Free estimator

How much water do you need? Estimate it from the biggest water study

Eight glasses a day was never measured. In 2022 researchers tracked how much water actually passes through the body in 5,604 people in 23 countries, and turned it into an equation. Put in your own details and weather to see your estimate.

Works out in your browser. Nothing you type is stored or sent.

A few facts about you and the weather where you are most days.

Age (years)Weight (kg)

Sex

Pregnancy or breastfeeding

How active is a usual day?

Mostly sitting · desk job, little walking, no exerciseLightly active · some walking or standing, light exercise once or twice a weekModerately active · on your feet a good part of the day, or exercise most daysVery active · physical job, or hard training about an hour a dayExtremely active · hard training several hours a day, or heavy manual work all day

I train for a sport at competition level

Usual temperature (°C)Humidity (%)Altitude (m)

Use a typical day's outdoor average; a weather app shows both. Leave altitude at 0 unless you live in the mountains.

Where you liveThe study grouped countries by the UN Human Development Index; people in less developed countries turned over more water, likely from more manual work and less air conditioning.

Where the numbers come from

The study (Yamada and colleagues, Science, 2022) gave people water labelled with harmless heavy isotopes and measured how fast it left their bodies. That "water turnover" is the water you lose each day through urine, sweat, breath and stools, which you have to replace. This page uses the study's first equation, built from body weight, sex, age, activity, temperature, humidity, altitude and how developed the country is. For its own example, a 20-year-old man of 70 kg with average activity living at sea level at 10 °C, it gives 3.2 litres a day; a 60 kg woman in the same place, 2.7 litres.

Turnover is not the same as what you should drink. About a tenth is made inside you when you burn food and a little comes in through skin and breath, so drinks and food cover about 85%. Food then supplies 20 to 50% of that in diet surveys. That is why the estimate of drinks is a range. For comparison, the European Food Safety Authority advises 2.0 litres a day for women and 2.5 litres for men from drinks and food together in a mild climate.

Limits: the equation explains 47% of the differences between people, so treat the result as a middle estimate. It was built on healthy people and doesn't apply if a doctor has given you a fluid target. Activity is estimated from a description, and the study's pregnancy figure covers only the last three months.

General information, not medical advice. If you have heart failure, kidney disease or another condition where fluids are limited, follow your doctor's advice. Get help quickly for confusion, fainting, very little dark urine, or after drinking a lot of water in a short time with headache, nausea or vomiting.