01How much water by weight
Perhaps the simplest method for determining how much water a person should consume on a daily basis is to utilize their body weight as a metric. Larger individuals hold more water within their bodies and lose more water as the day progresses, thus requiring more water to rehydrate to optimal levels. The most widely used formula for ascertaining an individual’s optimal water consumption is to divide their body weight, in pounds, by 2, and consume the result in the form of ounces of water. For metric system users, this translates into approximately 33 ml of water per kilogram of body weight.
Of course, a normal (sedentary) person requires a certain amount of water for basic needs. When you engage in sport, or in hot weather, your needs will of course be higher. This page first explains the water calculator for people with normal (sedentary) lives. Then there is a weight-by-weight table for hot and sporty people, plus some practical advice. Under the Formula tab you will find the simple mathematics behind the water calculator, with an example calculation for a normal (sedentary) person.
02What this calculator does
This water intake calculator asks for your body weight in either kilograms or pounds. It also asks for the number of minutes that you exercise in a day. This calculator then estimates the amount of water that you should be taking in each day, and displays this amount in cups, 250mL glasses, and 500mL bottles, to make the numbers more relatable. For related targets, try our BMI calculator and BMR calculator; broader guidance on daily fluid needs comes from the Mayo Clinic and the U.S. National Library of Medicine.
This is a sensible starting point for a healthy adult and is not hard and fast rules. The body fat percentage calculator is designed to be as logical as possible and will reject input that is impossible, for example: a blank field, zero, a negative figure or a weight that is outside the parameters for that height. This means you will always get an answer that is as accurate as possible.
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 based on the amount of drinks you are having. Then for every 30 minutes of exercise you need to add about 350ml (12oz) and for hot/humid weather add a bit more. Also about 20% of the average person’s water intake comes from food. So you don’t have to get all of it from a 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 the imperial system, your weight in pounds is divided by two to give the amount of water you should have from drinks, and that amount is measured in ounces – which are nearly equal in value to pounds divided by two.
02Worked example
Take a 70 kg adult who exercises for 30 minutes a day. Work through it one step at a time:
That’s the target intake from drinks. And an additional 20% or so from food. It’s quite easy to achieve this by having a glass of water with every meal, and a few between times. Especially on very hot days, or for people who have done a lot of exercise.