Water Intake Calculator

How much water should you drink a day?

Water makes up roughly 60% of an adult's body weight and is lost continuously through urine, sweat, breath and stool. The amount you need to replace depends mainly on your body size, how much you sweat and whether you are pregnant or producing milk, which is why a single figure such as "eight glasses a day" fits few people well. This calculator starts from your weight and adds allowances for the factors that raise your losses, giving a daily target for fluids from drinks.

The formula behind this calculator

The baseline is 35 ml of water per kilogram of body weight, a rule of thumb used by dietitians and in clinical nutrition textbooks for healthy adults. It sits between the 30 ml/kg often used for older adults and the 40 ml/kg sometimes applied to active young people. On top of that:

• Exercise: + 350 ml for every 30 minutes, pro-rated. That is about 0.7 liters per hour, in the middle of the 0.4 to 0.8 liters per hour sweat rates typical of moderate exercise.
• Hot or humid climate: the total so far is multiplied by 1.1 to cover higher sweat losses at rest.
• Pregnancy: + 300 ml a day, matching the increase in the official adequate intake.
• Breastfeeding: + 700 ml a day, the extra recommended by the European Food Safety Authority to cover milk production.

Example: a 70 kg person who exercises for 30 minutes a day in a temperate climate needs 70 × 35 = 2,450 ml as a baseline, plus 30 ÷ 30 × 350 = 350 ml for exercise. The total is 2,800 ml, or 2.8 liters: about 11.8 US cups, 94.7 fluid ounces or five and a half 500 ml bottles. In a hot climate the same person would need 2,800 × 1.1 = 3,080 ml.

Official recommendations

The US National Academies of Sciences, Engineering, and Medicine (NASEM) set an adequate intake for total water, which includes the water in food. Food typically supplies about 20% of the total, so the amount to drink is roughly 80% of these figures:

These are averages for healthy, lightly active people in a temperate climate. The weight-based estimate above lands in the same range for average-sized adults and scales up or down for people who are larger, smaller or more active.

How to use this calculator

1. Choose metric or US units and enter your current body weight.
2. Enter the minutes of sweaty exercise you do on a typical day. Use 0 for a rest day.
3. Pick "Hot or humid" if daytime temperatures are regularly above about 30 °C (86 °F) or the air is very humid.
4. Select pregnant or breastfeeding if that applies, then read the target in liters, cups, ounces or bottles.

Signs of dehydration and when to drink more

• Early signs: thirst, dry mouth, dark yellow urine, urinating less than four times a day, headache and tiredness. Dizziness on standing and confusion are later, more serious signs.
• Illness: fever, vomiting and diarrhea raise losses sharply; add extra fluid and consider an oral rehydration solution.
• Altitude and dry air: breathing loses more water at altitude and in air-conditioned or airplane cabins.
• Diet: high-protein, high-fiber and high-salt diets all need more water to process, as does alcohol.
• Older adults feel thirst less strongly and may need to drink on a schedule rather than waiting to feel thirsty.

Can you drink too much?

Healthy kidneys clear roughly 0.8 to 1 liter of water per hour, so a daily target spread across the day is safe. Problems arise when people force down several liters in a short time, most often during marathons or long hikes, which can dilute blood sodium (hyponatremia). During exercise longer than an hour, drink to thirst and replace salt as well as water. If you have heart, kidney or liver disease, or take diuretics, your doctor may set a different limit. To see how your weight and activity affect your energy needs as well, try the calorie calculator.

This calculator gives a general estimate only and is not medical advice.