How the calculator works
The starting point is the Adequate Intake for total water set by the US National Academies: about 3.7 litres a day for men, 2.7 litres for women, 3.0 litres in pregnancy and 3.8 litres while breastfeeding. That total includes the water in food, which supplies roughly a fifth of it, so the calculator shows the remaining 80% as the amount to drink.
Exercise adds sweat losses. Sweat rates vary from about 0.5 to 2 litres an hour depending on the person, intensity and heat (American College of Sports Medicine, 2007), so the calculator adds a conservative 0.5–1 litre per hour of exercise. If you train hard in heat, you may need more.
A better check than any formula
These are population averages, not a personal prescription. Two simple checks work better day to day:
- Urine colour: pale straw means you are fine; dark means drink more.
- Weigh yourself before and after a long workout: each kilogram lost is about a litre of sweat to replace.
Does coffee count?
Yes. Tea, coffee, milk and other drinks all count toward the total. Caffeine has a mild diuretic effect, but regular drinkers adapt and the net effect on hydration is small.
When to be careful
People with heart failure, kidney disease or on fluid restrictions should follow their clinician's limit. Drinking very large volumes quickly, especially during long endurance events, can dilute blood sodium; drink to thirst and sweat loss rather than forcing fluid.