How this instrument works
A body is mostly fluid — roughly 60% of an adult's mass — and it loses some every hour through urine, breath, and skin. Those losses scale with size, because a larger body has more tissue to perfuse and burns more energy keeping it alive. That is why this sheet multiplies rather than prescribes: 33 millilitres for every kilogram you weigh, which puts a 60 kg adult near 2 litres a day and a 90 kg adult near 3.
The coefficient is not arbitrary. Clinical nutrition has long put adult maintenance fluid at 30 to 35 ml per kilogram per day, and 33 sits in the middle of that band. It agrees with the older bedside rule of one millilitre per kilocalorie eaten, and it converges on the population figures published by the European Food Safety Authority — 2.0 litres daily for women, 2.5 for men, counting every source — once you feed it typical adult weights.
Two honest caveats sit around the result. Food supplies perhaps a fifth to a third of everything you take in — soup, fruit, yoghurt, even bread — so the litre figure is the total your body settles at, not a debt owed to the tap. And the number moves: heat, altitude, fever, hard exercise, pregnancy, and breastfeeding all push it up, while certain heart and kidney conditions call for the opposite. Read it as a starting guideline, never as a prescription.
- Enter Body weight — the unit menu takes kilograms, pounds, or stone and converts before any arithmetic runs.
- Read Daily water, the 33 ml/kg target for one full day; switch its unit to millilitres, US cups, or gallons if those suit you better.
- Check Glasses (250 ml) for the same quantity as a count you can actually keep track of between morning and bed.
- Add roughly half a litre to a litre for every hour of hard exercise or real heat, on top of what the sheet returns.
Worked example — a 70 kg adult
Take a 70 kg adult. Multiply: 70 × 33 ml = 2310 ml, which is 2.31 litres — the figure the instrument returns, and 0.00231 cubic metres in the canonical units underneath. Divide by a 250 ml glass and the day works out to 2310 ⁄ 250 = 9.24 glasses, so nine of them plus a mouthful.
Nine glasses sounds like a lot until you count what already arrives uninvited: two mugs of tea at 250 ml each, a 330 ml can with lunch, and the moisture inside an apple, a bowl of soup, and a yoghurt. Subtract those and the deliberate drinking left over is nearer three or four glasses. The 2.31 litres is the total the body settles at, not a separate quota poured on top of everything else.
Questions
Is eight glasses a day the right target?
Not for everyone — it is a round number with no clear origin in the research literature. Eight 250 ml glasses is 2 litres, which happens to land close to the requirement of a 60 kg adult and well under it for anyone larger. Scaling by body mass keeps the same spirit while acknowledging the obvious: a 55 kg person and a 95 kg person are not running the same plumbing.
Do tea, coffee, and juice count toward the total?
Yes. Caffeine is a mild diuretic, but the fluid arriving in a cup of coffee comfortably outweighs the extra passed because of it, and controlled trials find no meaningful dehydration at ordinary intakes. Milk, juice, soup, and soft drinks all count as well. Plain tap water simply arrives without sugar, acid, or calories, which is why it stays the default suggestion rather than the only permitted answer.
How much of my daily fluid comes from food?
Roughly 20 to 30% on a typical mixed diet. Cucumber, melon, and lettuce are over 90% liquid by mass; soup, milk, yoghurt, and cooked rice or pasta carry a great deal too. Because this instrument reports the total daily requirement, that dietary fraction already sits inside the number — so the amount you consciously drink can fall meaningfully below the litre figure shown and still be enough.
Can you drink too much?
Yes, though it takes real effort. Healthy kidneys clear somewhere around 0.8 to 1.0 litres per hour, and pushing well past that for hours — most often during endurance events, or in a contest — can dilute blood sodium far enough to cause hyponatremia, which is genuinely dangerous. Spreading intake across the day and stopping once thirst and pale-straw urine agree that you are topped up avoids the problem entirely.
How much extra should I add for exercise or hot weather?
Plan on half a litre to a litre for every hour of sustained sweating, and more in genuine heat — sweat rates of 1 to 2 litres per hour are ordinary during a hard session in warm conditions. Weighing yourself before and after a workout gives the exact answer: each kilogram lost is close to a litre of fluid to replace, since sweat is mostly water.
Why does the target scale with weight rather than height?
Because fluid turnover tracks tissue mass and energy expenditure, both of which weight captures better than height does. A tall, light person and a short, heavy one of equal mass lose fluid at broadly similar rates, whereas a 100 kg body has more cells to perfuse, more surface to cool, and a larger blood volume than a 50 kg one. Weight is also the input a bathroom scale reports honestly, with no tape measure required.
References
- EFSA — Scientific Opinion on Dietary Reference Values for water (2010)
- National Academies — Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate
- CDC — Nutrition
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.