How many litres of water does an adult need per day from all sources?

How to get close to this number

Reason from losses rather than from advice. An adult loses water through urine, sweat, breathing and digestion at a rate of a couple of litres a day in a temperate climate, and intake has to replace it. Balancing a system against its known outputs is more reliable than recalling a guideline, and it produces the same answer.

The correction in the explanation is the one that matters: a substantial share of intake comes from food rather than drinks. Fruit, vegetables, soup and even bread carry significant water, so the amount you need to drink is meaningfully less than the amount you need to take in. Guidelines quoted as "water to drink" routinely omit this and overstate the requirement.

The eight-glasses rule has no traceable scientific origin, which is worth saying plainly because it is one of the most repeated health claims in existence. Requirements also vary enormously with climate, body size and activity, so a single figure describes a reference adult in a temperate climate and nobody in particular — which is exactly how the source itself frames it.