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Adds 0.4–0.8 litres an hour.
2.46 L – 2.76 L
Roughly 10.4–11.7 US cups. Food supplies the rest.
An estimate, not medical advice, and not a target to hit. These are Adequate Intakes: the median intake of a healthy population, published because the evidence was not strong enough to set an actual requirement. Thirst and pale urine remain the practical guide, and drinking well past thirst is not harmless — endurance athletes have died of the low blood sodium it causes. Anyone on a fluid restriction, or with heart or kidney disease, should use the figure their clinician gave them instead.
The headline figures are the US Institute of Medicine’s 2004 Adequate Intakes for total water, less the fifth of it that arrives in food. Europe’s EFSA read similar evidence in 2010 and landed about a litre lower for adults, which is shown alongside rather than averaged away — the gap is real and neither panel is wrong. The exercise addition is the American College of Sports Medicine’s 0.4–0.8 litres an hour, a range because a heavy runner in the heat sweats at several times the rate of a light one in the cold. Heat, altitude, fever and a very high protein or salt intake all raise the requirement by an amount no formula can pin down, so none is applied here. There is no evidence base at all for “eight glasses a day”.
Almost every hydration target you have seen traces back to one document: the US Institute of Medicine’s 2004 report on dietary reference intakes, which set Adequate Intakes for total water at 3.7 litres a day for men and 2.7 for women. The same report notes that around a fifth of total water arrives in food rather than in a glass, so the figure that belongs on your drinking is four fifths of the headline.
This calculator applies exactly that, by life-stage group, and then adds an allowance for time spent exercising. It shows the result in litres or US fluid ounces, alongside the European figure for comparison.
The Adequate Intake is 2.7 litres of total water. Take off the fifth that comes from food and 2.16 litres is left to drink. Three quarters of an hour of exercise adds between 0.3 and 0.6 litres, giving a target of 2.46 to 2.76 litres — about 83 to 93 US fluid ounces.
The range is not vagueness. The American College of Sports Medicine puts fluid loss during endurance exercise at 0.4 to 0.8 litres an hour, and the spread is real: a heavy runner in humid heat loses several times what a light one loses in the cold. A tool offering one number for exercise has picked a point inside that band without telling you.
An AI is not a requirement and not a minimum. It is set when the evidence is too thin to derive an actual requirement, and it records the median intake observed in a healthy population — a description of what people already drink, promoted to a reference value. Meeting it proves nothing and missing it proves nothing.
The clearest evidence for that is the disagreement between expert panels. EFSA reviewed comparable evidence in 2010 and landed on 2.5 litres for men and 2.0 for women, about a litre below the American figures. Both are shown here rather than averaged, because averaging two honest disagreements produces a number nobody published.
Eight eight-ounce glasses is 1.89 litres, and no study produced it. Heinz Valtin traced the advice in 2002 to a 1945 recommendation of roughly one millilitre of water per calorie eaten. The very next sentence of that recommendation said most of the quantity is already contained in prepared foods — and that sentence did not survive the journey into popular advice.
In the other direction, drinking far past thirst is not free. Diluting blood sodium causes hyponatraemia, which has killed marathon runners and fraternity pledges. Heat, altitude, fever and a high salt or protein intake all raise the requirement, by amounts no calculator can put a figure on, so nothing is applied for them here.
Yes. Total water means all of it — tea, coffee, milk, juice, soup and the water inside food. Caffeine has a mild diuretic effect at high doses but nothing close to enough to make a caffeinated drink a net loss, and the reference intakes were set from populations drinking all of these.
It is the most practical one available without a laboratory. Pale straw suggests you are fine; consistently dark suggests you are behind. It is thrown off by B vitamins, beetroot, some medications and the first sample of the morning, so read it across a day rather than from a single visit.
Almost certainly, and the same goes for altitude, fever and hard physical work. The evidence is clear that the need rises; what nobody has produced is a defensible multiplier, because it depends on temperature, humidity, clothing, acclimatisation and how much you are moving. Drink to thirst and check the colour.
Because milk is mostly water and producing it draws directly on maternal stores. The Adequate Intake rises from 2.7 to 3.8 litres a day while breastfeeding, and to 3.0 during pregnancy. Both figures are set for the general case and are not a substitute for what a midwife or doctor tells you.