Emergency Water Purification Calculator (Bleach Drops, Tablets & Boiling)

How it works

CDC dose: 8 drops (≈ 0.4 mL) of 6% bleach per US gallon = 0.106 mL per litre ≈ 6 mg/L; scale by 6 ÷ strength; × 2 for cloudy or cold water; 30 min contact · 1 mL ≈ 20 drops; 1 tsp ≈ 4.93 mL · boil 1 min (3 min above 2,000 m) · minimum supply 4 L per person per day

Household bleach is a concentrated chlorine solution, so a few drops in a gallon brings water to the few milligrams per litre that kill bacteria and viruses in half an hour; the dose scales with the volume, up with weaker or older bleach and doubles for cloudy or cold water where particles and low temperature slow the chemistry. It is the fastest way to treat large stored quantities, tablets do the same job in measured form, and boiling — which needs no measuring and also defeats the chlorine-resistant parasite Cryptosporidium — is the fallback whenever fuel allows. None of them removes chemicals, which is why source matters as much as treatment.

Worked example

Treating a 20-litre jerry can of clear water with regular 6% bleach takes about 2.1 mL — roughly 42 drops or 0.43 teaspoon — for about 6 mg/L of chlorine; cloudy water would need double. Four people for two weeks need about 224 L at the 4 L minimum, which is 24 mL of bleach from a single bottle, 224 one-litre tablets, or about 7 L of stove fuel to boil.

Frequently asked questions