Daily Water Intake Calculator

Daily water intake: 35 mL per kg

The reference most guidelines fall back on is 35 mL of water per kilogram of body weight per day, the figure used by the U.S. National Academy of Medicine and the Brazilian Federal Council of Medicine. Apply it to a 70 kg adult and you land at roughly 2.45 L/day counting every fluid combined. The well-known 8 × 8 rule (eight 250 mL glasses = 2 L) is an older rule of thumb that never had much science behind it, though it still works as a ballpark. Then come the adjustments. Each hour of hard exercise calls for another +0.5 to +1.0 L to make up for sweat; pregnancy adds +0.3 L, breastfeeding +0.7 L, and a hot climate pushes the number higher still. As an example, an 80 kg person training for an hour in summer needs about 2.8 + 0.7 + 0.5 = 4.0 L. Coffee and tea do count toward this (the diuretic effect is mild at the doses people actually drink), but they're usually tallied on their own to keep things clear.

Physiology: dehydration and hyponatremia

Shed just 1% of body mass in water and you already see your thinking slow, your mood dip, and your physical endurance fall off; at 2% the body starts struggling to regulate its own temperature. The opposite extreme matters too. Drinking well over 1 L/hour for long stretches can trigger exercise-associated hyponatremia, a rare but dangerous dilution of blood sodium that's turned up in marathon runners. Staying well hydrated also cuts the risk of kidney stones (getting urine output above 2 L/day is the main thing that helps) and keeps skin, joints, and the bowels working as they should. Elite athletes step on the scale before and after a session, then drink back about 1.0–1.5 L for every kilogram they lost.