Daily Water Intake Calculator
A daily water target from your weight, climate and exercise, in glasses.
Work out a daily water target from your weight, sex, climate and exercise, and see it next to the published reference intakes. Two reference points are shown, because no single number fits everyone: the US National Academies adequate intake for total water, and a weight-based estimate you can adjust. About a fifth of daily water normally comes from food, so the page splits the total into a drinks figure and a glasses count. This is general information, not medical advice: kidney, heart and other conditions change fluid needs, and a clinician's instruction beats any calculator.
Source: the common 30 to 35 ml per kg planning rule. Change it and the estimate moves.
Source: National Academies, about 20% of total water from food.
Planning figure only: sweat rates vary widely, and heavy or long sessions need salt as well as water.
Honest limits. The National Academies figures are adequate intakes, the level healthy people were observed to drink, not a minimum requirement and not a personal prescription. The weight-based figure is a planning rule, not a measurement of your sweat or your kidneys. Illness, fever, vomiting, diarrhoea, medicines, pregnancy, breastfeeding, and heart or kidney conditions all move fluid needs, sometimes in opposite directions, so a clinician's instruction always beats this page. Drinking far beyond thirst for hours on end can itself be harmful. Use thirst and pale-yellow urine as a rough daily check, and treat a big gap between the two reference figures as a sign to pick the middle and watch how you feel, not as an error to resolve by forcing water.
Two reference points, and why they differ
The US National Academies of Sciences, Engineering, and Medicine set an adequate intake for total water of 3.7 litres a day for men and 2.7 litres a day for women, counting all beverages and food. About 20% of that normally comes from food, which leaves about 3.0 litres (13 cups) of drinks for men and 2.2 litres (9 cups) for women. The European Food Safety Authority sets lower totals, 2.5 litres for men and 2.0 litres for women, again from all sources. Both are population observations, not prescriptions, and both move with climate, activity and body size, which is why a fixed "eight glasses a day" rule, 2 litres, sits below either reference for men and is only a reminder, not a requirement.
The weight-based rule gives a personal starting point: 30 to 35 ml of total water per kg of body weight a day. An 80 kg adult at 35 ml per kg gets 2,800 ml of total water, close to the women's reference and below the men's, which is exactly the kind of gap the page shows rather than hides. Exercise adds a planning figure of 500 ml per hour on top, and hot weather adds 500 ml, or 1,000 ml when it is very hot or humid. Those additions are editable because sweat rates differ by an order of magnitude between people, and a fixed addition would pretend otherwise.
Total water is not drinking water
Tea, coffee, milk, juice, soup and the water inside fruit and vegetables all count toward total water. Counting only plain water overstates what you still need, by roughly the food share, about 20%. The page's drinks figure already subtracts that share, so the glasses count is glasses of drinks, not glasses on top of everything you ate and drank.
When more water is the wrong answer
Some conditions restrict fluids, some medicines change salt balance, and long hard exercise in heat needs salt with the water, not water alone. If a clinician has given you a fluid limit, or you are ill, pregnant, or breastfeeding, this page's number does not apply. The National Academies pregnancy figure is 3.0 litres of total water, and breastfeeding 3.8 litres, but those are population figures too, and your clinician's figure wins.
Frequently Asked Questions
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