Water Intake Calculator

Baseline (ml) = weight (kg) × 35, + 350 ml per 30 min exercise, + 500 ml if hot/humid climate

Worked example: a 160 lb (72.6 kg) person with 30 minutes of daily exercise in a temperate climate: baseline = 72.6 × 35 ≈ 2,540 ml, plus 350 ml for exercise = 2,890 ml (about 2.89 liters, 97.7 fl oz, or roughly 12.2 glasses). The same person exercising 60 minutes in a hot climate instead needs about 3.74 liters (126.5 fl oz) — the extra exercise and heat add up.

The classic "8x8" rule — eight 8-ounce glasses, about 1.9 liters — is easy to remember but treats everyone the same regardless of body size or activity. A larger or more active person typically needs meaningfully more than 8 glasses, while a smaller, sedentary person in a cool climate might genuinely need close to that amount.

A weight-based estimate like this calculator's gives a more individualized starting point than a flat rule, though actual needs still vary by individual factors like sweat rate, diet, and overall health that no general calculator can fully capture.