7 Hydration Myths, and What's Actually True

Most of the hydration rules people repeat, like 8 glasses a day, half your body weight in ounces, and coffee dehydrating you, don’t hold up against the evidence. The official US guidance is simpler: most healthy people meet their needs by drinking when they’re thirsty and with meals, and every drink counts, coffee included.

These myths are not fringe. When we read 177 pages that rank on Google’s first page for common water questions, 34 of them repeated at least one hydration myth as fact. You can see the full breakdown in our research on hydration myths. Below are the five myths we tracked, plus two more, each with what’s true and where that comes from.

Myth 1: You need 8 glasses of water a day

What’s true: There is no scientific basis for “8 x 8” as a rule. A 2002 review in the American Journal of Physiology found no scientific studies supporting the advice to drink eight 8-oz glasses of water a day, for healthy, largely sedentary adults in a temperate climate.

What the National Academies set instead is an Adequate Intake of total water: about 2.7 liters (91 fl oz) a day for women and 3.7 liters (125 fl oz) for men. That total includes food, which supplies about 20% of it, and every kind of drink, not just plain water.

On Google’s page one: 16 pages we read repeated 8 glasses as a rule; 24 corrected it.

Is 8 glasses of water a day a myth?

As a rule everyone must follow, yes. The review found no evidence behind it. But it isn’t far off for many women: the National Academies expect about 2.2 liters (74 fl oz, roughly 9 cups) of a woman’s daily intake to come from beverages, according to their Dietary Reference Intakes. The catch is that those cups include coffee, tea, milk and other drinks, and that your own needs move with heat, activity and health. Very active people in hot weather can need 6 liters (about 200 fl oz) or more of total water a day, the National Academies note.

So 8 glasses is a reasonable rough picture for many people, not a target you fail by missing.

Myth 2: Drink half your body weight in ounces

What’s true: No health body sets water needs by body weight. The National Academies give fixed daily amounts by age and sex. Size does affect what you need, along with activity and climate, so your weight can be a personal starting point. It just isn’t the official rule. We explain how to use it sensibly in how much water to drink based on your weight.

On Google’s page one: 4 pages we read gave half your body weight in ounces as advice, and none corrected it.

Myth 3: 75% of Americans are chronically dehydrated

What’s true: This statistic has no study behind it. The claim that “75% of Americans are chronically dehydrated” circulated in a 2011 chain email, and Snopes rates its “chronically dehydrated” premise False.

What survey data does show is how people actually drink: US women drink an average of 43 oz of plain water a day, according to CDC data, and get the rest of their fluid from other drinks and food. That’s a fact about habits, not proof of mass dehydration. More figures are in our hydration statistics.

On Google’s page one: 3 pages we read still presented the 75% figure as fact; none corrected it.

Myth 4: Coffee dehydrates you

What’s true: Coffee counts toward your daily water. The National Academies concluded that “caffeinated beverages can contribute as much as noncaffeinated options” to daily water intake. We go deeper in does coffee count as water.

Caffeine has its own limit for other reasons: the FDA cites 400 mg a day, about two to three 12-fl-oz cups of coffee, as an amount not generally linked to negative effects for most adults.

On Google’s page one: this is the myth that’s losing. 7 pages we read said coffee dehydrates you; 27 corrected it.

Myth 5: If you’re thirsty, you’re already dehydrated

What’s true: Thirst is how your body tells you to drink, and it works well for most people. The National Academies found that “the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide,” along with drinks at meals.

Feeling thirsty on an ordinary day is the signal to drink, not proof that something has gone wrong. Thirst is also listed as a symptom of dehydration by the NHS, which is where the myth gets its grain of truth: if thirst comes with dark urine, dizziness or peeing less than usual, take it seriously.

On Google’s page one: this was the most stubborn myth in our study. 10 pages we read stated it as fact, and not one corrected it.

Myth 6: Your urine should be completely clear

What’s true: The aim is pale, not colorless. The NHS advises drinking enough during the day “so that your pee is a pale colour,” and lists dark yellow, strong-smelling urine as a sign of dehydration. MedlinePlus also lists dark-colored urine among the signs.

Myth 7: You need a sports drink for every workout

What’s true: For most everyday workouts, water is enough. The National Athletic Trainers’ Association says exercise “lasting less than 1 hour should require no substances other than water.” Carbohydrate-electrolyte drinks may help in longer or more intense sessions, especially in heat. Our electrolytes vs water guide covers when they make sense, and the workout hydration guide covers the rest.

How much water do you actually need?

For most healthy adults, the honest answer is: drink when you’re thirsty, have something with meals, drink more in heat and when you sweat, and count all your drinks. If you’d like a number to start from, our water intake calculator shows the official figure first and a personal estimate next to it.

When to see a doctor

Mild thirst is normal, but dehydration can become serious. MedlinePlus lists the usual signs as feeling very thirsty with a dry mouth, peeing and sweating less than usual, dark-colored urine, dry skin, tiredness or dizziness. Get medical help right away if dehydration comes with confusion, fainting, little or no urination, a rapid heartbeat, rapid breathing, or signs of shock. If you’re pregnant, have a health condition, or take medicines that affect fluids, ask your doctor what’s right for you rather than relying on any rule of thumb.

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Hydrate counts every drink you log, coffee included, and adjusts your goal for workouts and hot days. Join the waitlist.