Daily Water Intake Calculator
Enter your weight, activity level and whether it is hot to see how much water to drink each day.
This tool is for information only and does not replace a doctor. If you have kidney disease or another medical condition, ask your doctor about your daily fluid needs.
Why the '8 Glasses' Rule Doesn't Apply to Everyone
The "8 glasses a day" rule ignores the difference between a 60 kg and a 100 kg person, activity level and hot weather. A common method is about 30–35 ml of water per kilogram of body weight; this calculator uses 35 ml per kilogram to set a personal target.
Effect of Exercise and Weather
You lose a lot of fluid through sweat during exercise. The calculator adds 350 ml for light activity, 700 ml for moderate and 1,050 ml for active, plus 500 ml if you tick hot weather.
Signs of Dehydration
Dark yellow urine, headache, tiredness and trouble concentrating are early signs of dehydration. Losing fluid equal to just 2% of body weight already lowers physical and mental performance.
Health Benefits of Adequate Hydration
Drinking enough water supports kidney function, lubricates joints, keeps energy up and helps concentration. Even mild dehydration, a loss of just 1–2% of body weight, noticeably lowers mental performance and physical endurance. Part of this need is met by water from food.
How Do Temperature and Exercise Affect Water Needs?
Sweat loss rises in direct proportion to air temperature and exercise intensity. In high heat or during intense activity, you can lose 0.5 to 2 litres of sweat an hour. Replacing it promptly prevents both dehydration and muscle cramps. If you train, drink at regular intervals before, during and after exercise.
How to Calculate Daily Water Needs Based on Body Weight?
The most common way to estimate daily water needs is a body-weight formula: about 30–35 ml of water per kilogram. This calculator uses 35 ml. Example: 70 kg × 35 ml = 2.45 litres. Water from tea, coffee, juice and food can count toward this, but caffeine and alcohol lower net fluid balance because of their diuretic effect.