Hydration by Activity Calculator
Estimate daily water needs from body weight plus extra fluid for exercise, so your intake scales with how active your day is.
How it works
Baseline = weight(kg) × 35 ml · Exercise = minutes ÷ 30 × 400 ml · Hot climate adds 15% of baseline
A common baseline for fluid is about 30–35 ml per kilogram of body weight, covering losses through breath, skin, urine and stool. Exercise raises the need because sweat can cost several hundred millilitres every half hour, so this tool adds roughly 400 ml per 30 minutes of activity, plus a margin in hot or humid conditions. Food and other drinks also contribute, and thirst plus pale-yellow urine remain the best day-to-day guides — needs vary with health, medication and pregnancy.
Worked example
A 70 kg person: baseline 70 × 35 = 2,450 ml. Add 45 minutes of exercise → 45 ÷ 30 × 400 = 600 ml. In a temperate climate the total is about 3,050 ml, roughly 3.1 L or 13 cups.
Frequently asked questions
Does coffee and tea count toward hydration?
Yes. Despite a mild diuretic effect, caffeinated drinks still add net fluid, and food provides around 20% of intake. The target here is total fluid, not plain water only.
How much extra should I drink when exercising?
Sweat losses vary, but adding roughly 400–600 ml per 30 minutes of moderate exercise is a reasonable starting point. Weigh yourself before and after long sessions to fine-tune, replacing about 1.5 L per kg lost.
Can I drink too much water?
Rarely, but yes — over-drinking far beyond needs can dilute blood sodium (hyponatraemia), which is dangerous. Spread intake through the day and let thirst guide you rather than forcing large volumes.
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Educational estimate using standard published formulas — not medical, dietary or nutritional advice. Individual needs vary with health status, medication and activity; consult a qualified clinician or dietitian before making changes.