How Much Water Should You Drink a Day?

Is "8 glasses a day" right?

The "8 glasses" rule is a handy reminder, not a scientific target. Real needs vary with body size, activity level, temperature and diet. A widely used estimate is about 30–35 ml of water per kilogram of body weight per day, counting all sources.

Estimate your daily need

Multiply your weight in kilograms by roughly 33 ml. A 70 kg adult needs around 2.3 litres a day from drinks and food combined — food typically supplies about 20% of that. Add 0.5–1 litre for every hour of intense exercise or hot weather.

Signs you are drinking enough

Pale-yellow urine and rarely feeling thirsty are good signs. Dark urine, headaches or fatigue can mean you need more. Spread your intake across the day rather than drinking a large amount all at once.

What changes your water needs

Several factors push your target up or down. Exercise and heat increase losses through sweat, so add fluid on active or hot days. A larger body simply holds and uses more water. Pregnancy and breastfeeding raise needs, as do fever, vomiting and diarrhoea. Altitude and dry indoor heating quietly increase losses too. Older adults often feel thirst less reliably, so drinking on a schedule helps more than waiting for the signal.

Common hydration myths

A few beliefs are worth retiring. "Eight glasses" is a rough guide, not a rule — your size and your day decide the real number. Coffee and tea do not dehydrate you; their fluid outweighs caffeine's mild diuretic effect. Clear urine is not the goal — pale yellow is ideal, and completely clear can mean you are overdoing it. And thirst is a normal, useful signal, not proof you have already fallen behind.

Building a hydration habit

Hitting your target is easier with small routines than with willpower. Start the day with a glass of water, keep a bottle where you can see it, and drink alongside meals so it piggybacks on an existing habit. If plain water feels dull, add slices of fruit, cucumber or a splash of juice — flavour helps most people drink more without noticing. On hot or active days, pre-fill a marked bottle so you can see how much is left, and sip steadily rather than downing a litre at once.

Frequently asked questions

Do coffee and tea count toward hydration?

Yes. Despite caffeine's mild diuretic effect, tea and coffee still add a net positive amount of fluid. Water, milk and most foods count too.

Can you drink too much water?

Rarely, but yes — drinking extreme amounts in a short time can dilute blood sodium (hyponatremia). For healthy adults, spreading normal amounts through the day is safe.

Should I drink more when exercising?

Yes. Add roughly 0.5 to 1 litre per hour of intense activity, and more in the heat, to replace what you lose through sweat.