Water Intake Calculator
Estimate how much water you should drink each day based on body weight, exercise and climate.
How much water do you need?
There is no single number that fits everyone. A practical rule used by many dietitians is about 35 ml of water per kilogram of body weight per day (roughly half an ounce per pound), plus extra for exercise and heat. This calculator adds about 350 ml for every 30 minutes of exercise and 500 ml for hot weather.
A 70 kg person who exercises for 30 minutes needs about 70 × 35 = 2,450 ml plus 350 ml, or 2.8 liters a day from all drinks and food.
Official guidance
The US National Academies set an "adequate intake" of total water (from drinks and food) of about 3.7 liters a day for men and 2.7 liters for women. The European Food Safety Authority suggests 2.5 liters for men and 2.0 liters for women. Around 20% of daily water usually comes from food such as fruit, vegetables and soups.
Signs you need more water
- Dark yellow urine. Pale straw color usually means you are well hydrated.
- Thirst, headache, tiredness or difficulty concentrating.
- Dizziness or cramping during exercise.
Coffee and tea count toward your intake; their mild diuretic effect does not cancel out the water they contain. People with heart or kidney conditions may need to limit fluids and should follow their doctor's advice.
Frequently asked questions
Do I really need 8 glasses of water a day?
The "8 × 8" rule (eight 8-ounce glasses, about 1.9 liters) is a reasonable minimum for many adults but is not based on strong evidence. Needs vary with body size, activity and climate.
Does coffee count toward water intake?
Yes. Studies show moderate coffee and tea consumption hydrates about as well as water.
Can you drink too much water?
Yes. Drinking very large amounts in a short time can dilute blood sodium (hyponatremia). This is rare and mostly seen in endurance events.
Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.