Water Intake Calculator
How much water do you really need? Not necessarily eight glasses, that figure was never based on science. Your needs depend on your size, how much you move and your climate. This gives a realistic estimate, and the honest rules of thumb that beat any number.
How the estimate works
A common guide is about 30–40 ml of water per kilogram of body weight, plus extra for exercise and heat.1 Roughly 20% of your water comes from food, so the calculator subtracts that to show how much to actually drink.
Total water ≈ 35 ml × weight(kg) + exercise + climate
The '8 glasses' myth
The famous "eight glasses a day" has no solid scientific origin, and it ignores your size, activity and diet. A large, active person in a hot climate needs far more than a small, sedentary one in a cool one. Use your body, not a fixed number.
The rules that beat any calculator
- Watch your urine. Pale straw means you're hydrated; dark yellow means drink more. This is the most reliable everyday signal.
- Drink to thirst. For most healthy people, thirst is a good guide, contrary to the myth that thirst means you're 'already dehydrated'.
- Add for sweat. Hard exercise and heat raise needs a lot; drink extra around workouts.
Common questions
Does tea, coffee or milk count?
Yes. All fluids count toward hydration, including tea and coffee. The mild diuretic effect of caffeine is small and doesn't cancel out the water in the drink. So do watery foods like fruit and soup.
Can I drink too much water?
Rarely, but yes, drinking huge amounts very fast can dangerously dilute your blood sodium. For normal people this isn't a concern; just don't force litres beyond thirst.
Does drinking more water help weight loss?
A little, indirectly. Water is filling, has no calories, and swapping sugary drinks for it cuts calories. It won't 'boost metabolism' meaningfully, but it helps you eat less.
Want the full plan, not just a number?
Lean28 turns these numbers into a 28-day meal and workout plan, free.
Build my free planSources
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific opinion on dietary reference values for water. EFSA Journal. 2010;8(3):1459. Source
This calculator is general information, not medical advice, and its results are estimates for healthy adults. Speak to a doctor or registered dietitian before making significant changes, especially if you are pregnant or breastfeeding, under 18, or manage a medical condition.
