How much water do you need?
There's no single number that fits everyone. Your needs depend on your size, how active you are, the weather, and your health. As a reference, the US National Academies of Sciences, Engineering, and Medicine set an adequate total water intake for healthy adults of about:
- 3.7 litres a day for men
- 2.7 litres a day for women
That total includes water from all drinks and food. Food usually provides about 20% of it, so the amount you actually drink is lower, roughly 3 litres for men and 2.2 litres for women.
How this calculator estimates it
To fit the estimate to your body and routine, the calculator uses a common weight-based guideline and adds extra for sweat:
water (ml) = weight (kg) × 33
+ 350 ml for every 30 minutes of exercise
+ 500 ml in a hot or humid climate
Example: you weigh 70 kg and exercise for 30 minutes a day.
70 × 33 = 2,310 ml. Add 350 ml for exercise = 2,660 ml, about 2.7 litres.
That's around 11 cups (8 oz each) or 90 fluid ounces.
Signs you're drinking enough
- Pale yellow urine. Dark yellow often means you need more. Clear all the time can mean you're drinking more than you need.
- You rarely feel thirsty. For most healthy adults, drinking when thirsty and with meals is enough on normal days.
- No headache or tiredness from mild dehydration, especially in the afternoon or after exercise.
When you need more
- Exercise: drink before, during and after. Long or intense sessions may need a drink with electrolytes as well as water.
- Heat and altitude: you lose more water through sweat and breathing.
- Illness: fever, vomiting and diarrhea all increase fluid loss.
- Pregnancy and breastfeeding: the National Academies suggest about 3.0 litres total a day in pregnancy and 3.8 litres while breastfeeding.
Important: this is a general estimate for healthy adults, not medical advice. If you have kidney, heart or liver disease, or your doctor has limited your fluids, follow their advice. Drinking a very large amount of water quickly can be dangerous.
Frequently asked questions
Is "8 glasses a day" right?
It's a reasonable easy-to-remember target, about 1.9 litres, but it isn't based on a specific study. Many people need more, especially larger or active people, and some need less. Food and other drinks count too.
Do tea and coffee count?
Yes. Although caffeine has a mild diuretic effect, regular tea and coffee drinkers still gain most of the fluid. Water is still the best everyday choice, since it has no sugar or calories.
Can you drink too much water?
Yes, though it's rare. Drinking several litres in a short time can dilute the sodium in your blood (hyponatremia), which can be serious. Spread your drinking through the day.
Last reviewed November 2026.