What is a water intake calculator?
A water intake calculator estimates how much fluid you should drink each day. Water keeps your body temperature stable, carries nutrients, and helps your kidneys clear waste; even mild dehydration can bring on headaches, tiredness and poor concentration.
This calculator starts from 35 ml per kilogram of body weight, adds 500 ml for each hour of exercise and another 500 ml for hot or humid weather, and shows the total in litres and 250 ml glasses with a breakdown of each part. It's a practical target for healthy adults, not a medical prescription.
How to use the Water Intake Calculator
1.Enter your weight
In kilograms or pounds.
2.Add exercise and weather
Minutes of exercise on a typical day, and tick hot or humid weather.
3.Spread it through the day
See the total in litres and glasses, and how each part adds up.
Example: 70 kg, 45 minutes of exercise, summer
Each part of the estimate:
- Body weight
- 70 × 35 ml = 2.45 L
- Exercise
- 45 min = 0.375 L
- Hot weather
- 0.5 L
- Total
- about 3.3 L, or 13 glasses
Water = 35 ml × kg + 500 ml per hour of exercise + 500 ml if hot
On a cool rest day the same person would need about 2.45 L; heat and exercise add almost a litre.
Where the numbers come from
There's no single official figure. The European Food Safety Authority suggests about 2.5 litres of total water a day for men and 2.0 for women, including the roughly 20% that comes from food. A common practical rule is 30–35 ml of fluid per kilogram of body weight, with extra for exercise and heat, which is what this calculator uses.
Your body is a good guide: thirst, and pale yellow urine, show you're drinking enough for most healthy adults.
Frequently asked questions
Is 8 glasses a day right?
It's a rough average. Larger, more active people in hot weather need more; smaller, less active people may need less.
Do tea and coffee count?
Yes. Caffeinated drinks still hydrate overall; only very large amounts of caffeine have a noticeable effect.
Can I drink too much water?
Rarely, but drinking many litres in a short time, for example in endurance events, can dangerously lower blood sodium.
How much extra for exercise?
About 400–800 ml per hour depending on sweat; this calculator adds 500 ml per hour.
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