This Is Exactly How Much Water You Need to Drink in a Day

Although you may prefer wine, water makes up roughly 60 percent of your body, where it seriously pulls its weight: It helps transport nutrients to your cells, moves waste out of your body, and plays an important role in respiration and energy metabolism, according to the National Academy of Sciences’s Institute of Medicine.

The thing is, you lose liquid when you breathe, go to the bathroom, and sweat — bad news if you don’t replace it. “Dehydration is damaging to our tissues and decreases our blood volume, which can reduce blood flow to vital organs,” says Dr. Irwin Rosenberg, M.D., Senior Scientist at Tufts University’s Neuroscience and Aging Laboratory. It’s why even mild dehydration can trigger headaches, darken urine, and cause mouth dryness, says Melissa Majumdar, a registered dietitian at the Center for Metabolic and Bariatric Surgery with Brigham Health and spokesperson for the Academy of Nutrition and Dietetics.

Drink too little — or too much — and you can throw off your body’s concentration of electrolytes, a mix of minerals such as sodium that enable nerves to send messages throughout the body for proper functioning, according to MedlinePlus. Good news: It’s not hard to get your hydration levels just right:

How to Calculate Your Daily Fluid Needs

Most adult women need 11 cups of fluid per day, while most men require about 15 cups— but it largely depends on your body weight and activity levels, says Majumdar.

If you want to get technical, she says you can estimate how many fluid ounces to drink each day by multiplying your body weight in pounds by .5 or, if you plan to exercise or spend time in extreme heat or cold, use .66. Remember: There are 8 fluid ounces in one cup.

When to Step Up Your Hydration Game

Climate and altitude can affect how much fluid you need, according to the Institute of Medicine: In the heat, your body loses more water and electrolytes through sweating, which evaporates to keep you cool. And in cold temperatures or at high altitudes, you lose extra water every time you exhale. To prevent dehydration in these scenarios, Majumdar recommends keeping a water bottle on hand at all times, and refill it regularly. “The best way to hydrate is to sip small amounts consistently throughout the day so your body can absorb the water more efficiently,” she says.

Sickness can also affect your body’s fluid balance: Your body expels a lot of water when you vomit or have diarrhea, according to the Centers for Disease Control. To recover, they recommend sipping on broth or a sports drink, which, unlike water, contains restorative electrolytes.

How to Tell Whether You’re Drinking Enough

You don’t need to count cups — just listen to your body: “Our systems are built to tell us when we’re thirsty,” Dr. Rosenberg says. The first sign you’re behind on fluid intake is a decrease in saliva, which kicks in when you’re two cups short of being hydrated and leads to dryness in the mouth, according to Majumdar. Drinking that much fluid can bring you back to baseline, she says.

To check whether you’re sipping enough throughout the day, glance in the toilet after you pee, suggests Dr. Rosenberg. “If it’s light yellow it means you’re hydrated and your system is working well,” he says — but look out for dark urine, which means your body is so short on water that it’s holding on to what it’s got.

Which Liquids Count?

If you can’t stand the taste of plain old water, which is ideal since it contains no added sugars, according to Majumdar, milk, plus sugar-free options like fruit-infused or carbonated water can count toward your hydration goals. (La Croix please?)

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