mg/L: dose = (100,000 × 50) ÷ 650,000 = 7.7 kg of 65% HTH. Always pre-dissolve in a separate mix tank, then add to the empty volume before filling.
HTH 65% has 5–6× the active chlorine per kg of NaOCl 12% — better for storage-limited sites or cold/remote applications where NaOCl freezes. HTH stays stable for 1–2 years in dry, cool storage. NaOCl is faster to dose (already liquid) but loses 1%/week strength and freezes at −20°C.
Calcium hypochlorite contains ~33% calcium by mass — every kg of HTH dosed adds ~330 g calcium hardness to the water. For soft-water sources or RO feed, this can be significant; for hard water it is negligible. Use NaOCl if calcium addition must be avoided.
Yes, but cyanuric acid (CYA) reduces chlorine activity. With CYA at 30–50 ppm, target FAC = 2–4 ppm (2–4× higher than uncovered pools). HTH adds calcium so monitor calcium hardness — keep below 400 mg/L to prevent scale formation on heater tubes.