How the target is built
The calculator uses the common ~33 ml per kg of body weight baseline, then adds roughly 350 ml per 30 minutes of exercise to cover sweat losses. For a 70 kg person training 45 minutes, that lands near 2.8 litres — in line with general adequate-intake guidance once you account for the ~20% of fluids food provides.
Reading your own gauges
- Pale-straw urine is the classic adequate-hydration signal; dark yellow suggests drinking more.
- Needs climb with heat, altitude, illness and heavy sweat rates — the formula can't see your weather.
- Thirst lags slightly behind need during long exercise; drinking to a plan beats drinking to thirst in sessions over an hour.
Frequently asked questions
Is 8 glasses of water a day right?
Eight 250 ml glasses is 2 litres — a reasonable middle for smaller adults but light for larger or active people. Weight-based estimates personalize it better than a fixed glass count.
Does coffee count toward water intake?
Yes — caffeine's diuretic effect is mild and habitual coffee drinkers retain most of the fluid. Caffeinated drinks count, just less efficiently than water.
Can you drink too much water?
Yes — excessive intake without electrolytes can cause hyponatremia, mainly a risk in endurance events. For everyday life, spreading intake through the day makes overdoing it very hard.
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Sources
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press. 2005. [link]