Water Intake Calculator: How Much Water Do You Need a Day?

Water makes up more than half of an adult’s body weight, and every organ depends on it. This tool estimates your daily fluid needs from your weight and physical activity and shows the official reference values of the European Food Safety Authority (EFSA) alongside, so you know in numbers how many glasses you actually need.

Why does your body need water?

Water makes up about 50 to 60% of an adult’s body weight and is the medium for almost every vital process: it carries nutrients and oxygen in the blood, regulates body temperature through sweat, removes waste through the kidneys and lubricates joints and membranes. The body does not store water, so what we lose every day through urine, sweat and breathing has to be replaced continuously.

Even mild dehydration, losing about 1–2% of body weight as water, can be linked to headache, fatigue, poorer concentration and lower exercise performance, as a review in Nutrition Reviews describes.

How does the calculator estimate your water needs?

The calculator combines two approaches:

1. The weight-based clinical rule: 30 to 35 ml per kilogram of body weight, a common rule of thumb in nutrition practice, plus extra water for exercise depending on duration and intensity.
2. The European reference value (EFSA): shown for comparison. It is an “adequate intake” for total water from drinks and food together.

Water need (L) = weight × (0.030 to 0.035) + exercise water + (0.3 if pregnant or 0.7 if breastfeeding)

Worked example: a man who weighs 75 kg and does 30 minutes of moderate activity. 75 × 30 = 2,250 ml and 75 × 35 = 2,625 ml; adding 350 ml for exercise gives a range of 2.6 to 3.0 litres a day, or about 10 to 12 glasses. The EFSA reference for men is 2.5 litres from all sources, about 2.0 litres of it from drinks.

EFSA reference values for water

In 2010 the European Food Safety Authority set an “adequate intake” for total daily water. Usually 20 to 30% comes from food and the rest from drinks:

These values apply to moderate temperatures and moderate activity. In hot weather, such as a Gulf summer, or with long training sessions, needs rise clearly.

Water and exercise

The American College of Sports Medicine (ACSM) advises keeping sweat losses during exercise below 2% of body weight. The simplest way to know your needs is to weigh yourself before and after training: replace each kilogram lost with about 1.5 litres of fluid, spread over the following hours. For sessions longer than an hour with heavy sweating, a drink containing sodium helps.

Are you drinking enough? Signs of dehydration

• Urine colour: pale yellow (straw-coloured) means good hydration; dark yellow means you need more water.
• Thirst and dry mouth: relatively late signals, and weaker in older adults, who benefit from a regular drinking schedule.
• Headache, fatigue and dizziness on standing: can be signs of dehydration, especially in the heat.
• Infrequent urination: fewer than 4 times a day can point to low fluid intake.

When to be careful with large amounts of water

Drinking very large amounts in a short time can lower blood sodium (hyponatraemia), a serious condition sometimes seen in long endurance events. More importantly, some patients need fluid restriction on medical advice, such as people with heart failure, advanced kidney failure or dialysis, liver cirrhosis or SIADH. If that applies to you, follow your doctor’s amount, not the calculator.

Practical hydration tips

1. Start your day with a glass of water and keep a bottle in sight at work.
2. For people with diabetes: high blood sugar increases urination and thirst. Water is the best drink; avoid juices and sugary soft drinks that raise blood sugar quickly.
3. During Ramadan: spread water between iftar and suhoor instead of drinking a large amount at once, and limit very salty foods and caffeinated drinks at suhoor.
4. Coffee and tea: they count as fluid. A study in PLoS One found that four moderate cups of coffee a day hydrated people about as well as water.
5. For weight management: a randomised trial found that drinking about 500 ml of water before meals on a low-calorie diet modestly increased weight loss in middle-aged and older adults. Work out your calories with the TDEE calculator.

Water-rich foods from our food guide

Part of your water comes from food. These foods are high in water and low in calories:

Check your other health markers with the BMI calculator and the waist-to-height ratio calculator, or browse the food guide.

Frequently asked questions

Related calculators and tools

Scientific sources

1. EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA). Scientific Opinion on Dietary Reference Values for water. EFSA Journal. 2010;8(3):1459.
2. Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutr Rev. 2010;68(8):439-458.
3. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, et al. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390.
4. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154.
5. Dennis EA, Dengo AL, Comber DL, et al. Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity. 2010;18(2):300-307.

This calculator is an educational estimate for healthy adults. If you have heart failure, kidney or liver disease, or your doctor has told you to restrict fluids, follow their instructions rather than the calculator.