Daily Water Intake Calculator
A rough daily hydration range from body weight, activity level, and climate. Individual needs vary widely — thirst and light-yellow urine color are usually better guides than any formula.
Daily Water Range
Where the "8 glasses a day" rule came from
The famous “8 × 8 oz glasses” recommendation has no strong scientific origin. It's traceable to a 1945 National Research Council recommendation of about 2.5 liters of water per day — but that same document noted that most of this comes from food and other beverages, not deliberate water intake. The “8 glasses” version dropped the food-and-beverages qualifier and became folklore.
A more evidence-based baseline
The Institute of Medicine (2005) suggested adequate intake of about 3.7 liters/day for men and 2.7 liters/day for women, including water from all sources (food is roughly 20% of intake). This calculator produces a body-weight-scaled range (roughly 0.5 oz per lb, adjusted for activity and climate) as a starting point.
Better guides than the calculator
Thirst is a functional signal in healthy adults — drink when you're thirsty. Urine color is a fast check: pale yellow = well hydrated; dark yellow or amber = drink more. If you're working out hard in the heat, weigh yourself before and after; a pound of body-weight loss is roughly 16 oz of water to replace.
When to be careful
Heart failure, kidney disease, SIADH, and some medication regimens require restricted or increased fluid intake set by a clinician. Very heavy exercise + very high water intake without electrolytes can cause hyponatremia (dangerously low blood sodium) — endurance athletes should supplement with electrolytes, not just water. This calculator is not for anyone in these categories.