Water intake

Total water is not water you must drink

This is the mistake that runs through almost every calculator for this. The official figures are for total water: everything you drink plus everything in your food. Fruit, vegetables, soup, yoghurt, rice and potatoes are all mostly water, and together they supply roughly 20% of the total — some sources put it as high as 30%.

So a tool that takes the 2.5 litre figure and tells you to drink 2.5 litres has overstated the drinking target by half a litre before accounting for anything else. This page separates the two and makes the drinkable figure the headline.

Why the UK, EU and US figures differ

EFSA sets adequate intake of total water at 2.5 litres a day for men and 2.0 for women, in a temperate climate at moderate activity. The US National Academies set 3.7 and 2.7. That is a big gap for the same species, and the reason is methodological rather than biological: the US figures are essentially the median observed intake from national survey data, not a derived requirement. People drank that much, so that became the adequate intake.

The NHS, meanwhile, suggests six to eight glasses of fluid a day — roughly 1.2 to 2 litres — which is lower again, and is about fluid rather than plain water. All three are shown above. When three expert bodies differ by 70%, treating any one of them as a precise personal target is a mistake.

Does tea, coffee and milk count?

Yes. The diuretic effect of caffeine at normal intakes does not offset the fluid the drink contains, so tea and coffee count towards your total. So does milk, so do soft drinks, so does the water in soup. Alcohol is the exception that genuinely works against you.

For a calorie deficit specifically, the practical point is not hydration but what the fluid brings with it — a few sweetened drinks a day can be several hundred calories that never register as eating. Thedeficit calculator has the arithmetic.

When you need more

Sweating is the big one: roughly 350 to 700 ml per hour of exercise, though the between-person variation is enormous. Heat and humidity add more, as does fever — about 10% per degree above normal. Pregnancy and breastfeeding raise requirements. Older adults need slightly less per kilogram, and also feel thirst less reliably, which is its own risk.

You can drink too much

Worth saying plainly, because almost no calculator does. Drinking far beyond what you need can dilute blood sodium to dangerous levels — hyponatraemia — and the people it happens to are disproportionately those drinking to a target rather than to thirst, including endurance athletes. Do not force fluids to hit a number off a web page.

And the most important caveat on this page: anyone with heart failure, kidney or liver disease, or taking diuretics may be under a prescribed fluid restriction. If a clinician has given you a limit, that limit replaces everything here.