Health & fitness
Water intake calculator
The eight-glasses rule has no research behind it. Actual needs scale with body size, activity and heat — and a fifth of your intake arrives as food.
Where "eight glasses a day" came from
The figure traces back to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day. The next sentence of that document noted that most of this quantity is contained in prepared foods — a qualification that dropped out of the advice as it was repeated.
Modern guidance from the European Food Safety Authority and the US National Academies is expressed as total water from all sources, and lands around 2.0 to 2.5 litres a day for women and 2.5 to 3.7 for men, with wide individual variation.
Food is a real contributor
Around 20% of typical fluid intake comes from food, and considerably more on a diet heavy in fruit and vegetables. Cucumber, lettuce, watermelon, tomatoes and strawberries are over 90% water by weight. Soup, yoghurt and most cooked grains contribute substantially too.
Coffee and tea count as well. The mild diuretic effect of caffeine does not offset the fluid in the drink, and habitual consumers develop tolerance to it. A cup of coffee is a net positive for hydration, and the belief that it is dehydrating is one of the more durable nutrition myths.
Thirst works
For healthy adults with normal kidney function, thirst is a well-calibrated signal and drinking to it is generally sufficient. The main exceptions are older adults, in whom the thirst response weakens with age, young children, people who are ill with fever or vomiting, and anyone exercising hard in heat, where fluid can be lost faster than thirst registers.
Urine colour is the simplest practical check. Pale straw suggests adequate hydration; dark amber suggests drinking more. Completely clear urine throughout the day may mean you are drinking more than you need, though this is harmless for most people.
More is not always better
Drinking far beyond need can dilute blood sodium, a condition called hyponatremia. It is rare but genuinely dangerous, and it has occurred in endurance events where participants drank large volumes of plain water over several hours, and in water-drinking contests. Symptoms include headache, nausea, confusion and, in severe cases, seizures.
For most people the risk is negligible, because healthy kidneys can excrete around 0.8 to 1.0 litres per hour. The situations to be careful about are prolonged endurance exercise, where fluids containing sodium are more appropriate than plain water, and any deliberate attempt to drink a large volume quickly.
Common questions
Does coffee dehydrate me?
No. The caffeine has a mild diuretic effect, but the fluid volume of the drink more than compensates, and regular drinkers develop tolerance. Coffee and tea count toward daily fluid intake.
How much should I drink during exercise?
A common guideline is 400–800 ml per hour of activity, adjusted for heat and sweat rate. To be more precise, weigh yourself before and after a session: each kilogram lost is roughly a litre of fluid. Sessions over about an hour, especially in heat, benefit from electrolytes rather than plain water.
Do I need electrolytes?
Not for everyday hydration — a normal diet supplies plenty of sodium and potassium. They become relevant for exercise lasting more than an hour, heavy sweating in heat, or illness involving vomiting or diarrhoea.
Should I drink more if my urine is dark?
Usually yes, though some vitamins, medications and foods change urine colour without indicating dehydration — B vitamins in particular turn it bright yellow. Persistently dark urine alongside thirst, fatigue or dizziness is worth mentioning to a doctor.