Water Purification Guide
Interactive reference for emergency water purification methods, calculators, and storage planning.
Water Needs Calculator
FEMA recommends 1 gallon per person per day (drinking + sanitation).
Boiling
Boiling is the most reliable method to kill pathogens. Bring water to a rolling boil.
- Filter cloudy water through a clean cloth first.
- Bring to a vigorous rolling boil.
- Boil for the time indicated by your altitude (see calculator below).
- Let cool naturally. Do not add ice.
- Store in clean, sanitized containers.
Boil Time by Altitude
At higher altitudes water boils at lower temperatures, requiring longer boil times.
Chemical Treatment
Bleach Disinfection
Use regular unscented liquid chlorine bleach (5.25–8.25% sodium hypochlorite).
If water is cloudy or very cold, double the amount. If no chlorine smell after 30 min, repeat the dose and wait 15 more minutes.
Iodine Tablets
- Follow package directions (typically 1 tablet per liter for clear water).
- Wait at least 30 minutes before drinking (longer in cold water).
- Not recommended for pregnant women or people with thyroid conditions.
- Effective against bacteria and viruses; less reliable against Cryptosporidium.
- Water may have a slight taste; vitamin C tablets can neutralize it after treatment time.
Filtration Methods Comparison
| Type | Pros | Cons | Est. Cost |
|---|---|---|---|
| Ceramic Filter | Removes bacteria & protozoa; long lifespan; no power needed | Slow flow rate; does not remove viruses; fragile | $25–$80 |
| Activated Carbon | Removes chemicals, chlorine, bad taste & odor; affordable | Does not remove bacteria or viruses; requires replacement | $15–$50 |
| UV Purifier | Kills bacteria, viruses & protozoa quickly; no chemicals | Requires batteries/power; ineffective in cloudy water; no filtration of particles | $50–$130 |
| Reverse Osmosis | Removes virtually all contaminants including heavy metals | Expensive; wastes water; slow; requires pressure/power; bulky | $150–$500+ |
Tip: For best results in emergencies, combine methods (e.g., filter sediment first, then boil or use UV).
Solar Disinfection (SODIS)
SODIS uses UV-A radiation from sunlight to destroy pathogens. Endorsed by the WHO for emergency use.
- Use a clean, transparent PET plastic bottle (up to 2 liters). Remove labels.
- Fill with clear water. If cloudy, pre-filter through cloth until you can read newspaper through the bottle.
- Shake the bottle vigorously for 20 seconds with a small air gap to oxygenate.
- Place horizontally on a reflective surface (corrugated metal roof is ideal) in direct sunlight.
- Sunny/partly cloudy: 6 hours minimum.
- Overcast: 2 consecutive days.
- Rain/heavy clouds: Do not use SODIS; boil instead.
- Drink directly from the bottle to avoid recontamination.
Does not remove chemical contaminants. Glass bottles block UV; use PET only.
Water Storage Checklist
Maintain a reliable emergency water supply with this checklist.
Further reading
Emergency Water to Store, by Household Size
Produced by the calculator above, at one gallon per person per day — the figure that covers drinking and basic sanitation together, not drinking alone. Find your household size on the left and the length of outage you are planning for across the top. Pets, hot weather, illness and anyone who is pregnant or nursing all push the requirement up.
| Household | 3 days | 1 week | 2 weeks | 30 days |
|---|---|---|---|---|
| 1 person | 3 gal | 7 gal | 14 gal | 30 gal |
| 2 people | 6 gal | 14 gal | 28 gal | 60 gal |
| 3 people | 9 gal | 21 gal | 42 gal | 90 gal |
| 4 people | 12 gal | 28 gal | 56 gal | 120 gal |
| 5 people | 15 gal | 35 gal | 70 gal | 150 gal |
| 6 people | 18 gal | 42 gal | 84 gal | 180 gal |
| 8 people | 24 gal | 56 gal | 112 gal | 240 gal |
표의 모든 값은 이 페이지의 계산기가 직접 계산한 결과입니다 · 1 gallon per person per day · drinking and sanitation · stored water needs rotating — check the EPA guidance linked below before relying on any of it, and treat your local emergency authority's advice as the one that counts
Interactive emergency water purification guide with boiling time calculator, chemical treatment ratios, filtration comparison, solar disinfection steps, and water storage checklist.
Related tools
Sources
Boiling times, chlorine dosing and filter pore sizes should be checked against the EPA's emergency disinfection guidance before you rely on them.
Links checked 2026-09-09.
Frequently asked questions
How long do I need to boil water, and why does altitude change it?
One minute at ordinary elevations, three minutes above 6,562 feet (2,000 m) — that is the split the boil-time calculator on this page uses. Altitude matters because water boils at a lower temperature as pressure drops, so the same rolling boil is doing less to the organisms in it and needs longer to finish the job.
Does boiling remove chemicals and heavy metals?
No. Boiling kills organisms, but it does not remove lead, nitrates, pesticides or other dissolved chemicals — and because water evaporates, it can concentrate them slightly. For chemical contamination the answer is a different water source, not more heat.
Will a filter alone make water safe to drink?
It depends on what is in the water and what the filter is rated for. Pore size decides which organisms are held back, and a filter that stops protozoa may pass viruses. Treat filtering and disinfection as two separate jobs rather than assuming one covers the other.