Where the Number Comes From
Total water need covers what the body loses daily, urine, sweat, breath, and stool, plus what metabolic processes demand. The 30–35 ml/kg heuristic tracks the Institute of Medicine's adequate-intake figures closely for most adults.
A 70 kg adult lands at 2.1–2.5 litres, of which roughly 0.4–0.5 litres typically arrives inside food. So the classic '2 litres of drinks' advice is, for once, close to the science.
Exercise, Heat, and Special Cases
Each hour of training costs roughly 500–1,000 ml of sweat on top of baseline. Hot or humid conditions add more; pregnancy adds about 300 ml daily and breastfeeding 700–1,000 ml.
Athletes in long events need electrolytes with the water, drinking litres of plain water under heavy sweat can dilute blood sodium into dangerous territory (hyponatremia).
The Signs That Actually Work
Thirst is a late signal; by the time it fires, you're already 1–2% down. Better proxies: urine color, pale yellow is on target, dark amber in the afternoon means drink, and morning body weight, which tracks fluid loss during hot training blocks.
Dehydration at just 2% measurably degrades concentration, mood, and endurance. Drink on a schedule in heat and sport, not on feeling.
Sources
FAQ
How much water should I drink daily?
About 30–35 ml per kg of body weight, 2.1–2.5 litres for a 70 kg adult, more with exercise, heat, or breastfeeding.
Does coffee dehydrate you?
No, moderate coffee and tea count toward daily fluids. Alcohol is the genuine dehydrator.
Can you drink too much water?
Yes, rapid litres of plain water can cause hyponatremia (low blood sodium). Spread intake through the day and use electrolytes during long events.
A note before you begin: this tool offers educational estimates only. It cannot replace advice from your doctor or another qualified professional.