How Much Water Should You Drink a Day?

Where "8 glasses" comes from

The rule is memorable but arbitrary — eight 8-ounce glasses is about 1.9 litres, which happens to be a reasonable baseline for many adults, but it was never a precise medical target. It also ignores that a lot of your water comes from food and other drinks, not just plain water.

A body-weight estimate

A common practical guide is about 30-35 ml of fluid per kilogram of body weight per day. For a 70 kg adult that is roughly 2.1-2.5 litres from all sources combined. It is an estimate, not a quota — a starting point to adjust from, not a number to force down.

What changes your needs

Exercise and heat raise needs through sweat; a dry climate or high altitude adds more; illness with fever or stomach upset increases them sharply. Caffeine and alcohol have a mild diuretic effect. Larger bodies and higher activity simply need more. There is no single number that fits every day.

How to tell if you are hydrated

The simplest signal is urine colour: pale straw means well hydrated, dark yellow means drink more. Thirst is a decent guide for most healthy people, though it lags in older adults and during hard exercise. You rarely need to overthink it — drink to thirst and check the colour.

Frequently asked questions

Does coffee count? Yes — the fluid in coffee and tea contributes despite the mild diuretic effect; it does not dehydrate you on balance.

Can you drink too much? Rarely, but yes — very high intake in a short time can dilute blood sodium dangerously. Spread intake through the day.

Do I need electrolyte drinks? For everyday activity, water and normal meals are enough; they mainly help during long, intense or very hot exercise.