CalcVitalsWater Intake Calculator — How Much Per Day?
A widely used clinical rule is 35 ml of water per kilogram of body weight: at 70 kg that is 2.45 litres a day. The second method on this page — two-thirds of your body weight in pounds, in ounces, plus more for exercise — usually lands a little higher.
How it works
There is no single experimentally ordained number of litres — water needs swing with temperature, activity, diet and body size — so honest calculators show you the methods rather than a verdict. The 35 ml/kg rule comes from clinical practice for estimating adult requirements; the bodyweight rule (drink in ounces two-thirds of your weight in pounds, adding 12 oz per half-hour of exercise) is the sports-medicine rule of thumb, and it usually lands slightly higher because it quietly bakes in activity.
Two facts calm the whole topic down. First, food counts: fruit, vegetables, yoghurt, rice and even bread are mostly water, and together meals typically supply 20–30% of the day's total. Second, for healthy adults in ordinary conditions, thirst is a well-engineered signal — drinking to thirst, with a bit extra around exercise and heat, keeps nearly everyone adequately hydrated without arithmetic.
Where the numbers do earn attention is at the edges: older adults, whose thirst signal dulls; endurance exercise in heat, where sweat losses can pass a litre an hour; illness with fever or vomiting; and anyone whose clinician has set a fluid target or restriction. Those cases follow professional guidance, not rules of thumb.
The method
Method A: 35 ml × body weight (kg). Method B: body weight in lb × ⅔ = fluid ounces, + 12 fl oz per 30 minutes of exercise. Conversions: 1 fl oz = 29.5735 ml; 1 lb = 0.45359237 kg. Total water includes food moisture in real life — these figures are drinking-water estimates by design of the original rules.
Frequently asked questions
Is the '8 glasses a day' rule real?
It is a memorable rounding of roughly 2 litres, with no direct experimental source — and it ignores food water entirely. The calculated methods here, scaled to your body weight, are a better guide, and thirst is a better referee than either.
Can you drink too much water?
Yes, though it is uncommon: forcing down very large volumes faster than the kidneys can clear (roughly 0.8–1.0 L/hour sustained) dilutes blood sodium — the hyponatraemia that has harmed some endurance athletes who drank to a schedule rather than to thirst.
Does coffee count toward hydration?
Yes. Caffeine is mildly diuretic, but the water in the drink outweighs the effect at normal intakes — habitual coffee and tea drinkers hydrate from them about as well as from water in trials.