"Eight glasses a day" is repeated so often it feels like medicine. It traces back to a 1945 US recommendation that — in the same sentence — noted most of the water comes from food. The sentence survived; the caveat did not.
What the Evidence Supports
Total fluid needs (all drinks plus food moisture): roughly 2.0L/day for women and 2.5L for men — the EFSA guideline — with 20–30% typically arriving via food. Tea and coffee count: the diuretic effect of normal caffeine intake is far too weak to cancel the fluid.
Scaling Factors
- Body size: ~30–35ml per kg of bodyweight is a better baseline than any fixed number
- Exercise: add 400–800ml per hour of sweat-inducing activity
- Heat: hot days can double losses; so can air-conditioned dryness
- Life stages: pregnancy and breastfeeding add 300–700ml/day
The Best Gauge Is Built In
Thirst plus urine colour outperforms any app: pale straw = fine; dark amber = drink. The kidneys of a healthy person manage the balance to within 1% daily — the body is not waiting for a hydration influencer.
Can You Overdo It?
Yes — drinking litres rapidly around endurance events can dilute blood sodium (hyponatraemia), which is genuinely dangerous. More is not better; matched to losses is better.