You've probably heard the "eight glasses a day" rule since you were in grade school. It’s one of those health myths that just won’t die, even though it’s not exactly based on hard science. Honestly, figuring out how many milliliters of water should you drink a day is way more personal than a single number on a plastic bottle.
If you’re looking for a quick answer, the U.S. National Academies of Sciences, Engineering, and Medicine suggests about 3,700 milliliters for men and 2,700 milliliters for women. But wait. Before you go chugging a two-liter bottle, you need to realize that about 20% of that usually comes from the food you eat.
Water is everywhere. It’s in your morning coffee. It’s in that crisp apple you had at lunch. It’s even in the steak you grilled last night.
Why the 8x8 rule is kinda nonsense
The "8x8" rule (eight ounces, eight times a day) roughly equals 1.9 liters. While that’s a clean, easy-to-remember goal, it lacks nuance. It doesn’t account for your weight, your sweat rate, or where you live. If you’re hiking in the humid heat of Florida, your needs are going to be vastly different than someone sitting in an air-conditioned office in Seattle.
Dr. Heinz Valtin, a kidney specialist from Dartmouth Medical School, spent years looking into this. He found no scientific evidence to support the idea that everyone needs those specific eight glasses. In fact, for most healthy people, the body’s thirst mechanism is incredibly precise. You get thirsty when you need water. It’s a survival instinct that’s worked for thousands of years.
Thirst kicks in when the concentration of your blood (osmolality) increases by less than 2%. Your body is basically a high-tech sensor.
Calculating how many milliliters of water should you drink a day for your body
If you want to get specific, there are better ways to calculate your needs than just guessing. A common baseline used by nutritionists is 30 to 35 milliliters of water per kilogram of body weight.
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Let’s do the math. If you weigh 70 kilograms (about 154 pounds), you’d be looking at roughly 2,100 to 2,450 milliliters.
But it gets complicated.
Are you active? If you’re smashing out a HIIT workout or training for a marathon, you need to add significantly more to that total. The American College of Sports Medicine suggests drinking about 500 to 600 milliliters a few hours before exercise and then "topping off" based on how much you sweat.
The humidity factor
Climate changes everything. High altitudes—anything over 8,000 feet—can double your fluid loss through respiration. You’re breathing faster and the air is drier, so you’re literally exhaling your hydration. If you've ever felt a "high-altitude headache," there's a good chance it's just dehydration masquerading as altitude sickness.
Then there’s the heat. In extreme temperatures, the body can lose up to 1.5 liters of sweat per hour. You can't just stick to a "daily average" when the weather is actively trying to dry you out.
Don't forget the food
People often forget that moisture-rich foods count toward their milliliters. Watermelon and spinach are almost 90-99% water. When you eat a big salad, you're basically eating your hydration. This is why some people feel perfectly fine drinking "less" water than the guidelines suggest—they're getting a massive infusion of fluids through their diet.
On the flip side, if your diet is high in sodium or processed snacks, you'll need more plain water to help your kidneys flush out the excess salt. Salt pulls water out of your cells. It's a tug-of-war happening inside your veins.
Does coffee actually dehydrate you?
This is a big one. For years, we were told that caffeine is a diuretic and therefore "doesn't count" toward your daily intake. That's mostly a myth.
While caffeine has a mild diuretic effect, the water used to brew the coffee more than compensates for the fluid lost. A study published in PLOS ONE by researchers at the University of Birmingham monitored 50 men who drank either four cups of coffee or four cups of water daily. The results? No significant differences in hydration markers.
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Basically, your latte counts. So does your tea.
Alcohol, however, is a different story. It suppresses the antidiuretic hormone (ADH) that tells your kidneys to hold onto water. That’s why you pee so much when you drink beer. For every standard drink, you might lose significantly more fluid than you took in. That’s the "hangover" in a nutshell—your brain is literally shrinking from lack of water.
Signs you aren't hitting your milliliter goals
Checking the color of your urine is the easiest "lab test" you can do at home. You’re aiming for a pale straw color. If it looks like apple juice, you’re behind. If it looks like clear water, you might actually be overdoing it.
Hyponatremia is a real thing. It happens when you drink so much water that you dilute the sodium levels in your blood. It’s rare for the average person, but it happens to endurance athletes who drink gallons of plain water without replacing electrolytes. It can be fatal. Moderation matters.
Other signs of "under-hydrating" include:
- A dull headache that won't go away.
- Feeling "brain fog" or being unable to focus on simple tasks.
- Dry skin that doesn't "snap back" when you pinch it (the skin turgor test).
- Sudden cravings for sugar (sometimes your liver needs water to release glycogen, and if it doesn't have it, it signals for fuel instead).
Actionable steps to optimize your hydration
Stop trying to hit a "perfect" number and start listening to your biology.
- Start with 500 milliliters immediately upon waking. You’ve been breathing out moisture for eight hours. You're naturally depleted.
- Buy a marked bottle. If you don't know the volume of your container, you'll never know how many milliliters of water should you drink a day. Use a 1,000 ml (1 liter) bottle so the math is easy.
- Adjust for "hidden" losses. If you had three cups of coffee, great. If you had three glasses of wine, add an extra 500 ml of water before bed to compensate.
- Eat your water. Incorporate cucumbers, celery, and oranges into your snacks. It’s a slower, more sustained way for the body to absorb fluids because of the fiber content.
- Trust your thirst, mostly. If you are elderly or an extreme athlete, your thirst mechanism might be slightly dampened. In those cases, a schedule helps. For everyone else, if your mouth is dry, drink.
The "right" amount of water is a moving target. It changes when you're sick, it changes when you travel, and it changes as you age. Aim for that 2,700 to 3,700 ml range as a loose framework, but give yourself permission to deviate based on how you actually feel. Your kidneys are remarkably good at managing the rest.
To get the most out of your hydration, monitor your energy levels across three days while hitting a consistent 2,500 ml goal. If the mid-afternoon slump disappears, you've found your baseline. If you're still feeling sluggish or your urine remains dark, increase the intake by 250 ml increments until you reach a steady state of clarity and physical comfort.