How Many Ounces of Water Should You Drink a Day?

The calculator shows two estimates side by side. The first comes from the reference values for water set by the National Academies for healthy adults. The second is a popular rule of thumb: drink half your body weight in pounds as fluid ounces.

Both results are shown in fluid ounces, cups, liters and bottles of 16.9 fl oz, the size of a common 500 ml bottle of water. The table under the results splits the daily amount evenly over the day, as an example rather than a rule.

The National Academies set an Adequate Intake of total water of 3.7 liters a day for men and 2.7 liters for women. Total water counts everything: plain water, other drinks and the water in food.

About 80% of it usually comes from drinks. That is about 101 fl oz (3 L) of drinks a day for men and 74 fl oz (2.2 L) for women, and it is the number the calculator shows first.

The values are based on how much healthy people actually drink, not on a measured need. The report says they "should not be interpreted as a specific requirement", and that most healthy people get enough by drinking when thirsty and with meals.

The reference goes up to 3 liters of total water a day during pregnancy, about 2.3 L of it from drinks. While breastfeeding it is 3.8 liters, about 3.1 L from drinks.

These values apply from age 14 to 50. For anything specific to your pregnancy, ask your doctor or midwife.

Many extension services and health columns repeat a simple rule: drink half your body weight in pounds as ounces of water, for example Rutgers Cooperative Extension and the University of Georgia. The calculator uses 0.5 fl oz per pound.

We found no health agency that sets a water target by body weight, so treat this as a rough guide. Michigan State University Extension gives a wider range of half an ounce to one ounce per pound, which shows how loose the rule is.

Exercise and heat both raise how much water you need. For exercise, the American College of Sports Medicine suggests a starting range of 0.4 liter to 0.8 liter per hour for marathon runners, or about 14 to 27 fl oz.

The lower end fits slower, lighter people in cool weather and the upper end faster, heavier people in warm weather. Sweat rates differ a lot, so the ACSM recommends a personal plan based on weighing yourself before and after exercise, and you should not drink more than you sweat.

For work in the heat, NIOSH advises one cup (8 fl oz) of water every 15 to 20 minutes and never more than 48 fl oz in an hour. The National Academies note that very active people who are always in hot weather often need 6 liters or more of total water a day.

This calculator is for healthy adults. People with heart failure, or with kidney failure treated by dialysis, often have to limit how much they drink, as MedlinePlus and the NIDDK explain. Advanced liver disease can call for limits too.

Drinking far more than you need is rare but can be dangerous. The kidneys can clear only about 0.7 L to 1 L of water an hour, and drinking much faster than that can lower the salt in your blood.