Purify Water With Bleach in an Emergency

The CDC Dosing Table (Copy This Somewhere Useful)

Most guides tell you to add "a few drops" of bleach per gallon. That vagueness gets people sick. The CDC publishes exact figures, and the numbers differ depending on your bleach concentration — which matters because the two most common household bleach concentrations require different doses.

Regular bleach (6% sodium hypochlorite): 8 drops per gallon, 16 drops per 2-liter bottle, 1/3 teaspoon per 5 gallons.

Concentrated bleach (8.25% sodium hypochlorite, common in Clorox): 6 drops per gallon, 12 drops per 2-liter bottle, 1/4 teaspoon per 5 gallons.

Always check the label. If your bleach is scented, color-safe, or has any added cleaners, do not use it for drinking water. Plain sodium hypochlorite only.

One practical note on measuring drops: a standard medicine dropper or oral syringe produces drops of roughly 0.05 mL each. That calibration is consistent enough for this purpose. Eyedroppers built into some bleach bottle caps are less reliable — the drop size varies by how hard you squeeze. For anything beyond a gallon, switch to measuring spoons: 1/8 teaspoon = roughly 8-9 drops.

For large batches — say, a 5-gallon bucket of stored tap water that you're re-treating before a storm — the math is straightforward: 5 gallons × 8 drops (for 6% bleach) = 40 drops, or just under 1/4 teaspoon. Stir, cover, wait 30 minutes, smell. The same ratios scale linearly to 55-gallon barrels and IBC totes if you're building serious storage capacity.

Step-by-Step: Treating Water With Bleach

1. 1
   Check Your Bleach Concentration

   Find the sodium hypochlorite percentage on the label. 6% = 8 drops/gallon. 8.25% = 6 drops/gallon. If the label doesn't list the concentration or you can't read it, assume 6% and use the higher dose.

2. 2
   Filter Out Visible Particles First

   If the water is cloudy or has debris, strain it through a clean cloth, paper towel, or coffee filter into your container. Bleach works on dissolved microorganisms — suspended solids physically block it from reaching pathogens.

3. 3
   Measure and Add Bleach

   Use a medicine dropper or syringe for accuracy. Add the correct number of drops for your container size, then stir or shake the container to distribute the bleach evenly throughout the water.

4. 4
   Wait the Full Contact Time

   Let the treated water sit for 30 minutes before drinking. If the water is cold (below 40°F) or still looks cloudy after filtering, double the dose and wait 60 minutes. Cold water slows the disinfection reaction significantly.

5. 5
   Smell Test Before Drinking

   After waiting, the water should have a slight chlorine odor. If it has no smell at all, the bleach may have been too old or degraded — add another dose and wait another 15 minutes. A faint chlorine smell confirms the treatment worked.

6. 6
   Store in a Clean, Sealed Container

   Treated water stays safe for up to 24 hours in a covered container at room temperature. Direct sunlight degrades residual chlorine faster. For longer storage needs, see our guide on long-term home water storage.

How to Know If It Actually Worked

The chlorine smell is your primary indicator. Treated water should smell faintly of chlorine after the wait period — similar to what you'd expect from a lightly chlorinated swimming pool. If you smell nothing, something went wrong: either the bleach was too degraded, you under-dosed, or the water was too cold for adequate contact time.

If you're unsure whether the bleach worked — particularly with cloudy water, murky tap water during a boil-water advisory, or suspect well water — treat it a second time or boil the water instead . Boiling kills everything bleach kills, plus Cryptosporidium (which bleach cannot). For a deeper look at how these methods compare, see our explainer on water filtration vs. purification .

Bleach Shelf Life and Potency Decay

Here's the part most preppers miss: household bleach loses roughly 20% of its active chlorine per year under normal storage conditions. A bottle that was 6% when purchased may be only 4.8% after 12 months on a shelf, and closer to 3.8% after 24 months.

Heat and light accelerate decay. Bleach stored in a garage in a hot climate degrades faster than bleach kept in a cool, dark cabinet. For emergency purposes, we recommend treating any bleach older than one year as if it's 1-2% weaker than labeled — which means bumping your dose by a few drops per gallon and extending the contact time to 60 minutes.

The safest approach is to rotate your bleach supply on the same schedule as your food storage. Write the purchase date on the bottle with a marker. Anything past 18 months should be replaced for emergency use — though it's still fine for laundry.

When Bleach Is Not Enough

Bleach is a disinfectant. It kills living organisms — bacteria, most viruses, Giardia — but it does not remove or neutralize anything that isn't alive. Three categories of contamination make bleach irrelevant:

Heavy metals (lead, arsenic, mercury): Bleach has no effect on dissolved metals. If your water is sourced from an area with known industrial contamination, old lead pipes, or a compromised municipal system, disinfection alone is insufficient. You need a filter rated for heavy metals — a reverse osmosis system or a certified carbon block filter.

Chemical spills and agricultural runoff: If there's been a train derailment, chemical plant fire, or significant agricultural contamination upstream, bleach will not help. These events require evacuation, bottled water, or a filter specifically rated for chemical removal. No amount of bleach makes chemically contaminated water safe.

Cryptosporidium: This chlorine-resistant parasite is the one biological threat that bleach cannot handle at household concentrations. Crypto is shed by infected animals and humans, survives in cold water for months, and was responsible for the 1993 Milwaukee outbreak that sickened 400,000 people. The only reliable household methods for killing Crypto are boiling (one minute at a rolling boil) or a filter rated to NSF Standard 53 for cyst removal.

For extended outages where treatment chemicals may run out or water source quality is unknown, a water generation device removes dependence on questionable sources entirely. If you're putting together a complete water plan, also read our guide to long-term home water storage and make sure water is covered in your 72-hour blackout checklist .