💧 Water Intake Calculator
Simple daily water intake estimate from weight and activity. Runs privately in your browser.
Where the per-kilogram rule comes from
The baseline rule of thumb — about 30–35 ml of fluid per kilogram of body weight per day — comes from clinical nutrition practice and lands close to official adequate-intake figures for most adults. The calculator adds a flat allowance for activity, because sweat losses during even moderate exercise are substantial, and reminds you that climate and health conditions shift needs further.
Note the word fluid: all drinks and moisture-rich food count toward the total. Coffee and tea contribute, despite the myth that caffeine dehydrates at normal intakes. You do not need to drink pure water for every millilitre.
Baseline, plus an activity allowance
Daily fluid ≈ weight (kg) × 0.033 litres + activity allowance. The calculator also shows the equivalent in fluid ounces (1 litre ≈ 33.8 fl oz) for US readers.
Hydration myths worth dropping
- Chugging large volumes at once. Absorption is limited; spread intake across the day.
- Counting only plain water. Tea, coffee, milk, soup and fruit all hydrate.
- Drinking to a fixed number while ignoring thirst and urine colour — pale straw urine is the practical everyday signal.
- Ignoring medical advice. Heart, kidney and some other conditions require fluid restriction; the rule of thumb does not apply.
A 75 kg adult, moderately active
A 75 kg adult with moderate activity: baseline is 75 × 0.033 = 2.48 litres, plus the activity allowance, giving roughly 3 litres a day — about 101 fl oz, or twelve and a half 240 ml glasses. On a rest day the same person needs closer to 2.5 litres.
Hydration questions
Is 8 glasses a day a real rule?
It is a rough folk benchmark (~2 litres) that ignores body size. Weight-based estimates like this calculator's are more useful, though still approximate.
Do coffee and tea count?
Yes. At normal intakes the net hydrating effect is positive; the old 'caffeine dehydrates you' claim only applies to large doses in unhabituated people.
What about exercise or hot weather?
Add roughly 0.5–1 litre per hour of sweating activity, more in heat. The activity field gives a baseline bump; adjust upward for intense sessions.
Can I drink too much water?
Rarely, but yes — extreme intake can dilute blood sodium (hyponatraemia). Spread intake; do not force litres beyond thirst during normal days.
Does the output differ for men and women?
Not in this model — it is weight-driven. Average body-size differences show up automatically in the weight input.
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Tip: double-check inputs before relying on results for important decisions.