How Many Ounces of Water Should You Drink a Day?

The figure on this page starts from your body weight, because size is the biggest driver of how much fluid you lose and need to replace. The calculator uses about 35 millilitres of water per kilogram, then converts to US fluid ounces at 29.6 ml each. That comes out close to the popular American shorthand of half an ounce per pound: a 120 lb person lands near 60 oz, a 154 lb person near 83 oz, and a 200 lb person near 108 oz. None of those are precise to the millilitre — they are estimates of a baseline — but they capture the important truth that a larger body genuinely needs more water than the flat "64 ounces for everyone" rule allows. From that baseline, the rest of your day adds on. To see exactly the same number expressed as litres, glasses and bottles, run the full Water Intake Calculator.

"Drink eight 8-ounce glasses a day" is the most repeated hydration tip in America, and the arithmetic behind it is simply eight times eight, or 64 ounces — about 1.9 litres. It is a tidy, memorable number, and for a smaller and fairly inactive person it is roughly right. The trouble is that it was never meant to be a one-size answer: it ignores body weight entirely, so it under-shoots for most adults of average size and badly under-shoots for larger or very active people. Treat 64 oz as a sensible minimum to clear on a quiet day, not the target. The calculator above shows how many ounces over that floor your own weight and activity put you, which is usually somewhere between 80 and 110 oz for an active adult. If you would rather work from the older "half your body weight in ounces" rule, the half your body weight in ounces page runs that version.

Once you have a baseline, two things push it up. Exercise is the first: you sweat out fluid, and the calculator adds about 350 ml — roughly 12 fluid ounces — for every 30 minutes of activity, so an hour of training adds nearly 24 oz on top of your resting figure. Heat is the second: a hot or humid climate lifts the whole total by ten percent, because you lose more even when sitting still. Put both together — a long workout on a hot day — and a number that started near 80 oz can climb past 110. Remember the figure is your total fluid from everything, not just plain water: food provides roughly a fifth, and tea, coffee, milk and juice all count. To see what those workouts cost in calories, use the TDEE Calculator; to plan eating alongside, the Calorie Deficit Calculator. These are general estimates, not medical advice.