The "eight glasses a day" rule has no real evidence behind it, and actual needs scale with body size, how much you sweat, and how hot it is. This calculator gives a weight-based baseline and adjusts for exercise and climate, then converts the result into glasses and bottles.
Yes. The diuretic effect of normal caffeine intake is modest and does not cancel out the fluid. Food also contributes meaningfully — fruit, vegetables, and soup can supply 20% of daily intake.
Urine colour is the simplest everyday indicator: pale straw suggests adequate hydration, dark amber suggests you are behind. Thirst is a lagging signal, not an early one.
Yes, though it is rare. Drinking litres in a short period can dilute blood sodium dangerously, a condition called hyponatraemia, seen occasionally in endurance events. Spread intake across the day.
Considerably. Heavy sweating can cost more than a litre an hour, and in that situation electrolytes matter as much as water volume.
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