Water Intake by Body Weight

Of all the things that change how much water a person needs, body weight is the most important, which is why scaling fluid to weight beats any flat number. The common guideline runs from about 30 to 40 millilitres per kilogram; this tool uses 35 as a sensible middle. The arithmetic is direct — multiply your weight in kilograms by 35 to get millilitres, then divide by a thousand for litres. A 50 kg person lands near 1.8 litres, a 70 kg person near 2.45, and a 90 kg person near 3.15. In US units it works out close to half a fluid ounce per pound, so the same people are near 60, 83 and 108 fluid ounces. These are baselines: they assume an ordinary, lightly active day, and they move up from there once you add a workout or hot weather.

The reference chart in the result turns the formula into something you can scan. Each row takes a common body weight and shows the matching baseline in litres, glasses and fluid ounces, so you can find the row nearest your own and read across. The pattern it reveals is a steady climb: every ten kilograms adds roughly a third of a litre, or about one and a third glasses, to the baseline. That gentle slope is the whole point of a by-weight approach — it explains why two people can both be "drinking enough" at quite different volumes, and why copying someone else's litre target rarely fits. Pick the row closest to you for a quick answer, or enter your exact weight above for a figure to the decimal. To see the same number expressed purely in glasses, the how many glasses of water a day page runs the same baseline.

The by-weight figure is a floor, not the final word, because it deliberately leaves out the day's activity. Exercise is the main thing it does not include: you lose fluid as sweat, and the full tool adds about 350 millilitres for every 30 minutes of activity on top of the weight-based baseline. Heat is the other: a hot or humid climate raises the whole total by ten percent, which is why the chart lets you switch climates and watch every row rise. So treat your row as the amount to drink on a quiet, cool day, and add to it when you train or when the weather turns. As always, food and other drinks count toward the total, so the number is your whole fluid intake, not a column of plain water. For the complete estimate with exercise and a sensible range folded in, run the Water Intake Calculator; to see what your training burns, the TDEE Calculator; and to plan eating alongside, the Calorie Deficit Calculator. These are general estimates, not medical advice.