“Drink eight glasses of water a day” is one of the most repeated health rules. It isn’t wrong exactly, but it isn’t based on strong evidence either.
What the guidance actually says
The US National Academies set an adequate total water intake for women of about 2.7 litres a day — but that’s from all sources, including food, which typically provides around 20%. Tea, coffee, milk and other drinks count toward fluid intake too. Despite the old myth, moderate coffee drinking doesn’t dehydrate you overall.
Thirst works well for most people
For healthy adults, thirst is a reliable guide. Pale yellow urine is a good sign you’re getting enough.
When to pay closer attention
- Hot weather and exercise, when sweat losses rise
- Hot flashes and night sweats, which add fluid loss
- Older age, when the sense of thirst becomes less sensitive
- Constipation or kidney stones, where more fluid helps
- Illness with fever, vomiting or diarrhoea
Can you drink too much?
Yes. Drinking very large amounts quickly can dilute sodium in the blood, which can be dangerous. People with heart or kidney conditions may be advised to limit fluids.
The simple version: drink when you’re thirsty, a bit more when it’s hot or you’re sweating, and keep water to hand.