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Daily Water Intake Calculator

A weight-based estimate of how much to drink, plus the two signals that tell you more than any number on this page.

—litres a day

Your estimated daily intake from drinks, on a 1 to 5 litre scale LowerTypicalHigherAthlete / heat1L2L3L4L5L —
Your estimate covers drinks only. Food adds roughly another fifth on top, which is why this number is lower than the "total water" figures you may have seen.

The eight-glasses rule was never a finding

It appears to trace back to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day — which included the sentence, widely dropped in later retellings, that most of that quantity is already present in prepared foods. The number survived; the caveat did not.

Roughly a fifth of your daily water comes from food, not drinks Where a day's total water actually comes from ~50%Plain water~30%Other drinks~20%Food Proportions vary with diet — a salad-and-soup day sits well above 20% from food.
Based on National Academies of Sciences intake data. Fruit, vegetables, soup, yoghurt and cooked grains all carry substantial water — which is why a fixed glass count has never matched what anyone actually needs.

The National Academies' reference values are about 3.7 litres of total water a day for men and 2.7 for women, food included. Strip out the food share and you land near 3.0 and 2.2 litres from drinks — close to what this calculator produces for an average-sized adult, and a long way from a rigid eight glasses.

Two signals that beat any formula

Your body regulates fluid far more precisely than a weight-based estimate can. Thirst is a genuine physiological signal, not a sign you already failed — and for healthy adults under normal conditions, drinking to thirst is enough. Urine colour is the second check: pale straw is fine, persistently dark suggests you are behind.

Where deliberate tracking helps is when those signals are blunted or outpaced — older adults, whose thirst response weakens with age; anyone exercising hard in heat, where sweat losses outrun thirst; and during illness with fever, vomiting or diarrhoea.

This estimate is for generally healthy adults. Kidney disease, heart failure and some medications come with specific fluid limits that can be lower than anything here — if you have been given a fluid target by a clinician, that one is the one that counts.

Questions

Do coffee and tea count?

Yes. The mild diuretic effect of caffeine does not cancel out the fluid in the cup — controlled studies find moderate caffeinated drinks hydrate about as well as water. The same is true of milk, juice and soft drinks, though those bring sugar and calories with them.

Can you drink too much water?

Yes, though it is rare outside endurance sport. Drinking far more than the kidneys can excrete — very roughly a litre an hour, sustained — dilutes blood sodium, a condition called hyponatraemia that can be serious. It is a real risk for marathon runners who drink at every station, and the reason sports drinks contain sodium.

Is pale urine really the test?

It is the most practical one. Pale straw means you are generally well hydrated; consistently dark yellow suggests you are behind. It is imperfect — B vitamins colour urine bright yellow and some medications alter it — but for everyday purposes it beats counting glasses.

References

  1. National Academies of Sciences — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate
  2. CDC — Water and healthier drinks
  3. EFSA — Scientific opinion on dietary reference values for water
  4. Maughan & Shirreffs — beverage hydration index research, American Journal of Clinical Nutrition

Reviewed September 2026. VitalTally calculators are informational and are not medical advice.