Calculate exactly how much bleach, iodine tablets, filter capacity, or UV treatment you need to purify emergency drinking water for your household — based on your people, duration, and water source.
Access to clean drinking water is the most critical element of any emergency preparedness plan. FEMA recommends storing at least one gallon per person per day for drinking and basic sanitation, but two gallons is a more realistic target when you factor in cooking, basic hygiene, and pet needs. Knowing your purification method — and having enough supplies for it — is just as important as the storage containers themselves.
Select purification method(s) to include in your plan:
Every water purification method has tradeoffs in cost, availability, effectiveness, and convenience. Layering two or more methods — called multi-barrier treatment — is the gold standard because no single method eliminates every threat. For example, filtering removes sediment and many pathogens, while chemical treatment or UV kills remaining viruses. Here is what you need to know about each approach.
Boiling water for one full minute (three minutes above 6,500 feet elevation) kills bacteria, viruses, and parasites including Giardia and Cryptosporidium. It is the most reliable purification method available when you have fuel. The main downsides: it requires a heat source, does nothing for chemical contaminants or heavy metals, and leaves water flat-tasting. Filter water through a clean cloth or coffee filter before boiling if it is visibly cloudy — sediment does not affect safety after boiling, but pre-filtering improves taste.
Regular 8.25% sodium hypochlorite bleach (unscented, no added cleaners) is a powerful, inexpensive disinfectant. The CDC-recommended dose is 8 drops per gallon (about 1/8 teaspoon) for visually clear water and 16 drops per gallon for cloudy or turbid water. A standard 64-ounce bottle of bleach contains roughly 384 teaspoons, which is enough to treat approximately 2,300 gallons of clear water at 8 drops per gallon. After adding bleach, wait 30 minutes before drinking; treated water should have a faint chlorine smell. Bleach loses potency over time — replace your stock every 6 to 12 months.
Potable Aqua, Aquatabs, and similar products each treat one quart (0.25 gallons) of water per tablet. That means four tablets per gallon. Tablets work well against bacteria and viruses but are less effective against Cryptosporidium. Iodine-based tablets should not be used long-term by pregnant women or those with thyroid conditions. Combine with a follow-up neutralizing tablet to reduce iodine taste. Keep tablets sealed and dry — once opened, they degrade within a year.
Hollow-fiber membrane filters physically remove bacteria and protozoa (Giardia, Cryptosporidium) but do not remove viruses — a critical limitation for international travel or severely contaminated sources. The LifeStraw Personal filter is rated for 1,000 liters (264 gallons). The Sawyer Squeeze and Sawyer MINI are rated for 1 million gallons. For emergency preparedness, a Sawyer Squeeze is the far better value per gallon. Always pre-filter visibly turbid water through a bandana or coffee filter to extend membrane life. Backflush Sawyer filters regularly with the provided syringe.
UV pen devices like the SteriPen kill bacteria, viruses, and protozoa by disrupting their DNA. One treatment cycle purifies one liter in about 90 seconds. Battery life varies — most models treat 50 liters per set of AA batteries or 150 to 200 liters per charge cycle on rechargeable models. UV treatment only works on clear water; cloudiness blocks UV light and dramatically reduces effectiveness. Always pre-filter turbid water before UV treatment. UV pens are fast, lightweight, and taste-neutral, but they depend on functioning batteries.
For stream, river, or rainwater sources, pre-filtering through a clean cotton bandana, coffee filter, or improvised sand-and-gravel filter before chemical treatment or UV sterilization dramatically improves both effectiveness and taste. Sediment clogs filters faster, reduces UV penetration, and can shield pathogens from bleach. This step costs nothing and should never be skipped with turbid water.
Treated water must be stored in food-safe containers — HDPE plastic labeled with recycling code 1, 2, or 7 (BPA-free), food-grade stainless steel, or glass. Avoid milk jugs and juice containers; they harbor bacterial residue. Keep stored water out of direct sunlight and away from chemical fumes (gasoline, pesticides). Replace chemically treated water every 6 months; commercially sealed water is good for 2 years. Label containers with the treatment date.
FEMA and the Red Cross cite one gallon per person per day as the absolute minimum for drinking and basic sanitation. That covers roughly half a gallon for drinking and half for food preparation and minimal hygiene. Two gallons per person per day is a more realistic target if you want to include basic washing, pet water, and cooking needs. People in hot climates, pregnant or nursing women, and those doing physical labor need more. This calculator uses one gallon as the minimum and two gallons as the recommended figure — the difference in storage and treatment supplies between those two targets is significant over 30 or 90 days.
Probably not at full strength. Sodium hypochlorite degrades to salt and water over time, especially in heat or direct light. Bleach older than one year may have lost 50% or more of its disinfecting potency. If your bleach smells noticeably weaker than fresh bleach, replace it. You can double the dose with old bleach, but the safest move is to rotate your stock yearly. Store bleach in a cool, dark location and buy the smallest practical size so you replace it more frequently.
Yes, in most emergency scenarios. Flooding, power outages (if your pump is electric), earthquakes, and nearby chemical spills can all compromise well water. A boil advisory from your county health department is the most obvious signal, but you should treat well water any time you suspect contamination even without an official notice. A private well is not tested continuously the way municipal water is — during a prolonged grid-down scenario, your well water quality may degrade without any visible indication. Use bleach or filtering as a precaution when in doubt.
No. Rainwater picks up pollutants, bird droppings, roof debris, and atmospheric contaminants as it falls and runs off collection surfaces. It is not potable without treatment. Filter it first to remove particulates, then disinfect with bleach or UV. Rainwater from a clean metal roof with proper first-flush diverters is a good emergency source after treatment — but never drink it straight. In areas with significant air pollution, heavy industrial activity, or near agricultural spray, rainwater may also contain chemical contaminants that standard purification does not address.