A weight-based estimate of daily fluid, adjusted for exercise and heat.
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Time spent actually sweating, not time at the gym.
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Total fluid a day
2.7 L
Includes water from food, which is roughly a fifth of the total
About 9 glasses of 250 ml
From body weight alone
Replacing what you sweat out
Pale straw is about right
Tea and coffee count toward this. So do fruit, soup, and yoghurt. The number is a starting target, and your own urine colour is a better day-to-day guide than any calculator.
Not from research. It is a piece of folklore that has been repeated so widely it acquired the sound of a guideline, and it takes no account of body size, activity, or climate. A 50 kilogram office worker in a cool climate and a 100 kilogram labourer in summer heat do not need the same amount of anything.
Weight-based estimates are a better starting point, which is what this tool uses. They are still estimates.
Roughly a fifth of most people's fluid intake arrives as food. Fruit, vegetables, soup, and yoghurt all carry meaningful water. So the number here is total fluid, not glasses of plain water you must drink on top of everything else.
Tea and coffee count too. The idea that caffeine dehydrates you is overstated at normal intakes: the fluid in the cup more than offsets the mild diuretic effect.
Urine colour. Pale straw is about right. Dark yellow means drink more, and completely clear usually means you are drinking more than you need.
Thirst is also more reliable than it gets credit for in healthy adults. The main exceptions are older people, whose thirst signal weakens with age, and anyone exercising hard in heat, where losses outrun the sensation.
Water is one of the most commonly tracked habits and one of the most commonly abandoned, because the target is usually set as a daily volume that requires counting. Counting is the part people stop doing.
Attaching it to something you already do works better than a number. A glass with each meal and one when you sit down at your desk is four without any arithmetic. That is the version that survives a busy week.
The baseline estimate scales with body mass, in either kilograms or pounds.
Hard or long sessions add meaningful losses, especially in heat. Add roughly what you actually did, not what you planned.
Hot or humid conditions raise the estimate. So does altitude, which is often forgotten.
Use the figure as a starting target, then adjust by urine colour and thirst over a few days.
Estimates how many calories you burn in a day, and what to eat to hold, lose, or gain weight.
Enter height and weight. The number appears as you type, and nothing leaves your browser.
Work backwards from your alarm, or forwards from bedtime, in whole sleep cycles.
A calculator tells you where you are once. If the thing you actually want is to keep something up, that takes a record rather than a result.
