Body Surface Area and Daily Water Intake: The Formulas Explained
The Mosteller body surface area formula and the weight-based water intake guideline, explained with real numbers.
Body surface area and daily water intake are both estimated from body size, but for very different practical purposes — one for medical dosing, the other for everyday hydration guidance.
Body surface area: the Mosteller formula
BSA = √((height in cm × weight in kg) / 3600). For someone 175cm tall weighing 70kg: BSA = √((175×70)/3600) = √(12,250/3600) = √3.403 ≈ 1.845 m². This is the version most commonly used in clinical settings today for its simplicity and reasonable accuracy across a wide range of body sizes.
Why BSA matters more than weight alone for some medications
Certain medications — most notably chemotherapy drugs — are dosed per square meter of body surface area rather than by weight alone, because BSA correlates more closely with metabolic rate and organ function than weight does on its own. Two people of identical weight but different heights can have meaningfully different BSA, and therefore different appropriate doses.
Daily water intake: a simple weight-based guideline
A commonly cited guideline suggests roughly 33ml of water per kilogram of body weight per day, with additional water for exercise. For a 70kg person: 70 × 33 = 2,310 ml (about 2.3 liters) as a baseline, before accounting for activity. Adding 30 minutes of exercise (roughly 350ml bonus per the common estimate) brings the total closer to 2,660ml.
Why "8 glasses a day" is a rough approximation at best
The commonly repeated "8 glasses a day" guideline doesn't account for body size at all — a 50kg person and a 100kg person have very different fluid needs, which the simple weight-based formula captures far better than a fixed, one-size-fits-all number.
Common mistakes to avoid
- Applying BSA-based dosing guidance from a general source rather than a prescribing clinician — medication dosing should always come from a qualified professional, not a general calculator
- Forgetting that roughly 20% of daily hydration typically comes from food, not just direct fluid intake, when comparing a "water intake target" to actual daily drinking habits
- Assuming water needs stay constant regardless of climate, altitude, or illness — all of which meaningfully increase fluid requirements beyond the baseline formula
Calculate your own numbers with the body surface area calculator and water intake calculator.