How much water you should drink a day — what the evidence says, and where 8 glasses came from

"Eight glasses a day" is the most repeated health rule in English and has no source anyone can find. Actual needs vary by a factor of two or three between people and days, and the body has a good gauge for it. This guide covers what the evidence says and how the water intake calculator estimates a starting point. General information, not medical advice.

Where 8 glasses came from

The likeliest origin is a 1945 US Food and Nutrition Board note that adults need about 2.5 litres of water a day — followed by the sentence "most of this quantity is contained in prepared foods", which everyone dropped. Total water includes what's in food (fruit, vegetables, soup — roughly 20–30% of intake for most diets) and in all drinks, including tea and coffee, whose caffeine is not dehydrating at normal doses despite the folklore. The 8 × 8 oz rule is about 1.9 litres of plain water, which is more than many people need and less than some.

What health bodies recommend

The European Food Safety Authority's adequate intake is 2.0 litres a day of total water for women and 2.5 for men, from all sources; the US National Academies give 2.7 and 3.7 litres total, of which about 80% from drinks. The UK NHS suggests 6–8 glasses (1.2–1.5 litres) of fluid on top of food. These are population averages for temperate climates and moderate activity, not targets to hit daily. The consistent message underneath: drink to thirst, more in heat and exercise, and the colour of your urine is a better gauge than a counter.

What changes your needs

• Body size: larger bodies need more; a common rule of thumb is 30–35 ml per kg per day, which the calculator uses as its base.
• Exercise: sweat losses of 0.5–1.5 litres per hour in hard exercise; drink during and after, with electrolytes for sessions over an hour or in heat.
• Heat and humidity, altitude, dry cabin air: all raise losses.
• Diet: high-salt and high-protein diets need more; fruit- and vegetable-heavy ones supply more.
• Pregnancy and breastfeeding: add about 0.3 and 0.7 litres respectively.
• Illness: fever, vomiting and diarrhoea raise needs sharply; this is when dehydration becomes dangerous, especially in children and older adults.
• Age: thirst weakens with age, so older adults may need to drink by schedule rather than by thirst.

Signs you're drinking enough

Pale-straw urine several times a day; not feeling thirsty most of the time; no headache or fatigue that a glass of water fixes. Dark yellow urine, dry mouth, dizziness on standing and infrequent urination point the other way. A single litre-bottle on the desk refilled once or twice, plus meals and hot drinks, gets most office workers there; the habit tracker helps if remembering is the problem, and the unit converter sorts out ounces and cups.

Can you drink too much?

Yes. Drinking litres of plain water quickly — endurance events, hazing, some psychiatric conditions — dilutes blood sodium (hyponatraemia), which causes confusion, seizures and has been fatal in marathons. The kidneys handle roughly a litre an hour; beyond that, especially while sweating out salt, water becomes dangerous. Athletes should drink to thirst during long events and replace sodium. For the vast majority of people the risk is nil and the advice simple: keep water available, drink when thirsty, drink more when it's hot or you're working, and don't count. How daily calorie needs are calculated — BMR, activity multipliers and what a deficit means covers the other side of intake; the calorie calculator the numbers.