Water Intake Chart by Body Weight

The figures below use 32.5 ml per kilogram, the midpoint of the 30 to 35 ml/kg range used in clinical nutrition practice for healthy adults. This is total fluid for a day with no deliberate exercise and a temperate climate.

At 70 kg this comes to about 2.3 litres a day, which lands close to the two most-cited authoritative figures: the European Food Safety Authority's adequate intake of 2.0 litres a day for women and 2.5 for men, and the US National Academies' 2.7 and 3.7 litres. That those numbers sit either side of the body-weight estimate is reassuring, and also a hint at how loose the target really is.

Exercise is the largest modifier. Sweat losses of 0.5 to 1.0 litre per hour of moderate training are typical, and considerably more in heat. The table below adds a conservative allowance for weekly training volume, spread across the week rather than loaded onto training days.

On a heavy training day, replace fluid to the session rather than to the daily average: weigh yourself before and after a long or hot session and drink roughly 1.25 to 1.5 litres for every kilogram of body mass lost. That measurement is far more accurate than any table, because sweat rate varies several-fold between individuals.

These are totals for all fluid, not for plain water alone. Everything you drink contributes, and so does food.

• Food supplies about 20 percent of total water intake in a typical mixed diet — more if you eat a lot of fruit, vegetables, soup, or yoghurt. Both EFSA and the National Academies figures are for total water including food.
• Tea and coffee count. The diuretic effect of caffeine at normal intakes does not offset the fluid in the drink. Moderate coffee consumption is hydrating on net.
• Milk and juice count, along with their calories. Sugar-sweetened drinks hydrate perfectly well and are a poor way to do it if calories matter to you.
• Alcohol is the exception. It suppresses vasopressin and increases urine output, so strong drinks are net dehydrating even though they are liquid.

Healthy adults with free access to fluid are already well regulated: thirst is a reliable signal, and the kidneys handle a wide range of intakes without difficulty. Urine colour is the simplest check — pale straw suggests adequate hydration, consistently dark yellow suggests drinking more, and completely colourless all day usually means more than you need.

Three groups should not rely on thirst alone. Older adults have a blunted thirst response, so a schedule works better than a signal. Anyone exercising hard in heat can lose fluid faster than thirst prompts replacement. And in some medical conditions — heart failure, kidney disease, on certain medications — fluid intake is deliberately restricted, in which case a clinician's limit overrides any chart.

It is also possible to drink too much. Overdrinking during endurance events can dilute blood sodium to dangerous levels, a condition called exercise-associated hyponatraemia, which has caused deaths in marathons. Drinking to thirst rather than to a fixed schedule is the current advice for endurance athletes for exactly this reason.

The honest summary is that no authority publishes a requirement, only an adequate intake — a figure derived from what healthy populations actually consume rather than from a measured need. EFSA says as much explicitly. Requirements vary with climate, humidity, altitude, diet, body size, and sweat rate, and the healthy range for one person is wide.

The familiar "eight glasses a day" rule has no identifiable scientific source; at 250 ml a glass it comes to 2 litres, which is coincidentally close to the EFSA figure for women, which is probably why it has survived. Our guide on how much water you should drink a day traces where the rule came from, and the water intake calculator applies the same 32.5 ml/kg baseline to your exact weight and training volume.