Where the numbers come from
The calculator uses two official sets of reference values. EFSA, the European Food Safety Authority, published its adequate intakes for water in 2010: 2.0 litres a day for women and 2.5 litres for men, the same values from age 14, 2.3 litres in pregnancy and 2.7 litres when breastfeeding, with lower values for children. The US Institute of Medicine of the National Academies published its values in 2005: 2.7 litres for women and 3.7 litres for men aged 19 and over, 3.0 litres in pregnancy and 3.8 litres when breastfeeding. An adequate intake is the amount that healthy groups of people normally take in while staying well hydrated, not a precise requirement for one person.
Total water, drinks and food
Both sets of values are for total water: plain water, every other drink, and the water contained in food. EFSA counts beverages of all kinds, including tea, coffee and mineral water. In the US survey behind the National Academies figures, drinks provided about 81% of total water and food about 19%, so the calculator shows the part from drinks as the National Academies give it, and for EFSA estimates it with the same 81% share. A diet with lots of fruit, vegetables and soups provides more water from food.
Heat, exercise and sweat
Neither body gives a formula for hot weather or exercise; both say people who are physically active or in hot surroundings need more. The National Academies report that people working in very hot climates often sweat 0.3 to 1.2 litres an hour, so the difference can be large and personal. A practical way to measure it is the one the report describes: body weight changes are used to estimate sweating. Weigh yourself before and after exercise or a shift outdoors; each kilogram lost is roughly a litre of water. Enter that figure and the calculator adds it to what you should drink.
Can you drink too much?
Yes, if a large amount is drunk very quickly. The National Academies note that healthy kidneys can excrete at most about 0.7 to 1.0 litre an hour, and that water toxicity has been reported when people drank far more than that in a short time. Spread drinks through the day, let thirst and the colour of your urine guide you, and do not force down large volumes to hit a number.
When to ask a doctor instead
This is a general estimate, not medical advice. People with kidney, heart or liver disease are sometimes told to limit fluids, and some medicines change how the body handles water. If a doctor has given you a fluid target, follow it. Ask a doctor or pharmacist about babies under one year, older people who feel little thirst, and anyone who is unwell with vomiting, diarrhoea or fever.