“Drink more water” is one of those pieces of advice that gets repeated so often nobody stops to ask where the number comes from. Some people swear by eight glasses a day, others by two liters, others insist you should just drink whenever you feel thirsty. So where does the guidance actually come from, and how much does it apply to you personally?

Where the “eight glasses” rule came from

The familiar rule of eight 8-ounce glasses — roughly 1.9 liters — isn’t a strict scientific threshold so much as a tidy, memorable number that stuck in popular culture. It happens to land in a reasonable ballpark for an average-sized, moderately active adult, which is probably why it spread so widely, but it was never meant to be a one-size-fits-all target. Actual fluid needs can differ quite a bit between individuals.

Water isn’t just what you drink

One detail that often gets missed: a meaningful share of daily fluid comes from food, not from glasses and bottles. Vegetables, fruit, soups, and broths all contribute to your overall fluid balance. Cucumber and watermelon are roughly 90 percent water, so a serving of either is effectively another glass of fluid, just one that doesn’t look like one. That means someone eating plenty of fresh produce may need less from drinks than someone whose diet leans heavily on dry, processed foods.

What actually drives your fluid needs

Fluid requirements depend on body size, activity level, ambient temperature and humidity, and what you’re eating. A larger person generally needs more fluid than a smaller one. Heat and intense exercise increase sweat losses that need replacing. Certain conditions — fever, vomiting, diarrhea — raise fluid needs as well, and in those cases it’s worth checking with a doctor rather than relying on generic guidelines.

How to tell if you’re drinking enough

One of the simplest, most reliable indicators is urine color: pale yellow generally signals adequate hydration, while dark yellow or amber suggests you should drink more. Thirst itself is also a reasonably good guide for most healthy adults, despite the popular myth that “by the time you’re thirsty, it’s too late.” The exception is older adults, whose sense of thirst tends to become less reliable with age — for them, sipping fluids throughout the day rather than waiting to feel thirsty is a sounder strategy.

Can you actually drink too much water

Yes, though in practice this is rare and usually tied to extreme situations — for example, trying to consume very large volumes of fluid quickly during an endurance event. Excess water can dilute sodium levels in the blood, a condition known as hyponatremia. For most people going about ordinary daily life, drinking according to thirst without chugging liters at a sitting, this risk is essentially not a concern.

What else affects fluid balance

Caffeine and alcohol have a mild diuretic effect, which is where the idea that “coffee doesn’t count” toward fluid intake comes from. In practice, for someone who drinks coffee moderately and regularly, this effect is minor, and a cup of coffee still contributes net fluid. A significant amount of alcohol, on the other hand, does produce noticeably more dehydration, which is one reason a glass of water after a night of drinking tends to help.

Practical habits beat precise numbers

Rather than tracking milliliters, it’s usually more useful to build a few simple habits: keep a water bottle within reach, drink a glass with meals and in the morning, glance at urine color now and then, and don’t override thirst when it shows up. This kind of approach tends to work better than a fixed daily target, because it naturally adjusts for the things that actually matter — body size, climate, activity, and diet — without requiring anyone to count anything.