How much water a day, really
The honest answer is a range, not a number. The major authorities — the US Institute of Medicine, WHO, EFSA, the classic 1945 “8 glasses” rule and a weight-based sports-medicine figure — land anywhere from about 1 to 3.5 litres a day for the same person. We show all five and let you read the spread, instead of trusting one tidy figure.
Total water vs. drinking water
Two of the sources (IOM and EFSA) count total water — including what your food already provides, which is roughly a fifth of the total. The other three count plain drinking water. That single distinction explains most of the gap between the numbers, and it is why “drink 3 litres” and “drink 2 litres” can both be right.
Adjust for your day
Coffee, alcohol, a high-protein diet, hard workouts, a hot climate, pregnancy, breastfeeding or illness all raise your needs. Toggle the ones that apply and the whole range shifts up. These are consumer rules of thumb — useful nudges, not clinical prescriptions.
FAQ
How much water should I drink a day?
There is no single right number. The major authorities land between roughly 1 and 3.5 litres a day for an adult, depending on body weight, activity, climate and how much water your food already provides. We show all five as a range and suggest the mid-point — then thirst and urine colour are your daily guide.
How many litres of water should I drink a day?
For most adults the authorities land between about 1 and 3.5 litres of total water a day — that is roughly 1–3.5 liters, or about 34–118 US fl oz, food included. A common midpoint is around 2–2.5 litres of plain drinking water per day, but the right figure moves with your body weight, activity and climate. Enter your details above to see your own range in litres per day rather than one fixed number.
Why do the sources disagree?
They measure different things. The US Institute of Medicine and EFSA give total water including what you get from food; the 1945 “8 glasses” rule and the sports-medicine figure count plain drinking water only. WHO scales with body weight. Same question, honestly different answers — so a range is the truthful reply.
Do coffee and tea count?
Mostly yes. At normal intake, caffeine’s mild diuretic effect is outweighed by the fluid in the drink, so coffee and tea count toward your total. Alcohol is the exception — it increases urine output, so add water alongside it. Turn on the “Coffee” or “Alcohol” toggle to nudge the range.
Is 8 glasses a day a real rule?
It comes from a 1945 US Food and Nutrition Board note: about 1 ml of water per calorie eaten, which at a ~2000 kcal diet is roughly 2 litres, or eight 250 ml glasses. The catch the original note added — and most people forgot — is that much of that water already comes from food.
How do workouts and hot weather change it?
Both raise your needs through sweat. A moderate session or a hot day adds roughly half a litre; hard or long training more. A simple check: weigh yourself before and after exercise and drink about 1.5 litres for every kilogram lost. Use the “Workouts”, “Hot climate” and other toggles to shift the range up.