The "eight glasses a day" rule fits nobody in particular. Water needs scale with body size, rise sharply with exercise and heat, and change in pregnancy. This calculator starts from your weight and adjusts for the rest, then separates the water you get from food from the water you actually need to drink.
How it is calculated
- Base need: about 35 ml per kilogram of body weight for adults, 30 ml/kg from age 65, when thirst signals weaken but total need falls slightly. For a 70 kg adult this gives 2.45 L of total water, close to the 2.0–2.5 L guidance from the European Food Safety Authority and the US National Academies.
- Exercise: about 350 ml for every 30 minutes of moderate activity, the amount an average person loses in sweat.
- Climate: 500 ml more in hot, dry weather; 700 ml in hot, humid weather where sweat evaporates poorly.
- Pregnancy adds about 300 ml a day; breastfeeding about 700 ml, roughly the volume of milk produced.
- About 20% of total water comes from food (fruit, vegetables, soups, yogurt). The main number is what remains to drink.
Example
A 75 kg, 30-year-old who exercises 30 minutes a day in a temperate climate: 75 × 35 = 2,625 ml base, plus 350 ml for exercise, total 2,975 ml. Minus 20% from food, about 2.4 L to drink, or nine to ten glasses of 250 ml.
What counts as water
Everything you drink: water, tea, coffee, milk, juice, soup. Caffeine’s diuretic effect is too small to cancel the fluid in a normal cup. Alcohol is the exception; it removes more water than it adds.
Signs you are getting it right
Urine that is pale yellow most of the day, rarely feeling thirsty, no afternoon headaches. Dark yellow urine or thirst means drink more; clear urine all day and frequent night trips mean you can ease off.
Frequently asked questions
How much water should I drink a day?
Most adults need about 2–3 litres of total water, of which roughly 20% comes from food. A 70 kg person typically drinks about 2 litres; more with exercise, heat or pregnancy. Enter your details above for your own number.
Is it true you need 8 glasses of water a day?
It is a rough average, not a rule. Eight 250 ml glasses is 2 litres, right for a moderately active 60–70 kg adult in mild weather, too little for a 100 kg person who trains, more than a small sedentary person needs.
Does coffee or tea count towards water intake?
Yes. Regular coffee and tea drinkers are not dehydrated by them; the caffeine effect is minor compared with the fluid. Only alcohol causes a net loss.
Can you drink too much water?
Yes, though it is rare. Drinking many litres in a few hours can dilute blood sodium (hyponatremia), mostly a risk for endurance athletes who drink far beyond thirst. Spread intake through the day and include electrolytes on long, sweaty sessions.
How much water should I drink to lose weight?
Water itself does not burn fat, but drinking 500 ml before meals modestly reduces how much people eat, and swapping sugary drinks for water cuts calories. Use your normal target; more is not better.
Sources
- EFSA Panel on Dietetic Products. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010
- Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies, 2005
- Sawka MN et al. Exercise and fluid replacement. ACSM position stand. Med Sci Sports Exerc, 2007
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked