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At 80 kg you need about 2.8 L — plus 350 mL per 30 minutes of exercise and three-quarters of a liter more in heat. The 8-glasses rule was never based on you.
The "8 glasses a day" rule fits nobody — a 55 kg office worker and a 100 kg landscaper have wildly different needs. The weight-based formula (35 mL per kilogram) tracks actual physiology, then adjusts for the two big dehydrators: exercise and heat. This calculator gives the target in liters, ounces, and cups so it maps to whatever bottle you actually carry.
Anyone past the 8-glasses myth uses it: 35 mL/kg says 80 kg needs 2.8 L — plus 350 mL per 30 minutes of exercise and more in heat.
The weight-based answer: 35 mL per kilogram — 2.1 L at 60 kg, 2.8 L at 80 kg, 3.5 L at 100 kg. Official references land in the same neighborhood: EFSA adequate intake is 2.0 L (women) and 2.5 L (men) from all sources including food, and since food supplies roughly 20%, drinking targets run slightly below the formula total. Exercise adds ~350 mL per 30 minutes of moderate sweating, doubled in heat or heavy gear. The honest fine-tuning signal is urine color: pale straw is hydrated, dark yellow says drink, and completely clear all day says ease off. Thirst is a decent guide for healthy adults at rest — it lags during exercise and in older adults, where scheduled drinking works better.
Yes — hyponatremia is the rare but real danger: blood sodium diluted below safe levels by extreme intake, typically several liters in a few hours. The cases that make news are endurance events and drinking challenges — marathoners who force-drink beyond thirst, hazing rituals, ecstasy-related cases. Symptoms start as nausea and headache and escalate to confusion and seizures. The safe ceiling for most adults is roughly 0.8–1.0 L per hour, the kidney's processing rate — daily totals under about 5–6 L spread across the day are fine for healthy kidneys. The paradox worth knowing: endurance athletes collapse from overhydration more often than dehydration, which is why current sports-medicine guidance is "drink to thirst, not to a schedule" during events.
Yes — the dehydration reputation is mostly myth. Caffeine is a mild diuretic, but studies show regular coffee and tea drinkers net-hydrate from those drinks almost as well as from water; the fluid volume outweighs the diuretic effect at normal doses (under ~400 mg caffeine/day). Alcohol is different — it suppresses vasopressin and dehydrates net-net, roughly 100 mL extra urine per standard drink. Sugary sodas hydrate but at a caloric price. What food contributes is underrated: watermelon, cucumber, and lettuce are 90%+ water; even oatmeal and rice carry meaningful water. Total it up and the glass-counting matters less than the habit of having water within reach — intake tracks availability more than intention.