Chlorine (NaOCl, Ca(OCl)₂, Cl₂ gas) Dosing Equation
Chlorine is the workhorse disinfectant of municipal water, swimming pools and cooling towers. Whether you dose 12% sodium hypochlorite, 65% calcium hypochlorite (HTH) granules or gaseous chlorine, the calculation comes down to mass balance — chlorine demand × flow ÷ stock strength.
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
- Calcium Hypochlorite (HTH) Dosing Calculator — HTH · Pool · Tank Disinfection
- Chlorine Dioxide (ClO₂) Dosing Calculator — ClO₂ · Legionella · Cooling
- Caustic Soda Dosing Calculator — pH Raise · Neutralisation
- All Chemical Engineering Calculators →
Frequently Asked Questions
WHO and most national codes target a free-chlorine residual of 0.2–0.5 mg/L at the consumer tap, which usually requires a 1–3 mg/L dose at the treatment plant to overcome chlorine demand from organics, ammonia and pipe biofilm. For highly contaminated raw water or breakpoint chlorination of ammonia, doses of 5–10 mg/L are common.
Shock chlorination targets 50–200 mg/L free Cl₂ for 30–60 minutes. For a 10,000 L tank at 100 mg/L target with 12% NaOCl: dose = (10,000 × 100) ÷ 144,000 = 6.94 L of 12% NaOCl. Always allow contact time before flushing and de-chlorinating.
Open-recirculating cooling towers typically need 1–4 mg/L free chlorine residual to control biofilm and Legionella. Allowing for 1–2 mg/L demand from organics and corrosion inhibitors, the applied dose is usually 2–6 mg/L, often shock-dosed at 5–10 mg/L weekly.
Yes. Calcium hypochlorite (HTH) is sold as 65–70% available chlorine, compared with 10–15% for liquid sodium hypochlorite. 1 kg of 65% HTH releases the same Cl₂ as ~5.4 L of 12% NaOCl. HTH is preferred where storage volume is limited; NaOCl is preferred where dust handling is a hazard.
12% NaOCl loses ~1% strength per week at 25°C, faster in heat or sunlight. Field strength is typically 9–11% even when labelled 12%. For accurate dosing, titrate stock strength monthly or assume 90% of nominal. Refrigerated storage and dark tanks extend life.