How to use this calculator
- Enter the flow and the hours you dose each day.
- Enter chlorine demand and the residual you want.
- Pick the chlorine product and check its strength.
- Read daily product use, dosing pump rate and CT.
How do you calculate chlorine dosage?
Dose (mg/L) = demand + residual
Cl₂ (kg/day) = Q (m³/day) × dose ÷ 1,000
Product = Cl₂ ÷ (available chlorine % ÷ 100)
CT = residual (mg/L) × contact time (min)
For sodium hypochlorite, a 10% w/v solution holds 100 g of available chlorine per litre, so kg of chlorine ÷ 0.1 gives litres.
Worked example
With flow 10 m³/h, operating hours per day 24 hours, chlorine demand 1.5 mg/L, free residual wanted 0.5 mg/L, chlorine product: Sodium hypochlorite solution, available chlorine in product 10 %, contact time before first user 30 minutes:
Sodium hypochlorite per day: 4.8 litres
The calculator also gives total chlorine dose 2 mg/L; chlorine needed 0.48 kg Cl₂/day; dosing pump rate 0.2 L/h.
Assumptions and limits
- Strength in % w/v for hypochlorite solution (g per 100 mL).
- Demand changes with season and source; recheck it regularly.
Frequently asked questions
How much bleaching powder is needed for 1,000 litres of water?
For a 2 mg/L dose, 1,000 litres needs 2 g of chlorine, which is about 6.7 g of 30% bleaching powder. Dissolve it as a paste first and let the lime settle.
How much sodium hypochlorite do I add per litre of water?
For 2 mg/L using 10% hypochlorite, add 0.02 mL per litre, or 20 mL per 1,000 litres.
What is the residual chlorine limit in drinking water in India?
BIS IS 10500:2012 requires at least 0.2 mg/L free residual chlorine at the consumer end, and up to 1 mg/L where disease risk is high.
References
- BIS IS 10500:2012, Drinking Water Specification
- WHO Guidelines for Drinking-water Quality, 4th ed.
- CPHEEO, Manual on Water Supply and Treatment (1999)