01How much water by weight
Your body weight is the simplest, most reliable way to estimate how much water you need, because bigger bodies hold more water and lose more of it through the day. The most widely used rule of thumb is to drink about half your body weight in pounds as ounces of water — which works out to roughly 33 mL for every kilogram you weigh.
On top of that baseline, exercise and hot weather raise your needs. This page covers what the calculator does, a weight-by-weight reference table, and practical guidance; the Formula tab shows the simple maths and a worked example.
02What this calculator does
This water intake calculator takes your body weight — in kilograms or pounds — and, optionally, how many minutes you exercise a day. From those it estimates your recommended daily water intake and breaks it down into cups, 250 mL glasses, and 500 mL bottles so the number is easy to picture.
The figure is a sensible starting point for a healthy adult, not a strict prescription. The calculator rejects impossible entries (a blank field, zero, a negative number, or an out-of-range weight) so you always get a meaningful result.
03Daily water by body weight
Using about 33 mL per kilogram, here is roughly how much water a day different body weights work out to — before adding anything for exercise:
These are baseline figures from drinks. Add about 350 mL (12 oz) for every 30 minutes of exercise, and a little more in hot or humid weather. Roughly 20% of most people’s water also comes from food, so you do not have to hit the whole number from the glass.
The by-weight rule is a useful guide, not a precise medical figure. A few things change how much you actually need:
- Climate and activity. Heat, humidity, altitude, and exercise all increase water loss through sweat, so your real need can be well above the baseline.
- Food and other drinks count. Around 20% of water intake comes from food, and tea, coffee, and milk all count toward the total — the figure here is total water, mostly from drinks.
- Health conditions. Pregnancy and breastfeeding raise needs, while some heart, kidney, and liver conditions require a doctor-set fluid limit — follow medical advice over any calculator.
- You can drink too much. Drinking very large amounts quickly can dilute blood sodium (hyponatremia), which is dangerous; spread intake through the day rather than forcing it.
- Out-of-range inputs are rejected. Weight must sit within sensible limits; the calculator flags anything outside them instead of returning a misleading number.
- Pick Metric (kg) or Imperial (lb).
- Enter your weight, and optionally the minutes you exercise a day.
- Tap Calculate to see your daily water in litres or ounces, plus cups, glasses, and bottles.
Tips for staying hydrated
- Use thirst and urine colour as your day-to-day guide — pale straw is well hydrated; dark yellow means drink more.
- Spread water across the day rather than drinking a lot at once; a glass with each meal and one between is an easy routine.
- Add extra on hot days and around exercise, and remember that tea, coffee, and watery foods all contribute.
01The formula
The calculator works from your body weight, then adds a little for exercise. The base comes from a simple per-kilogram rate (the metric version of “half your weight in pounds as ounces”):
Where:
- weight = your body weight in kilograms (the calculator converts pounds for you)
- 33 = millilitres of water per kilogram of body weight (the same as taking half your weight in pounds as ounces)
- exercise = minutes of activity a day, which adds about 350 mL (12 oz) for every 30 minutes
In imperial units the base is simply your weight in pounds divided by two, taken as ounces — the two work out to almost exactly the same amount. The result is your recommended water from drinks for the day.
02Worked example
Take a 70 kg adult who exercises for 30 minutes a day. Work through it one step at a time:
That is the target from drinks. With roughly a fifth of water coming from food as well, most people comfortably reach it with a glass at each meal and a few in between — more on hot days or after a longer workout.