How Much Water Should I Drink a Day?

Where the eight-glasses figure came from

The familiar "eight glasses a day" has no strong clinical origin. It is usually traced to a 1945 US Food and Nutrition Board note recommending roughly 2.5 litres a day for adults — a note whose very next sentence pointed out that most of that quantity is already present in food. That second sentence dropped out of circulation, and the number stuck.

Modern guidance is expressed as total daily water from all sources, food included.

Subtract the food contribution and the drinking target lands near 3.0 L for men and 2.2 L for women — the origin of the common 8–12 cup advice, though the range across individuals is wide.

By body weight

A more individual approach scales with body mass, commonly 30–35 ml per kilogram per day:

These are baselines for a sedentary day in a temperate climate. Almost everything you do adjusts them upward.

Adjustments

Sweat rates vary enormously between people — from roughly 0.5 to over 2 litres an hour during hard exercise. Weighing yourself before and after a long session gives a far better personal figure than any table: each kilogram lost is roughly a litre of fluid to replace.

What counts

Nearly all fluids contribute. Tea and coffee count despite their mild diuretic effect, which is more than offset by their water content at normal intakes. Milk, juice and soft drinks count too, though they bring sugar or calories with them.

Food supplies a real share: cucumber, watermelon, lettuce and tomatoes are over 90% water, and soups, yoghurt and fruit all add up across a day.

Reading your own signals

Urine colour is the practical everyday indicator. Pale straw suggests adequate hydration; dark amber suggests drinking more. It is imperfect — B vitamins and some medications tint urine regardless — but it needs no equipment.

Thirst is a reliable guide for most healthy adults under normal conditions. It becomes less dependable in older adults, during intense exercise, and in very hot weather, which is where a deliberate schedule helps.

More is not always better

Drinking far beyond need can dilute blood sodium, a condition called hyponatremia. It is rare but genuinely dangerous, and shows up mainly in endurance events where athletes drink heavily over many hours while losing sodium through sweat. The practical guidance is to drink to thirst plus a planned amount during long efforts, and to include electrolytes when sessions run beyond about 90 minutes.

If you have kidney or heart conditions, or take medication affecting fluid balance, target intake is a medical question rather than a general one — the numbers above assume an otherwise healthy adult.

Get a figure for your own weight and activity level in the water intake calculator, and if you are tracking training load, the TDEE calculator covers the energy side.