You can live weeks without food and about three days without water. This works out how much to store, how many
containers that is, and — the part people get wrong — exactly how to make a questionable supply
safe to drink: boil times, bleach dosing in drops and millilitres, tablet counts, filtration and
rainwater harvest. Works offline, because a water emergency is usually also a no-internet emergency.
This is general preparedness information, not medical advice. In the UK call 111 for advice
and 999 in an emergency. If your supplier has issued a boil-water notice, follow that notice
rather than anything here.
Make it safe — every method, in order of preference
Rainwater harvest — what your roof actually yields
Never do these five things
Never use scented, coloured, splash-less or "with detergent" bleach. Only plain, unscented sodium hypochlorite with nothing else added. Check the label: if it lists anything besides water, sodium hypochlorite and sodium hydroxide, do not drink what it has touched.
Never mix bleach with anything — especially ammonia, vinegar, toilet cleaner or descaler. That makes chlorine or chloramine gas, which is a genuine way to end up in hospital while trying to prepare for an emergency.
Never assume freezing, letting water stand, or a coffee filter makes it safe. Freezing does not reliably kill parasites; standing kills nothing; a filter removes dirt, not germs.
Never store water in containers that held milk, juice, fuel or chemicals. Milk proteins cling to the plastic and cannot be rinsed out. Use food-grade containers, and label them with the date — empty and refill every six months.
Never drink flood water, however clean it looks. It contains sewage, fuel, agricultural chemicals and dead animals, and chemical contamination is not removed by boiling or bleach.
For how much water a person needs per day in a crisis, see the Crisis Triage Guide in this
catalogue. This tool covers the other half: how to treat it and how to keep it.