Water Intake Calculator
There is no official per-kilogram water formula — so this calculator does not invent one. It shows the NASEM 2005 Adequate Intake reference values for total daily water, the share that typically comes from drinks, and a clearly labeled rough addition for sweaty exercise, along with the two things every hydration page should say: thirst is a reasonable guide for most healthy adults, and overdrinking carries real risk.
Daily water reference
Reference values: men 3.7 L/day, women 2.7 L/day total water (NASEM 2005).
What the Adequate Intake actually says
The National Academies' 2005 Dietary Reference Intakes report set Adequate Intakes for total water — everything you drink plus the roughly 20% that arrives in food — at about 3.7 L/day for men and 2.7 L/day for women aged 19+. That works out to roughly 3 L/day and 2.2 L/day from beverages respectively, computed by the same engine as the calculator. An AI is the observed average intake of adequately hydrated people — a reference point, not a requirement — and the report states plainly that the vast majority of healthy people meet their needs by letting thirst be their guide.
Exercise and heat: rough additions only
Sweat losses vary enormously — typical rates during exercise span roughly 0.5 to 2 litres per hour (ACSM's fluid-replacement position stand), and heat or humidity pushes them higher. One hour of sweaty training therefore adds somewhere between 0.5 and 2 L to the day — a band too wide to fake precision with, which is why the calculator labels it a rule of thumb. NASEM notes that very active people in hot climates can need 6 L/day or more. The accurate personal method is simple: weigh yourself before and after a session, and replace roughly a litre per kilogram lost.
The risk nobody puts on the bottle: overdrinking
More is not safer. Drinking far beyond thirst during long exercise can dilute blood sodium and cause exercise-associated hyponatremia (EAH) — a condition that has hospitalized and killed otherwise healthy marathoners and hikers. The international consensus statement on EAH identifies overdrinking as the primary cause and recommends drinking to thirst during long events rather than forcing fluids on a schedule. If a plan tells you to ignore thirst and drink "as much as possible", it is not a safety margin — it is the hazard.
Frequently asked questions
Is there an official "per kg" water formula?
No. Rules like "30–35 ml per kg" or "8 glasses a day" are folklore, not reference values — no major body publishes a per-kg water requirement for healthy adults. The citable anchor is the NASEM 2005 Adequate Intake: observed average intakes of adequately hydrated people, about 3.7 L/day total for men and 2.7 L/day for women, including roughly 20% from food.
Do I have to hit these numbers every day?
No — an Adequate Intake is a population reference level, not a personal requirement. The NASEM report itself notes that the vast majority of healthy people meet their hydration needs by letting thirst be their guide. Treat the numbers as context, not a quota.
Do coffee and tea count?
Yes. The NASEM report counts all beverages, including caffeinated ones, toward total water. The mild diuretic effect of caffeine at normal intakes does not cancel the fluid consumed.
How much extra do I need for exercise?
It genuinely varies too much for one number: typical sweat rates run roughly 0.5–2 L per hour (ACSM) depending on the person, intensity, and heat. The calculator shows that band as a rough addition. For a real answer, weigh yourself before and after a session — each kilogram lost is about a litre of sweat.
Can I drink too much water?
Yes, and it can be dangerous. Drinking far beyond thirst — classically during marathons and other long events — can dilute blood sodium and cause exercise-associated hyponatremia, which has killed otherwise healthy athletes. The consensus advice for long exercise is to drink to thirst, not to force fluids on a schedule.
Reference values, not prescriptions: the AI describes adequately hydrated populations of healthy adults; illness, medications, pregnancy, and medical conditions change fluid needs — consult a qualified professional in those cases. Values are processed locally in your browser and never transmitted. See the methodology page.