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Water Intake Calculator

Straight answer first: there is no precise formula for how much water you need — the research does not support one. Below is the common per-kg heuristic, clearly labeled, next to the actual published reference intakes.

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—L / DAY · 30–35 ML/KG HEURISTIC

National Academies adequate intake — total water, food included —

…of which beverages, roughly 80% —

EFSA adequate intake — total water —

Fill your details — every step of the arithmetic appears here.

The math, shown

The honest part first: claims like "everyone needs 8 glasses" have no solid scientific basis (Valtin 2002). For healthy adults in temperate conditions, thirst plus pale-straw-coloured urine is a genuinely reasonable guide. Needs climb with heat, altitude and training.

The heuristic: 30–35 ml per kg of bodyweight is a common clinical rule of thumb — useful as a sanity check, not a scientific requirement. It is labeled a heuristic here because that is what it is.

The published references: the US National Academies set adequate intakes of about 3.7 L/day (men) and 2.7 L/day (women) of total water — food included, with roughly 80% typically coming from beverages (Institute of Medicine 2005). EFSA’s European values are 2.5 L and 2.0 L total (EFSA 2010). Adequate intakes are population reference points, not personal requirements.

Around training: the practical method is weighing yourself before and after a session — each kilogram lost is about a litre of fluid to replace over the following hours.

These are population formulas — a starting point, not a prescription. Your training history, sleep, stress and cycle shift the real answer. That is what the 9-axis Body Scan measures.

Sources

Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 × 8"? Am J Physiol Regul Integr Comp Physiol. 2002;283(5):R993–R1004.
Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press; 2005.
EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for water. EFSA Journal. 2010;8(3):1459.

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