Why three days
Most people can survive around three days without drinking, and three days is also roughly how long UK utilities and councils take to stand up bottled water stations after a mains failure. Bridging that window yourself is the whole job.
Do the maths once
Three litres per person per day covers drinking and basic hygiene. Four people for three days is about 36 litres — two collapsible carriers, or a stack of supermarket bottles rotated every six months.
Have a treatment option
Storage covers a clean supply going away. Contamination is the other case: a filter bottle, purification tablets or a rolling boil for one minute all make questionable water drinkable.
Don't forget the toilet
No mains means no flush. A bucket of grey water — bath, butt or rainwater — refills a cistern and keeps a household civil far longer than people expect.
