The "8 glasses" myth
The famous "8 glasses a day" rule has no scientific origin — it appears to come from a 1945 US nutrition board note that most adults need about 2.5 liters of water total, most of which was already in food and other drinks. Actual needs vary hugely: a 50 kg office worker in a cool climate needs far less than a 90 kg marathoner in July. The calculator above starts from the common clinical rule of ~30–35 ml per kg of body weight, then adjusts for the big variables.
Signs you're dehydrated
- Dark yellow urine (light straw = well hydrated)
- Headache, fatigue, trouble concentrating
- Dry mouth, lips, skin
- Feeling thirsty — by then you're already mildly down
When you need more
Exercise (especially sweating), hot weather, fever or illness, high altitude, and alcohol (a diuretic). Pregnant women need roughly +300 ml/day; breastfeeding mothers +700–1000 ml/day because milk production is thirsty work.
Can you drink too much?
Yes, in extreme cases. Drinking several liters in a short window without electrolytes can cause hyponatremia (low blood sodium) — the main risk in marathon "water-loading". Normal daily habits won't get you there; your kidneys handle ~0.8–1 L/hour. This tool is general guidance, not medical advice — talk to a doctor about specific conditions.