📖 How To Use
How to Use This Chlorine Dosage Calculator
Getting the right chlorine dose for your water tank takes less than a minute:
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Choose your chlorine type
Select whether you're using granular calcium hypochlorite (most common for tanks), liquid household bleach (sodium hypochlorite), or chlorine tablets. Each has a different concentration of active chlorine, so the quantity needed will vary.
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Set your target dosage
Enter the chlorine residual you want to achieve in mg/L (same as ppm). WHO recommends 0.2–0.5 mg/L for safe drinking water. For tank disinfection or shock chlorination, use 5–10 mg/L. The default of 2 mg/L is a good general-purpose level.
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Hit Calculate
The result shows exactly how much product to add — in grams (or ml for bleach, number of tablets for tablets). Copy or print results before treating your tank.
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Add chlorine and wait
Pre-dissolve granular chlorine in a small bucket of water before adding to the tank. Allow at least 30 minutes contact time before the water is used. Test with a chlorine test kit before drinking.
Important: This calculator gives a theoretical dosage. Actual chlorine demand varies with water temperature, turbidity, organic load, and pH. Always verify residual chlorine with a test kit after treatment and before consumption.
📐 The Formula
Chlorine Dosage Formula Explained
The calculation is straightforward — convert volume to litres, then apply the dosage rate:
Total Chlorine Required (mg) = Volume (L) × Dose (mg/L)
Granular (g) = Total_mg ÷ (Purity × 10,000)
Bleach (mL) = Total_mg ÷ (Conc% × 10 × Density)
Tablets = Total_mg ÷ mg_per_tablet
For granular calcium hypochlorite at 65% purity: divide the total chlorine needed (in mg) by 6,500 to get grams of product. For liquid bleach, the volume depends on its active chlorine concentration — household bleach at 5% contains 50 g/L of available chlorine.
Dosage Reference Guide
| Application | Dose (mg/L) | Purpose | Contact Time |
|---|
| WHO Drinking Water | 0.2 – 0.5 | Maintenance residual | 30 min |
| EPA Tap Water | 0.2 – 4.0 | Distribution residual | Ongoing |
| General Disinfection | 2 – 5 | Tank sanitation | 30 min |
| Shock Chlorination | 5 – 10 | Heavy contamination | 60 min |
| Emergency Treatment | 10 – 50 | Unknown source water | 60–120 min |
Chlorine Product Concentrations
| Product | Active Chlorine | Notes |
|---|
| Calcium Hypochlorite (granular) | 65–70% | Most efficient for large tanks |
| Sodium Hypochlorite (liquid bleach) | 5% | Common household product |
| Pool Shock (granular) | 68–73% | Verify concentration on label |
| Chlorine Tablets (200 mg) | 200 mg/tablet | Easy for small volumes |
| Sodium Dichloro (NaDCC) | 55–60% | Emergency/humanitarian use |
pH Matters: Chlorine is most effective at pH 6.5–7.5. At higher pH, hypochlorite ion forms and disinfection efficiency drops significantly. Always test and adjust pH before chlorinating if possible.
🔬 Use Cases
When to Chlorinate Your Water Tank
Annual Tank Maintenance
All water tanks — rooftop, underground, or above-ground — should be fully cleaned and shock chlorinated at least once per year. Sediment, biofilm, and algae build up over time and can harbour pathogens even in treated water.
After Tank Installation or Repair
Any time a water tank is newly installed, repaired, or returned to service after standing empty, a full shock chlorination treatment (5–10 mg/L) should be performed before the water is consumed.
Following Contamination Events
If flooding, animal access, or suspected contamination has occurred, immediate shock treatment is essential. Use 10–50 mg/L for known contamination, allow full contact time, then flush and retest before use.
Emergency Water Preparedness
Water stored for emergency use should be treated with 2 mg/L chlorine when stored and re-treated every 6–12 months. Use our Emergency Water Storage Calculator to size your supply.
Rainwater Harvesting Systems
Harvested rainwater is not inherently safe to drink. A maintenance dose of 0.5–2 mg/L combined with physical filtration and UV treatment is recommended before potable use.
❓ FAQ
Frequently Asked Questions
How much chlorine should I add to my water tank per litre?
For drinking water maintenance, WHO recommends 0.2–0.5 mg of chlorine per litre (0.2–0.5 ppm). For tank disinfection, use 2–5 mg/L. For shock chlorination after contamination, use 5–10 mg/L. The actual amount of product needed depends on its concentration — granular calcium hypochlorite (65%) requires about 1.5 g per 1,000 litres at a 1 mg/L dose.
Can I use household bleach to disinfect a water tank?
Yes — unscented household bleach (5% sodium hypochlorite) is safe for water tank disinfection. For a 1,000-litre tank at a 2 mg/L dose, you need approximately 40 mL of 5% bleach. Never use bleach that contains added fragrances, surfactants, or thickeners, as these may be harmful. Check the label for active chlorine percentage and enter it in the calculator for precise results.
How long should chlorinated water sit before drinking?
Allow a minimum of 30 minutes contact time after chlorinating before consuming the water. For shock chlorination (5–10 mg/L) or emergency treatment (10+ mg/L), wait 60 minutes. After contact time, test the residual chlorine — it should be 0.2–0.5 mg/L for safe drinking. If the residual is too high, allow the water to stand or pass it through activated carbon filtration before use.
What is shock chlorination and when should I do it?
Shock chlorination is the deliberate addition of a high chlorine dose (5–50 mg/L) to kill all pathogens, biofilm, and algae in a water system. It should be done: when a new tank is first installed, after any repair or prolonged emptying, after a contamination event (flooding, animal access, visible algae), and at least once per year for all potable water tanks.
Is granular chlorine or liquid bleach better for tank disinfection?
Granular calcium hypochlorite is generally preferred for large tanks because it has a much higher active chlorine concentration (65–70%) compared to household bleach (5%). This means you need far less product. Granular chlorine also has a longer shelf life (1–3 years) compared to bleach (3–6 months). Liquid bleach is convenient for small tanks or when precise small doses are needed. Tablets are best for portability and emergency kits.
How do I calculate how much bleach to add per 1000 litres?
For 5% household bleach at a 2 mg/L dose: (1,000 × 2) ÷ (50 × 0.998) = approximately 40 mL of bleach per 1,000 litres. For a 5 mg/L shock dose, use 100 mL per 1,000 litres. For a 10 mg/L shock dose, use 200 mL per 1,000 litres. This calculator handles the math automatically for any bleach concentration and tank volume.
What chlorine level is safe for drinking water?
The WHO guideline for safe drinking water is a free chlorine residual of 0.2–0.5 mg/L (ppm). The US EPA allows up to 4 mg/L as a maximum residual disinfectant level (MRDL). Water at 0.5–1 mg/L is safe but may have a noticeable chlorine taste. Above 5 mg/L is too high to drink directly — this is only appropriate for shock treatment, after which the water must be tested and allowed to drop to safe levels before consumption.