How much water should you drink a day?

Where the eight-glasses idea comes from

The figure is a rough rounding of general public health guidance that total fluid intake for adults tends to fall in the range of about two to three litres a day. Crucially, that total includes fluid from food, tea, coffee and other drinks — not just plain water.

Many foods, especially fruit and vegetables, are largely water. For a typical diet, food contributes a meaningful share of daily fluid, which is why the amount you need to drink is usually less than the total fluid figure suggests.

Estimating your own baseline

A common starting estimate is roughly 30-35 millilitres of fluid per kilogram of body weight per day for a moderately active adult in a temperate climate. A 70 kg adult therefore lands around 2.1-2.5 litres of total fluid, some of which comes from food.

Then adjust for the obvious variables. Exercise increases needs, and so does heat. Altitude increases fluid loss through faster breathing. Illness with fever, vomiting or diarrhoea raises needs considerably and is a case where professional advice is appropriate.

• Body size — larger bodies generally need more fluid
• Activity — add fluid for exercise, more for long or sweaty sessions
• Climate — heat and dry air both increase losses
• Diet — more fruit and vegetables means more fluid from food

Simple signs you are drinking enough

The most practical day-to-day indicator is urine colour. Pale straw-coloured urine generally suggests adequate hydration; consistently dark yellow suggests you could drink more. This is a rough guide, not a medical test, and some supplements and foods alter the colour.

Thirst is also a reasonable signal for most healthy adults, particularly when water is freely available. It becomes less reliable during intense exercise, in very hot conditions and for some older adults, where a planned drinking schedule works better.

Can you drink too much?

It is possible, though uncommon, to drink more water than your kidneys can excrete, which dilutes the sodium in your blood. This is a genuine risk mainly during endurance events when large volumes of plain water are consumed over several hours without electrolytes.

For everyday life the risk is negligible, but the idea that 'more is always better' is not accurate. Spreading fluid across the day and including electrolytes around long, sweaty exercise is a sensible approach.

Practical habits that help

Rather than tracking to the millilitre, build a couple of anchors: a glass with each meal, and a bottle you refill during the day. Carry water when you exercise and drink to thirst with a little extra in the heat.

Coffee and tea count towards fluid intake for most people. The mild diuretic effect of caffeine does not typically outweigh the fluid the drink provides, though very high caffeine intakes are worth moderating for other reasons.