Water Intake Calculator
Daily hydration target from body weight.
Inputs
This calculator gives a general estimate and isn't medical advice. It doesn't account for your individual health history — talk to a doctor before making decisions based on it.
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Formula
Daily water (ml) = weight (kg) × 35 + activity bonus (0 / 250 / 500 ml)
Base intake scales with body mass at 35 ml per kilogram. Low activity adds nothing; moderate adds 250 ml; high adds 500 ml for extra sweat losses.
Worked example
A 70 kg person with moderate activity needs about 2.7 L per day (2,450 ml base + 250 ml bonus) — roughly 11 glasses at 250 ml each.
Where this can give the wrong answer
- Hot climate, altitude, pregnancy, and illness can require more fluid than this weight-based rule captures.
- The 'high' activity bonus is a flat 500 ml — marathon training in summer may need substantially more than the formula suggests.
- Glass count rounds to the nearest whole glass; partial glasses aren't shown, so 2.6 L and 2.7 L may both display as 11 glasses.
FAQ
- This calculator measures total fluid intake guidance, not plain water only. Moderate caffeine drinks still contribute to hydration for most healthy adults, though water remains the simplest baseline.