Daily Water Intake Estimator
This tool gives a rough daily drinking-water target based on your body weight, using the widely cited rule of thumb of about 35 ml per kilogram plus a small adjustment for how active you are.
How this estimate works
The estimate starts from a common clinical rule of thumb: roughly 35 ml of fluid per kilogram of body weight per day for a healthy adult. To that baseline it adds an allowance for physical activity — nothing extra for a light day, about 500 ml more for a moderately active day, and about 1,000 ml more for an intense one. The result is an approximate target for water and other drinks. For wider context, the National Academies (NASEM) set reference Adequate Intake values of about 3.7 litres of total water per day for men and 2.7 litres for women, but those totals also count the water you get from food, which is why a drinks-only estimate is usually a little lower.
Treat the number as a starting point, not a precise requirement. Real fluid needs shift with temperature and humidity, altitude, how much you sweat, your diet, pregnancy or breastfeeding, fever or illness, and simple day-to-day variation. A single weight-based figure cannot capture all of that, and thirst plus urine colour (pale straw is a reasonable everyday sign) are still useful cues that no formula replaces. The activity adjustments here are a general guide, not measured sweat losses.
This estimator is not medical advice and is not designed for anyone whose condition affects fluid balance. If you have heart failure, kidney or liver disease, are on a prescribed fluid restriction, or take diuretics or other medicines that change how your body handles water, do not use a generic target — follow the plan your clinician has given you. Both too little and too much water can cause harm, so if you are unsure how much you should drink, or you have symptoms like persistent dizziness, confusion, or swelling, speak with a healthcare professional.
Sources: NASEM — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate
Frequently asked questions
Where does the 35 ml per kilogram figure come from?
It is a long-standing rule of thumb used in nutrition and hospital practice to estimate maintenance fluid needs from body weight, usually quoted as 30 to 35 ml per kilogram per day for healthy adults. It is a practical approximation rather than an exact physiological requirement, and different guidelines land at slightly different numbers.
How is this different from the 3.7 litre figure I have seen elsewhere?
The National Academies (NASEM, formerly the Institute of Medicine) set Adequate Intake reference values of roughly 3.7 litres of total water a day for men and 2.7 litres for women. Those figures count all sources, including the water in food, which is about a fifth of the total. This tool estimates the water you take in from drinks specifically, so its target is normally somewhat lower than the full total-water reference value.
Does the 'eight glasses a day' rule still apply?
Eight 8-ounce glasses (about 1.9 litres) is easy to remember but ignores body size, activity, and climate, so it fits some people and not others. A weight-based estimate is a little more individualised. Neither rule is a hard requirement — for most healthy people, drinking to thirst across the day works well.
Can I drink too much water?
Yes. Drinking far more than you need over a short period can dilute the sodium in your blood, a condition called hyponatremia that can be serious. Use this figure as a loose guide, spread your intake through the day rather than forcing large volumes, and do not chase the number if you are not thirsty. Anyone with heart, kidney, or liver conditions, or on a fluid restriction, should follow their clinician's guidance instead of a general estimate.
