Water Intake Calculator

How to use the water intake calculator

1. Choose Metric (kg, L) or Imperial (lb, fl oz).
2. Select your Sex. For women, the Pregnancy or breastfeeding menu switches to the higher reference values (Pregnant or Breastfeeding).
3. Optionally enter your Weight. It is only used for the rule of thumb shown at the end; the reference values do not depend on it.
4. Enter your Exercise today (minutes) and tick Hot or humid weather if it applies.
5. Read Estimated drinks for today, with the equivalent in 250 mL glasses. The other boxes show how it is built: the Reference total water (EFSA), the part Of which from drinks (about 80%), the Extra for exercise (rough estimate) and the Extra for heat (rough estimate), plus the US reference and the weight-based rule of thumb for comparison.

Everything is calculated in your browser. Nothing is sent anywhere.

There is no single formula

Despite the popularity of “8 glasses a day” or “drink X mL per kilo”, there is no evidence-based formula that gives an individual’s exact water needs. Needs depend on body size, diet, activity, climate and health, and the body regulates them well through thirst in healthy people. What health authorities publish instead are adequate intakes: the average total water intake of healthy populations with no sign of dehydration.

This tool uses those references:

• EFSA (European Food Safety Authority, 2010): 2.0 L per day for women and 2.5 L for men, 2.3 L during pregnancy and 2.7 L while breastfeeding.
• IOM (US Institute of Medicine, 2004): 2.7 L per day for women and 3.7 L for men, 3.0 L during pregnancy and 3.8 L while breastfeeding.

These figures are total water, which includes the water in food (fruit, vegetables, soups, dairy). EFSA assumes food provides about 20%, so the tool shows 80% of the EFSA value as the amount to get from drinks: 1.6 L for a woman and 2.0 L for a man. All drinks count, including tea, coffee and milk. The two agencies differ because they studied different populations and methods; the gap is a reminder that these are orders of magnitude.

Exercise, heat and the weight rule of thumb

Sweat losses vary enormously, from about 0.3 to more than 2 L per hour depending on intensity, fitness, clothing and temperature (American College of Sports Medicine position stand, 2007). The tool adds a rough 0.5 to 1 L per hour of exercise and 0.5 to 1 L for a hot or humid day, shown as ranges because nobody can predict your sweat rate. For a better personal estimate, weigh yourself before and after a workout: each kilogram lost corresponds to roughly one litre of fluid.

The 30–35 mL/kg rule is widely quoted but is a rule of thumb, not an official reference. It is shown only for comparison. For a 70 kg adult it gives 2.1 to 2.45 L of total water.

Safety first

People with kidney disease, heart failure, liver disease or any condition requiring a fluid restriction must follow their doctor’s instructions, not a calculator. Drinking very large amounts in a short time can also be dangerous (hyponatraemia), particularly during long endurance events. Older adults may feel less thirsty and benefit from regular drinks. Signs such as dark urine, dizziness or confusion deserve medical attention. These values apply to adults; ask a health professional about children.