What next?
Hydration scales with body size and training load. These set both.
How to use this calculator
- Enter your body weight.
- Add your typical daily exercise and climate.
- Read your daily water target.
What your result means
The target estimates the total daily fluid to stay hydrated, adjusted for exercise and heat — and food and other drinks count toward it. Thirst and pale urine are good real-world checks; needs rise with illness, pregnancy and altitude.
Why this one is different
Three rows carry one target in different units — litres, millilitres, and 250 ml glasses — because which unit helps depends on whether you are filling a bottle or counting cups at a desk. A note under the figure states the target covers all fluids, so it is not misread as plain water alone.
Where the "8 glasses a day" rule actually came from
The famous rule has almost no science behind it — it likely stems from a 1945 note that also said most water comes from food, a line everyone forgot. Real needs scale with body size, activity and climate, not a round number.
Your body is a better guide than any rule: thirst, energy and urine colour tell you more than counting glasses. This tool just gives you a sensible starting target.
How it works
A common guideline is around 30–35 ml of water per kilogram of body weight. Exercise adds roughly 350 ml for every 30 minutes to replace sweat, and hot climates increase needs further. This is a starting estimate — thirst, urine colour and how you feel are your best day-to-day guides.
Formula
+ exercise adjustment + climate
Example calculation
A 70 kg adult in a temperate climate, no heavy exercise:
≈ 2.3 litres per day
Add more for exercise, heat or pregnancy.
Signs you are getting it roughly right
Urine colour is the most practical everyday check: pale straw suggests adequate hydration, dark amber suggests you are behind, and completely clear can mean you are drinking more than you need. Thirst is a reliable enough signal at rest for most healthy adults, but it lags during long or hot exercise, which is where a planned intake beats waiting. Weighing before and after a long session is the precise method — each kilogram lost is roughly a litre of sweat to replace over the following hours.
When more water is not better
Drinking far beyond need dilutes blood sodium, and in extreme cases — most often endurance events where athletes drink heavily on a hot day — that causes hyponatraemia, which is dangerous. Sodium replacement matters as much as volume during sessions over about an hour or in heat. In the other direction, several conditions require fluid restriction rather than encouragement, including some heart and kidney disease. This calculator gives a general adult estimate; anyone with a medical fluid target should follow that instead.
Frequently asked questions
Does tea and coffee count?＋
Mostly yes. Despite their reputation, moderate caffeinated drinks still hydrate you overall. Water and unsweetened drinks are the healthiest choices.
Can I drink too much water?＋
Very rarely, but drinking extreme amounts quickly can be dangerous. Spread intake through the day rather than forcing large volumes at once.
How do I know if I am drinking enough?＋
Urine colour is the simplest check: pale straw suggests you are well hydrated, dark yellow suggests you are not. Thirst is a reasonable guide for most healthy adults.
Does exercise change the target?＋
Yes. Sweat losses of 0.5-2 litres an hour are common depending on intensity and heat. Weighing yourself before and after a long session gives a personal figure, since each kilogram lost is roughly a litre of fluid.
Does food contribute to intake?＋
It does, typically 20-30 percent of total water intake, more if you eat a lot of fruit, vegetables and soups.
When do electrolytes matter?＋
Mainly for sessions over about an hour, or in heat where sweat losses are high. For everyday drinking, plain water and a normal diet cover it.
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Assumptions & limitations
Every figure here comes from a simplified model. Keep these limits in mind when reading your result:
- A general baseline from body weight and activity level.
- Real needs depend on climate, diet, illness and pregnancy.
- Thirst and pale-yellow urine are better day-to-day guides than any single number.