The "eight glasses a day" rule has no author, no study, and no real evidence

Ask someone how much water to drink and they will likely say eight glasses. It is one of the most repeated health claims in the English-speaking world. It is also, as far as researchers can tell, not based on any single piece of evidence — it appears to have drifted into public life without a clear scientific origin. That does not make staying hydrated unimportant. It makes the specific number untrustworthy.

Your body already has a hydration sensor

Before worrying about liters and glasses, it helps to notice that the body regulates its own water balance continuously. When you need more fluid, you get thirsty; when you have enough, the kidneys adjust output. This is not a perfect system — thirst can lag during intense exercise or in extreme heat — but for most people on most days, it works remarkably well.

The practical signal most people underuse is urine color. Pale yellow generally means adequate hydration; dark amber often means you are behind. It is cheap, immediate, and needs no app. The "clear urine is always better" idea is also overdone — clear urine can simply mean you are drinking past the point of need.

Where the weight-based formula comes from

The common "35 ml per kilogram" rule of thumb is a convenience, not a physiological law. It produces a reasonable daily figure for a typical adult and has the advantage of scaling with body size — a larger body generally needs more fluid than a smaller one. But it ignores everything else that moves the number: how much you sweat, the climate, your diet, and whether you are pregnant or breastfeeding.

You get water from food and other drinks, too

Hydration is not only about glasses of plain water. Roughly a fifth of daily fluid intake typically comes from food, especially fruit and vegetables. Coffee, tea, milk, and most other beverages also count toward fluid balance — the old claim that caffeinated drinks "dehydrate" you has been largely walked back, since the diuretic effect is mild and short-lived in regular consumers.

A more honest checklist

Rather than a single magic number, a useful daily check is: am I thirsty, what color is my urine, am I about to exercise or spend time in heat, and have I had any fluid recently? If the answers point toward "dry," drink something. The calculator gives you a sensible starting target based on your weight; your body gives you the correction.

Hydration needs vary widely, and some medical conditions require fluid restriction. Follow your clinician's guidance over any general estimate.