How much should you drink?
There is no single number that fits everyone, but there are good reference values. The National Academies of Sciences set an adequate intake of about 3.7 liters of total water a day for men and 2.7 liters for women. Around 20% of that comes from food, so drinks cover roughly 3.0 liters for men and 2.2 liters for women. This calculator starts from those values and adds water for exercise, hot weather, pregnancy and breastfeeding.
The method
- Base from drinks: 3.0 L for men, 2.2 L for women.
- Exercise: + 0.5 L per hour.
- Hot or humid weather: + 0.5 L.
- Pregnancy: + 0.3 L. Breastfeeding: + 0.9 L.
- Convert to fluid ounces (× 33.8), 8 oz cups and 16.9 oz bottles.
Worked example
A man exercises 45 minutes a day in mild weather.
- Base: 3.0 L.
- Exercise: 0.75 h × 0.5 = 0.375 L.
- Total: 3.4 L, or about 114 fl oz.
- That is 14.3 cups of 8 oz, or about 6.8 bottles of 16.9 oz.
Fluids by daily exercise (men, mild weather)
| Exercise | Fluid ounces | Cups |
|---|---|---|
| None | 101 | 12.7 |
| 30 min | 110 | 13.7 |
| 60 min | 118 | 14.8 |
| 90 min | 127 | 15.9 |
| 120 min | 135 | 16.9 |
For women, the base is about 74 fl oz, a little over 9 cups.
What counts
All drinks count toward the total: water, milk, juice, tea and coffee. Caffeine has a mild diuretic effect, but caffeinated drinks still hydrate overall. Alcohol does the opposite. Fruit, vegetables, soups and yogurt provide much of the water from food. Water is still the best everyday choice, because it has no calories or added sugar.
Signs you are drinking enough
- Pale yellow urine through the day.
- Rarely feeling thirsty, and no headache or fatigue from dehydration.
- Stable body weight before and after exercise: losing more than 2% of your weight during a workout means you should drink more next time.
When to adjust
You need more water with fever, vomiting or diarrhea, at high altitude, in very hot weather and during long endurance events, where drinks with electrolytes help replace sodium. You may need less if your doctor has asked you to limit fluids, for example with heart failure or kidney disease. Older adults feel less thirst and should drink regularly even when not thirsty.
Can you drink too much?
Yes, though it is uncommon. Drinking several liters within a few hours, especially during marathons, can dilute blood sodium (hyponatremia). Drink to thirst and spread intake over the day. This tool gives general guidance and does not replace medical advice.