Estimate water for a camping trip.
Water demand splits into drinking, which scales with climate and body count, and utility water for cooking and washing, which scales with the trip rather than the people. Drinking needs range from 2.5 litres per person per day in cool conditions to 6.5 in desert heat — a factor of more than two, and the single most important input here. Whether you carry the whole requirement depends entirely on having a reliable source on site: if you do, you carry one day plus a two-litre-per-person reserve against the source failing. If you do not, you carry everything plus 15 percent, and since water weighs a kilogram a litre that number governs how you travel.
Total requirement
Litres = climate rate x campers x days + utility litres per day x days
Carry-in volume
With a source: one day plus 2 L per person reserve. Without: total x 1.15
General planning guidance, not medical advice. Individual hydration needs vary with body size, exertion, altitude, medication and health conditions, and both dehydration and over-drinking carry real risks. Always treat water from natural sources by filtering, boiling or chemical purification, and seek medical guidance if you have a condition affecting fluid balance.
Because sources fail. Seasonal streams dry up, taps get turned off out of season, and reported springs turn out to be a trickle or contaminated. Two litres per person is a modest hedge that buys you the time to walk out or reach the next source without a genuine emergency.
Almost always, on weight alone. A filter weighs 100 to 300 grams and treats hundreds of litres; the water itself weighs a kilogram per litre. Carrying three days of water for four people is 42 kg. Carry a filter and only the water you need to reach the next source.