Water Intake Calculator

How much to drink a day — by the two official figures, which disagree by a litre, with the “half your body weight in ounces” rule beside them so you can see what it is actually doing. Exercise is handled the way the research says: by weighing yourself, not by a formula.

You are
lb
minutes
Is it hot where you are?
Which official figure?

To drink, on a day like this

9 cups

    Thirst is a good guide for most healthy adults, and these figures are population averages rather than a personal requirement. If a doctor has given you a fluid limit — for heart or kidney disease, or with certain medicines — that number wins over anything here.

    Figures checked 2026-09-23.

    Quick answer: the US National Academies put total water at 3.7 litres a day for men and 2.7 for women — food included, and about a fifth of it comes from food, so the drinks figure is nearer 3.0 and 2.2 litres. Europe’s food safety authority says 2.5 and 2.0 for the same people. Both are population averages, not a personal requirement, and for most healthy adults thirst is a better guide than either.

    How to use it

    Put in your weight, say how long you are training today and whether it is hot, and you get what to drink — in cups and ounces, or litres. The page shows the other official figure beside the one you picked, and what the popular “half your body weight in ounces” rule would give, so you can see how far apart three respectable-looking answers are.

    Why the official numbers disagree by a litre

    Because “how much water do I need” is not a question with a sharp answer. The US Adequate Intake was set by looking at what healthy, well-hydrated people actually consumed — a description of a population, deliberately generous. EFSA’s reference values were derived rather differently and land a litre lower for men. Neither body thinks a person drinking a bit less is dehydrated, and the same report that produced the US figure says plainly that most healthy people meet their needs by drinking to thirst.

    That is worth knowing before you force down a fixed number of glasses: the honest version of this advice is a range with a wide floor, plus attention to thirst and to the colour of your urine.

    The “eight glasses” rule, and its modern cousin

    Eight 8-ounce glasses is about 1.9 litres, which is close enough to the official figures to have survived for decades despite nobody being able to find where it came from. Its replacement — “drink half your body weight in ounces” — is what most water calculators on the internet are actually running, usually without saying so. It has no official source either, and it scales with body weight, which is not how water needs work: a heavier person does need more, but not proportionally more. This page shows that number too, labelled as the folk rule it is.

    Exercise is the part a daily number cannot handle

    Sweat rates differ several-fold between people, sessions and climates, so no formula can tell you what a training session costs. The American College of Sports Medicine’s position stand is to measure it: weigh yourself before and after, and replace about 1.5 litres of fluid for every kilogram lost. Keep the loss under about 2% of body weight. Before you have weighed anything, 400 to 800 ml an hour covers most people — the low end for an easy session in the cold, the high end for hard work in the heat.

    And too much water is dangerous

    This gets said far less often than “drink more”, and it should not. Drinking well beyond what you lose dilutes the sodium in your blood — exercise-associated hyponatraemia — and it has killed marathon runners who followed a schedule instead of their thirst. The early signs look like dehydration: nausea, headache, confusion, puffiness. If you have heart or kidney disease, or take a medicine that comes with a fluid restriction, your doctor’s number is the one that counts.

    Does drinking water help you lose weight?

    A bit, and not for the reason people think. Trials in which people drank water before meals show they eat somewhat less at that meal, which is a real and useful effect. Water does not raise your metabolism meaningfully, does not “flush out” fat, and the pounds that appear or vanish on the scale when you change how much you drink are water weight, not fat. Swapping sugary drinks for water is the version of this advice that actually changes a calorie total — often by several hundred a day.

    FAQ

    How much water should I drink a day?
    The US Adequate Intake is 3.7 litres a day for men and 2.7 for women, counting water in food as well as drinks — so about 3.0 and 2.2 litres of actual drinks. Europe’s figures are 2.5 and 2.0 litres total. Both are averages for populations; your own needs shift with heat, exercise, illness and what you eat, and thirst tracks all of that better than a fixed number.
    Does coffee or tea count?
    Yes. The caffeine in normal amounts has a mild diuretic effect, but the fluid in the cup more than covers it, and the official intakes count all beverages. So does the water in food — soup, fruit, yoghurt, vegetables — which is where roughly a fifth of the total usually comes from.
    Is “half your body weight in ounces” right?
    It is a folk rule with no official source, and for many people it lands surprisingly close to the published figures, which is why it persists. It goes wrong at the extremes: for a very heavy person it prescribes far more than any authority does, because water needs do not scale with body weight in a straight line.
    How do I know if I am drinking enough?
    Pale straw-coloured urine and no persistent thirst is the practical test, and it responds to heat, exercise and illness the way a fixed number cannot. Dark urine with a dry mouth is the signal to drink; clear urine all day usually means you are drinking more than you need, which is harmless for most people but not for everyone.
    Will drinking more water help me lose weight?
    Indirectly. Drinking water before a meal reduces how much people eat in trials, and replacing sugary drinks with water can remove several hundred calories a day. What it does not do is burn fat or speed up your metabolism to any meaningful degree — and the weight that moves on the scale when you drink more is water, which comes back the moment you stop.
    Can you drink too much water?
    Yes, and it is more dangerous than mild dehydration. Drinking much more than you lose dilutes blood sodium, and hyponatraemia has killed endurance athletes. It is a risk mainly for people drinking to a schedule during long events — which is precisely why the published advice is to drink to thirst, and to measure sweat loss rather than guess it.