Water intake calculator

There is no single right number. Hydration needs scale with body size, how much you sweat, and the heat you're losing it in. This calculator starts from a per-kg baseline and adds for exercise and climate:

Baseline           30 - 35 ml per kg bodyweight
Exercise           +500 - 1000 ml per hour of training (sweat replacement)
Hot climate        +500 - 1000 ml per day (heat / humidity sweat losses)

Total water vs. plain drinking water. The figure above is total water intake. Roughly 20% of it comes from food - fruit, vegetables, soups, yoghurt, even bread carry water. So the amount you actually need to drink is around 80% of the total target. The calculator shows both.

total_water_ml   = baseline + exercise + climate
drinking_water   = total_water × 0.80   (food supplies the other ~20%)

The "8 glasses" myth. Eight 8-oz glasses is about 1.9 L, a tidy number with no scientific basis. It likely descends from a 1945 US Food and Nutrition Board note recommending ~2.5 L of water per day - which explicitly added that "most of this quantity is contained in prepared foods." The food half got dropped, and a rule of thumb was born. Use thirst and urine colour as your real-time gauges; this number is a planning estimate, not a quota to force down.

Caveats. These are estimates for healthy adults in ordinary conditions. Pregnancy, breastfeeding, high altitude, fever, vomiting/diarrhoea, and some medications or kidney/heart conditions change requirements - follow medical advice over a calculator. Overdrinking far beyond thirst can dilute blood sodium (hyponatraemia), so more is not automatically better.