Water Intake Calculator
Estimate how much water to drink per day based on your weight.
A simple daily water guideline based on your body weight. Enter your weight to see an estimated daily intake in liters and cups. This is a general starting point — your real needs depend on activity, climate and health.
Formula / method
Examples
≈ 2.3 L
≈ 3.0 L
The weight-based estimate
A widely cited rule of thumb ties daily fluid needs to body weight, at roughly 30 to 35 milliliters per kilogram (about 0.5 to 1 ounce per pound). By that guideline a 70 kg adult lands near 2.1 to 2.45 liters a day. This is a starting baseline for a typical person, not a strict medical prescription, and the older 'eight glasses a day' saying is just a rough approximation of the same range.
Factors and caveats
Real needs shift with heat, exercise, altitude, pregnancy, breastfeeding, and illness, all of which raise them. You also do not have to drink every drop: food and other beverages supply around a fifth of total water intake. Practical signals beat any formula, so listen to thirst and aim for pale straw-colored urine. This estimate is not medical advice; anyone with kidney, heart, or other conditions that affect fluid balance should follow their clinician's guidance.
Where it's used
- Setting a rough daily hydration target from body weight
- Adjusting fluid goals upward for hot weather or exercise
- Planning how much water to carry on a hike or long flight
- A quick baseline for a general wellness or fitness routine
- Converting a litre target into cups or millilitres
Real-world examples
- A 70 kg adult lands around 2.3 litres a day by the weight-based guideline
- A 90 kg adult works out closer to 3.0 litres a day
- 2.3 litres a day is roughly nine 250 ml cups
A bit of history
The popular 'eight glasses a day' rule appears to trace to a 1945 US Food and Nutrition Board note recommending about 2.5 litres of water daily, whose very next sentence, widely forgotten, added that most of that comes from food.
Did you know?
In a 2002 review, kidney specialist Dr Heinz Valtin searched for evidence behind the '8x8' rule and found none, concluding it had never been shown that healthy adults must drink that much on top of the water already in their food.
FAQ
Is this medical advice?
No. It’s a rough guideline, not medical advice. Activity, weather, diet and health all change how much water you need — adjust accordingly.