Daily Water Intake Calculator
Find out how much water you should drink each day for your weight, exercise, climate and life stage, and get a drinking schedule that fits your waking hours.
How the water intake calculator works
A common rule of thumb for healthy adults is about 35 ml of water per kilogram of body weight per day, so a 70 kg person starts at 2,450 ml. This is an estimate, not a medical prescription: needs vary with diet, health, age and body composition, and water from food and other drinks also counts. The calculator uses the rule as its base and adds the extra amounts below. For reference, the European Food Safety Authority sets adequate total water intakes of 2.0 L per day for women and 2.5 L per day for men, including water from food.
Three add-ons adjust the base. Exercise adds 12 ml per minute, which is about 0.7 L per hour of activity. A hot climate adds 500 ml and a very hot one adds 750 ml. Pregnancy adds 300 ml and breastfeeding adds 700 ml, in line with the Institute of Medicine (IOM, 2004) adequate intakes of about 3.0 L of total water per day in pregnancy and 3.8 L during lactation, compared with 2.7 L for adult women. The sum is rounded to the nearest 10 ml and shown in liters, ml, glasses of 250 ml and, in the English version, fluid ounces.
The schedule spreads the goal over your waking hours. The first serving comes 30 minutes after you wake up and the last one 60 minutes before bedtime, so you are not woken at night. The number of servings is the goal divided by 250 ml, kept between 6 and 12, and the times are evenly spaced and rounded to 5 minutes. Each serving is the goal divided by the number of servings, rounded down to 10 ml, and the last serving takes the remainder. If your bedtime is earlier than your wake-up time, it is treated as the next day.
Example: 70 kg, 30 minutes of exercise, mild climate and neither pregnant nor breastfeeding gives 70 x 35 = 2,450 ml, plus 30 x 12 = 360 ml, for a total of 2,810 ml. Warning: people with kidney or heart conditions, or on fluid restriction, must follow their doctor's guidance instead of this estimate. Do not force yourself to drink more than you need, because excessive water in a short time can dilute blood sodium and cause hyponatremia. Spread the water through the day and drink to thirst when in doubt.