Last updated 23 August 2026
How the target is worked out
The baseline is roughly 33ml per kilogram of body weight — a commonly cited middle-ground figure from hydration research for a sedentary adult in a temperate climate. Activity adds to that: more exercise means more fluid lost to sweat, so the target rises with how active you say you are.
What this doesn't account for
Hot weather, altitude, illness, and pregnancy or breastfeeding all increase real fluid needs beyond what this calculator shows. It also only counts water specifically — food and other drinks contribute a meaningful share of daily fluid too, so this is a sensible target rather than the total amount you need to drink.
Method, limitations and sources
Formula. The base figure is 33 ml of water per kilogram of body weight per day, with a fixed addition for activity level: nothing for sedentary, 350 ml for light activity, 700 ml for moderate and 1,050 ml for active. Cups are shown at 237 ml and glasses at 250 ml.
Inputs it needs. Body weight and a self-reported activity level.
What it assumes. A healthy adult in a temperate climate. The 33 ml/kg rule is a widely used rule of thumb, not a clinical guideline — there is no official body-weight-based water requirement.
An important distinction. The National Academies set an Adequate Intake for total water, which includes water in food and in every other drink, not just plain water. Roughly a fifth of most people’s water intake comes from food. The number this tool gives is a drinking target, so treat it as generous rather than a minimum.
Where it is least reliable. Hot or humid climates, altitude, illness with fever or vomiting, breastfeeding, pregnancy, endurance exercise, and anyone with a heart, kidney or liver condition or on medication that affects fluid balance — several of these require a fluid limit rather than a target.
When not to use it. Do not use it to override medical advice, and do not force fluid intake to hit a number. Drinking far beyond thirst can be dangerous. Thirst and pale urine remain the practical guide for most healthy people.
- Institute of Medicine (2005), Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate — the source of the Adequate Intake figures for total water.
Common questions
Where does the 33ml per kg figure come from?
It's a commonly cited middle-ground estimate from hydration research, roughly 30–35ml per kilogram of body weight per day for a sedentary adult in a temperate climate. Individual needs vary with climate, health conditions and sweat rate.
Does this count water from food and other drinks?
No, this is a target for water specifically. In practice a meaningful share of daily fluid comes from food and other beverages, so this figure is a reasonable target rather than the only fluid you need to drink.
Is more water always better?
No. Drinking far beyond what your body needs can dilute sodium levels and cause genuine harm in extreme cases. This is a general target, not a floor to push past — thirst and urine colour are reasonable everyday guides too.