TOOLHow much water do you need?
Your daily fluid needs scale with your body size and how much you move and sweat. This calculator starts from about 35 ml of water per kilogram of body weight — a widely used baseline — and adds roughly 350 ml for every 30 minutes of exercise. The result is shown in litres, 240 ml cups and US fluid ounces.
Tips to hit your target
- Spread intake across the day rather than drinking a lot at once.
- Drink more in hot or humid weather and at altitude.
- Food and other drinks — tea, coffee, milk, fruit — all count toward the total.
- Pale-yellow urine is a simple sign you're well hydrated.
Planning training or nutrition too? See the TDEE calculator and macro calculator.
This is a general estimate for educational purposes only and is not medical advice. Individual needs vary with climate, health conditions and medication. People with heart or kidney conditions should follow their doctor's fluid guidance.
A worked example
Take someone weighing 70 kg who does 30 minutes of exercise. The baseline is 70 × 35 ml = 2,450 ml, and the workout adds about 350 ml, for a target near 2.8 litres — roughly twelve 240 ml cups, or about 95 fluid ounces. Add a second 30-minute session and the target rises to about 3.15 litres. The point isn't hitting a number to the millilitre; it's having a realistic, personalised ballpark instead of the one-size-fits-all "8 glasses" rule.
What changes how much you need
Body size and exercise are the big levers, but several other factors nudge the figure up:
- Heat and humidity — you lose more through sweat, so drink more on hot days.
- Altitude — higher elevations increase fluid loss through faster breathing.
- Pregnancy and breastfeeding — both raise daily fluid needs.
- Illness — fever, vomiting or diarrhoea increase losses and require replacing.
- High-protein or high-fibre diets — both draw on a little extra water to process.
Fuelling training as well as hydrating it? Set your calories with the TDEE calculator and split them into grams with the macro calculator.
Frequently asked questions
How much water should I drink a day?
A common guideline is about 30 to 35 ml of water per kilogram of body weight per day, with more added for exercise and hot weather. This calculator uses 35 ml per kg and adds roughly 350 ml for every 30 minutes of activity. Food and other drinks also contribute to your total fluid.
Does coffee or tea count toward water intake?
Yes. Despite their mild diuretic effect, tea, coffee, milk and most drinks still hydrate you, and watery foods like fruit and soup add fluid too. The figure here is a target for total water, not only plain water.
Can you drink too much water?
Yes, though it is uncommon. Drinking very large amounts in a short time can dilute blood sodium, a condition called hyponatraemia. Spread your intake through the day and let thirst and pale-yellow urine guide you rather than forcing large volumes at once.