Daily Water Intake Calculator
A starting figure rather than a prescription. Thirst and urine colour tell you more than any formula, and this is mainly useful as a sanity check on whether you are in the right region.
Water
3.24litres a day
- Litres
- 3.24
- US cups
- 13.7
- US fluid ounces
- 110
- From exercise
- 0.60 litres
- Base amount
- 2.64 litres
- This is a starting figure, not a prescription. Around 33ml per kilogram plus roughly 400ml for each half hour of exercise, which is the common rule of thumb.
- Food supplies a fair amount of it — often a fifth of the total — and tea and coffee count towards it despite the folklore. The diuretic effect of caffeine at normal intakes does not offset the water it comes in.
- Thirst and urine colour tell you more than any formula. Pale straw is about right; consistently dark means more, and completely clear all day probably means less.
- Heat, altitude, illness, pregnancy and some medications all move this substantially, and a few conditions require restricting fluid rather than increasing it.
About daily water intake
The rule of thumb is around 33 millilitres per kilogram of body weight, plus roughly 400 millilitres for each half hour of exercise. For an 80kg person training for 45 minutes, that is about 3.2 litres. The "eight glasses a day" advice is roughly two litres, which is in the same region for a smaller sedentary person and low for a larger active one.
A meaningful part of your intake comes from food — often around a fifth — and tea and coffee count towards it despite the folklore. The diuretic effect of caffeine at normal intakes does not offset the water it arrives in, so a cup of coffee is a net gain rather than a loss.
Urine colour is a better daily guide than any calculation. Pale straw is about right. Consistently dark suggests more, and completely clear all day suggests you are drinking more than you need — which is not usually harmful but is not doing anything either.
Heat, altitude, illness, pregnancy and breastfeeding all raise the requirement substantially, and some medications and a few medical conditions require restricting fluid rather than increasing it. Anyone in that last group should follow their own advice rather than a formula.
Drinking a very large amount very quickly is the one genuine risk here. It can dilute blood sodium dangerously, and it is a real phenomenon among endurance athletes who overdrink during long events. Spread intake through the day rather than front-loading it.
What it works out
- A daily target in litres, cups and fluid ounces
- The exercise portion shown separately
- Metric or imperial weight
The formula
Millilitres = (Weight in kg × 33) + (Exercise minutes ÷ 30 × 400)
Eighty kilograms at 33 millilitres each is 2,640 millilitres. Forty-five minutes of exercise is one and a half half-hours, which adds 600. The total is 3.24 litres — about 13.7 US cups, or 110 fluid ounces.
Both numbers are conventions rather than findings. The 33ml per kilogram scales intake to body size, which is more sensible than a fixed figure for everyone, and the exercise allowance is a rough replacement for sweat losses.
Sweat rate varies enormously — between about 0.5 and 2 litres an hour depending on the person, the effort and the heat. If you want a real figure rather than an estimate, weigh yourself before and after a session: each kilogram lost is about a litre of fluid.
Food supplies a fair amount on top of what you drink, so hitting this exactly from beverages alone is not the point. It is a region, not a target.
- Weight
- Body weight. Larger bodies need more.
- Exercise
- Minutes a day. Raise it further in heat or at altitude.
- 33ml
- Per kilogram. A convention that scales intake to body size.
A worked example
An 80 kilogram person training for 45 minutes.
That works out to 3.24 litres a day.
- Litres
- 3.24
- US cups
- 13.7
- US fluid ounces
- 110
- From exercise
- 0.60 litres
- Base amount
- 2.64 litres
Questions
How much water should I drink a day?
Around 33ml per kilogram of body weight plus about 400ml for each half hour of exercise. For an 80kg person training 45 minutes that is roughly 3.2 litres, including what comes from food and other drinks.
Is eight glasses a day right?
It is roughly two litres, which is in the right region for a smaller, sedentary person and low for a larger or active one. It has no particular scientific origin — it scales with nothing.
Does coffee count?
Yes. The diuretic effect of caffeine at normal intakes does not offset the water it comes in, so tea and coffee are a net gain. The idea that they dehydrate you is persistent folklore.
How do I tell if I am drinking enough?
Urine colour, which beats any calculation. Pale straw is about right; consistently dark means drink more. Thirst is a reasonable guide too for most healthy adults under normal conditions.
Can I drink too much water?
Yes, and it is the one real risk here. Drinking a very large volume quickly can dilute blood sodium dangerously — it happens to endurance athletes who overdrink during events. Spread intake through the day.
How much extra do I need when exercising?
Sweat rates run between about 0.5 and 2 litres an hour depending on effort and heat. Weighing yourself before and after a session gives your own figure — each kilogram lost is roughly a litre.
Does food count towards this?
A meaningful amount — often around a fifth of total intake, more on a diet heavy in fruit and vegetables. The target here is total water, not water you have to drink from a glass.