How much water should you really drink each day?

Where the 8-glass rule comes from

The famous advice to drink eight glasses a day was never based on a real measurement of human needs. It comes from an old recommendation that also counted the water contained in food, which most people forgot to include. In practice, roughly 20 to 30% of your daily water comes from what you eat: soups, fruit, vegetables, yoghurt.

What your body actually needs

An average adult loses about 2 to 2.5 litres of water a day through urine, sweat, breathing and digestion. That loss goes up in hot weather, at altitude, during fever, and with intense exercise. Rather than counting glasses, replace what you lose.

Two simple signals

• Thirst: in healthy adults it is a reliable alarm. Drink when you feel it, and do not force litres beyond it.
• Urine colour: pale straw means you are well hydrated. Dark amber means drink more. Completely clear all day long usually means you are drinking more than needed.

Who needs to pay more attention

Older adults feel thirst less clearly and should drink on a schedule. People prone to kidney stones benefit from a higher intake spread across the day. Anyone with heart failure or advanced kidney disease should follow the volume their doctor sets, not a general rule.

Can you drink too much?

Yes, though it is rare. Drinking several litres in a very short time can dilute the sodium in the blood, a dangerous state called hyponatraemia, sometimes seen in endurance runners. Spread your intake over the day instead of drinking huge amounts at once.