Enter your tank volume and target chlorine level to get the exact amount of granular chlorine, liquid bleach, or chlorine tablets needed for safe water disinfection.
Select your chlorine type, enter tank volume and desired dosage level
Safety notice: Always handle chlorine with gloves and eye protection. Never mix chlorine products. Allow 30 minutes contact time before consuming treated water.
Select Chlorine Type
Granular65–70% available Cl₂
Liquid Bleach6–8.25% sodium hypochlorite
Tablets200mg per tablet
Enter tank volume
Tank Volume
Target Dose
Bleach concentration
% Active Chlorine
%
Check your bottle's label. Most US household bleach is 6% or 8.25%.
⚠ Please enter a valid tank volume (greater than zero) and target dosage.
WaterTankCalculator.com — Chlorine Dosage Results
Your Results
Granular Chlorine Required
—
grams
Tank Volume
— L
Target Dose
— mg/L
Total Chlorine Mass
— mg
Teaspoons (approx)
— tsp
Contact Time
30 min
Chlorine Type
—
Dosage Visual Reference
📖 How To Use
How to Use This Chlorine Dosage Calculator
Getting the right chlorine dose for your water tank takes less than a minute:
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.
Enter your tank volume
Enter the total volume of water in your tank in litres, cubic metres, US gallons, or kilolitres. If you don't know your tank volume, use our Rectangular or Cylindrical Tank Volume Calculator first.
Set your target dosage
Enter the chlorine residual you want to achieve in mg/L (same as ppm). For drinking water, WHO guidance is a free chlorine residual of at least 0.5 mg/L after 30 minutes of contact and at least 0.2 mg/L at the tap. Your dose needs to be higher than the residual you want, because some chlorine is used up. For tank disinfection or shock chlorination, use 5–10 mg/L. The default of 2 mg/L is a good general-purpose level.
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.
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)
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 6% contains about 60 g/L (60 mg/mL) of available chlorine, and 8.25% bleach about 82.5 g/L.
Dosage Reference Guide
Application
Dose (mg/L)
Purpose
Contact Time
WHO Drinking Water
≥ 0.5 residual (≥ 0.2 at tap)
Minimum free chlorine 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
Contaminated Tank Shock
10 – 50
Disinfect tank and pipes only. Drain, refill and retest before drinking.
60–120 min
Chlorine Product Concentrations
Product
Active Chlorine
Notes
Calcium Hypochlorite (granular)
65–70%
Most efficient for large tanks
Sodium Hypochlorite (liquid bleach)
6–8.25%
Check the label; older bottles lose strength
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, WHO guidance is a free chlorine residual of at least 0.5 mg/L after 30 minutes of contact and at least 0.2 mg/L at the tap, so dose slightly above that to cover chlorine demand. 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 — regular unscented household bleach (usually 6% or 8.25% sodium hypochlorite) works for water tank disinfection. For a 1,000-litre tank at a 2 mg/L dose, you need approximately 33 mL of 6% bleach or 24 mL of 8.25% 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. After shock chlorination (5 mg/L and above), wait at least 60 minutes, then drain or dilute the tank before anyone drinks from it. Before drinking, test the free chlorine residual: it should be at least 0.2 mg/L and no more than 4 mg/L. 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 (6–8.25%). 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 6% household bleach at a 2 mg/L dose: (1,000 × 2) ÷ 60 = approximately 33 mL of bleach per 1,000 litres. For a 5 mg/L shock dose, use about 83 mL, and for 10 mg/L about 167 mL per 1,000 litres. With 8.25% bleach, use roughly a quarter less. This calculator handles the math automatically for any bleach concentration and tank volume.
What chlorine level is safe for drinking water?
WHO guidance is a free chlorine residual of at least 0.5 mg/L after 30 minutes of contact and at least 0.2 mg/L at the point of use, with 5 mg/L as the upper health-based guideline value. 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.
✅ Sources & Review
Sources and review
The dosage ranges and contact times on this page come from these public guidelines:
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
