Choose a disinfection method, enter your tank size and chlorine product, and get the exact amount to add, how long to hold it, and what residual to check for before you drain and refill.
Pick a method, enter your tank and product, and get the amount to add
Step 1 — Method
Step 2 — Tank & product
Tank Volume (to overflow)
Chlorine Product
Tank Condition (extra chlorine)
⚠ Please enter a valid tank volume greater than zero.
WaterTankCalculator.com — Disinfection Results
Dosage Results
Disinfectant Required
—
mL of bleach
In Millilitres
— mL
In Grams
— g
Teaspoons
— tsp
Chlorine Strength
— mg/L
Water to Dose
— L
Hold Time
— min
Disinfection Process Overview
📖 How To Use
How to Use This Disinfection Calculator
This calculator is for disinfecting the tank itself — after cleaning, repairs, contamination or before first use. For treating drinking water you'll drink right away, use the bleach to water ratio calculator or the chlorine dosage calculator instead.
Choose a methodFill & hold is simplest for household tanks. Surface spray uses far less chlorine and suits large tanks you can get inside safely. Partial fill is a middle ground.
Enter the tank volume and your productFor fill & hold and partial fill, enter the volume up to the overflow. For surface spray, enter how much solution you want to mix. Check your bleach label for its strength — most US household bleach is now 6% or 8.25%.
Follow the step-by-step planThe result gives the amount to add, the hold time, and a numbered plan, including the chlorine level to check with a test strip before you drain and refill.
Safety: Never mix bleach with ammonia, vinegar or acid cleaners — it releases toxic gas. Wear gloves and eye protection. Don't climb into an enclosed tank: chlorine fumes build up and confined spaces are dangerous — use a long-handled brush or sprayer from the hatch, or hire a professional.
📐 The Formula
Tank Disinfection Methods and Formula
The three methods come from AWWA C652, the US standard for disinfecting water storage tanks. It's written for public water system tanks; the same chlorine levels are widely used for household tanks.
Method
Chlorine
Hold time
Check before use
1 · Fill & hold
Whole tank at 10 mg/L
24 hours (6 hours if actively mixed)
At least 10 mg/L left at the end; then drain and refill
Example: 1,000 L tank, fill & hold at 10 mg/L, 6% bleach
= 1,000 × 10 = 10,000 mg ÷ 60 = 167 mL of bleach
Organic matter, sediment and biofilm use up chlorine, which is why the tank should be cleaned first and why the fill & hold method lets you add a little extra for an older tank. The test at the end tells you whether enough chlorine was left to do the job.
Chlorine product strengths
Product
Typical strength
Notes
Household bleach
6% or 8.25% (older bottles 5.25%)
Use plain, unscented bleach. It loses strength over time, so use a fresh bottle.
Pool liquid chlorine
10–12.5%
Stronger — handle with extra care.
Calcium hypochlorite
65–70%
Granules or tablets. Dissolve in a bucket of water first; never mix with other chemicals.
✅ When to Disinfect
When to Disinfect Your Water Tank
Before first use — a new tank, or one that's been out of service.
After cleaning, repairs or anyone entering the tank.
After contamination — floodwater, an animal or insects getting in, an open or broken lid.
After a positive bacteria test — for example a total coliform or E. coli result.
When you see or smell a problem — slime, sediment build-up, algae, or musty or sewage smells.
Clean first, then disinfect. Chlorine can't get through sediment and slime. Drain the tank, scrub the walls and floor, and rinse out the dirt before you add chlorine. After disinfecting a drinking-water tank, a bacteria test is the only way to confirm it worked.
❓ FAQ
Frequently Asked Questions
How much bleach do I need to disinfect a water tank?
For the fill-and-hold method (10 mg/L for 24 hours), about 167 mL of 6% bleach per 1,000 litres, or 121 mL of 8.25% bleach. In US units that's about ¼ cup (63 mL) of 6% bleach per 100 gallons. Add a little extra for an older tank, and check that at least 10 mg/L remains at the end.
How long should chlorine stay in a water tank?
It depends on the method: 24 hours for a full tank at 10 mg/L (6 hours if the water is actively mixed), at least 30 minutes for a 200 mg/L surface spray, and 6 hours plus a further 24 hours for the partial-fill method.
Can I drink the water after disinfecting the tank?
Not the strong disinfection water. Drain it, refill with clean water and run the taps until the strong chlorine smell has gone. The US EPA's limit for chlorine in drinking water is 4 mg/L. For a drinking-water tank, a bacteria test after disinfection confirms it worked.
Where should I drain the heavily chlorinated water?
Strongly chlorinated water can harm septic systems, plants, and fish in streams and ponds. Don't send large volumes into a septic tank, and don't drain it into waterways. Let it stand in the open to lose chlorine, spread it over a large area of ground away from plants, or check your local rules for disposal.
Can I use pool shock (calcium hypochlorite) instead of bleach?
Yes — use a plain calcium hypochlorite product with no algaecides, clarifiers or stabiliser (cyanuric acid). At 65–70% strength you need much less: about 15 g of 65% granules for 1,000 litres at 10 mg/L. Always dissolve it in a bucket of water before adding it, and never mix it with other chemicals.
How often should I clean and disinfect my water tank?
Inspect it regularly and clean it when sediment builds up — many owners do this once or twice a year, more often in dusty areas or where the supply is intermittent. Always disinfect after cleaning, repairs, contamination or a positive bacteria test, and follow any rules from your local health authority.
What's the difference between this and the chlorine dosage calculator?
US EPA maximum residual disinfectant level for chlorine in drinking water — 4 mg/L.
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.
