Daily Water Intake Calculator

Daily water needs: where the numbers come from

The familiar "eight glasses a day" has no identifiable scientific origin. Official reference values are considerably higher than that and, crucially, include water from food and all other drinks - not just plain water.

What the official reference values actually say

Two authorities publish adequate intake figures, and both refer to total water from all sources.

• EFSA (European Food Safety Authority): 2.0 litres a day for adult women, 2.5 litres for adult men, under moderate conditions and average activity.
• US National Academies: 2.7 litres for women and 3.7 litres for men, of which roughly 80% comes from beverages and 20% from food.
• Both explicitly include tea, coffee, milk, juice, and the water content of food. The moisture in fruit, vegetables, soup, and cooked grains is substantial.
• Neither figure is a minimum requirement. They are adequate intakes derived from observed consumption in healthy populations, which is a weaker form of evidence than a requirement derived from measured need.

The eight glasses myth

The claim has been traced to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day - which included the sentence, routinely omitted in retelling, that most of this quantity is contained in prepared foods.

Heinz Valtin's 2002 review in the American Journal of Physiology searched for scientific support for the eight-glasses rule and found none. No study has demonstrated benefit from drinking to a fixed target in healthy people who are not thirsty.

Caffeinated drinks were long excluded on the grounds of a diuretic effect. Controlled studies show the effect is modest at habitual intakes and that coffee and tea contribute net positively to hydration.

When needs rise, and when they should be limited

Requirements are not fixed. Several conditions raise them substantially, and a few make high intake dangerous.

• Raises need: heat, humidity, altitude above roughly 2,500 m, fever, vomiting or diarrhoea, high-protein or high-fibre diets, pregnancy and breastfeeding, prolonged exercise.
• Endurance athletes can lose 0.5 to 2 litres of sweat per hour, and losses above about 2% of body mass measurably impair performance.
• Restricted intake applies in some heart failure, advanced kidney disease, and hyponatraemia. Fluid limits in those conditions are set clinically and should not be overridden by a general calculator.
• Overdrinking is a real risk. Exercise-associated hyponatraemia occurs when plain water intake exceeds sweat losses and dilutes blood sodium; it has caused deaths in marathon runners and military recruits.

Worked example

A 72 kg person exercising 45 minutes in a temperate climate: baseline 72 x 33 = 2,376 ml, plus 45 x 12 = 540 ml for activity, giving 2,916 ml of total water.

Subtracting the 20 to 30% typically supplied by food leaves roughly 2,000 to 2,300 ml to drink - about eight to nine 250 ml glasses spread across the day.

In hot conditions, add 500 to 1,000 ml. Above 2,500 m altitude, add a further 500 ml or so, because respiratory water loss rises with the increased breathing rate in dry, thin air.

Adequate total water intake by reference body

All figures are total water from food and drink combined, not water consumed as a beverage.

Frequently Asked Questions

Related calculators

• TDEE Calculator
• Basal Metabolic Rate (BMR) Calculator
• BMI Calculator
• Volume Converter

References

• EFSA - Scientific opinion on dietary reference values for water
• Valtin H - "Drink at least eight glasses of water a day." Really? (2002)
• US National Academies - Dietary Reference Intakes for Water

Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.