How Many Glasses of Water Should You Drink a Day?

The famous "eight glasses a day" line is easy to remember and not far off for an average person, but it is a single number aimed at everyone, and people are not the same size. The honest version is a division: estimate how much water your body needs, then divide by the size of your glass. This calculator uses 250 ml — a quarter of a litre — so eight glasses comes to exactly two litres. A 70 kg adult, whose weight-based need is around 2.4 litres, actually lands closer to ten glasses before any exercise; a 90 kg adult is nearer thirteen. The point is not that eight is wrong, but that it is a floor pitched at a smaller, sedentary person, and your own figure scales up from there with your body and your day.

Three things move the number. The biggest is body weight, because a larger body has more cells and more surface losing water — the calculator uses about 35 ml per kilogram as its baseline, so heavier people simply start higher. The second is exercise: you lose fluid as sweat, and every 30 minutes of activity adds roughly 350 ml, which is about a glass and a half, so an hour's workout is three extra glasses on top of your resting figure. The third is climate: a hot or humid day lifts the whole total by ten percent because you sweat more even at rest. Stack a heavy session onto a hot day for a big person and the count climbs well into the teens, while a small, sedentary person on a mild day may sit comfortably around seven or eight. None of this is a quota to force down — food and other drinks count too, as the next section explains. If you would rather see the figure in ounces or as a comparison to the 8×8 rule, the is 8 glasses of water enough page runs the same numbers.

Glasses are a friendly way to picture a target, but litres are what the body actually deals in, so the two sit side by side in the result. Converting is easy: four 250 ml glasses make a litre, so ten glasses is 2.5 litres and twelve is 3. Remember that you do not have to drink every glass as plain water — fruit, vegetables and soups supply roughly a fifth of most people's fluid, and tea, coffee, milk and juice all count, so the figure is your total intake from everything, not a column of glasses to tick off. Use it as a direction for the day and let thirst and pale-straw urine confirm you are in range. For the full picture with a sensible range built in, run the Water Intake Calculator; to see what your workouts burn, the TDEE Calculator; and to plan eating alongside hydration, the Calorie Deficit Calculator. These are general estimates, not medical advice.