Hydration · lesson 2 of 9
How much water do you actually need?
A real per-bodyweight target — and why the '8 glasses' rule is a myth.
5 min read · reviewed October 2026
The famous "8 glasses a day" has no real scientific basis — it ignores that you're a different size from everyone else, and that food and other drinks count too (about 20% of intake typically comes from food). A better baseline scales with your body.
Roughly how much TOTAL water (from drinks + food) does a 70 kg / 154 lb adult need on an average day?
Somewhere between 1,000 and 4,500 mL.
Reveal the answer →
~2.5 L (35 mL/kg)
A common, defensible baseline is about 35 mL per kg of body weight per day of total water. For 70 kg that's ~2.45 L. Roughly 80% comes from drinks and 20% from food. Heat, exercise, altitude, illness, caffeine, alcohol, and high-protein or high-salt diets all push the number up.
Where 35 mL/kg comes from
Bigger bodies hold and turn over more water, so a per-kilogram rule fits better than a fixed number of glasses. ~35 mL/kg/day is a sensible starting point you then adjust for heat and activity.
How it works →
Daily water turnover is driven by obligatory losses: urine (to excrete the solute load from food), insensible loss through skin and breath, and sweat. These scale with body size, metabolic rate, and environment. Per-kg targets approximate that turnover; the kidneys then fine-tune by concentrating or diluting urine via ADH (vasopressin).
What the studies show →
The U.S. National Academies set an adequate intake (for young adults) of ~3.7 L/day total water for men and ~2.7 L for women from ALL sources — figures that already include water from food and that vary with body size and activity. The 35 mL/kg rule is a practical per-bodyweight approximation that lands in a similar range and matches observed adult water turnover. There's no high-quality evidence that pushing far beyond your needs improves health in well-hydrated people; the benefit is in avoiding a deficit, not in 'over-drinking.'
Formulate estimates your personal target from your body weight and shows today's intake against it.