How much water should you drink when it’s hot outside?

RRecord high temperatures are dangerous not only for the planet, but also for human health. Whether you’re in the middle of a heatwave or just trying to survive a scorching day, you’ll need strategies to stay cool and well.

One of the most important is to stay hydrated. But do eight 240 ml glasses really work in very high temperatures?

What you need in a normal day – and a killer day

Most adults should drink somewhere between 68 and 100 ounces of water per day, says Dr. Ashley Karpinos, associate professor of medicine, pediatrics and sports medicine at Vanderbilt University Medical Center. Another way to control your water intake is to halve your weight in pounds — and then drink that number of ounces per day, says Dr. Dana Cohen, an integrative medicine practitioner in New York City and co-author of To switch offa book about the science behind hydration.

But “in the heat, everyone needs more water,” says Karpinos. If you’re working, exercising, or doing physical activity in hot weather, a good rule of thumb is to drink 8 ounces of water every 15 to 20 minutes during the physical parts of the day, says Karpinos.

Sounds simple, right? Not a lot. Hydration is complex. Not only do some people need more water than others, but sometimes a glass of H20 isn’t even the best solution. The following expert tips can help ensure you’re properly hydrated when it’s hot outside.

Track your bathroom breaks

The best way to track how hydrated you are in hot weather is not through the total ounces of water you drink, but rather how often you urinate, says Cohen. “If you don’t get up and go to the bathroom every two to three hours, you’re not well hydrated,” she says. “That’s really the best way to do it: don’t look at the color of the urine, don’t pick at the skin.” (The skin pinch test is often used to assess skin turgor, or how elastic your skin is. If you are dehydrated, your skin may not recover as quickly after a gentle pinch.)

see more information: 8 ways to stay hydrated if you hate drinking water

Seth Feltheimer, a primary care physician and associate professor of medicine at Columbia University’s Vagelos College of Physicians and Surgeons, agrees. “It is not [about] How much you’re drinking is what your urine output is,” he says. “If you’re not urinating, you’re not drinking enough.”

Think beyond the water

Water is typically the best thing you can drink when it’s hot outside, says Karpinos. But other drinks can also do a great job of keeping you hydrated. She recommends drinking something with electrolytes after about an hour in the heat. Sports drinks like Gatorade do the trick, but Cohen recommends coconut water as it contains natural electrolytes.

Some research suggests that a surprising drink can also be hydrating: milk. A small 2016 study published in American Journal of Clinical Nutrition gave people 13 drinks and tested their urine output over the next four hours. All drinks tested (including still and sparkling water, soda, diet soda, hot and iced tea, beer, orange juice, coffee, sports drink, skim and whole milk, and oral rehydration solution) had more or less the same effect moisturizing, except for three highlights: oral rehydration solution, whole milk and skimmed milk. Cow’s milk contains sodium and electrolytes, which could prevent dehydration.

Avoid the Aperol spritz and iced coffee

Generally speaking, any liquid is good for hydration, except caffeinated drinks and alcohol, since both are diuretics, says Lina Begdache, a registered dietitian and associate professor at Binghamton University’s Decker College of Nursing and Health Sciences. It’s also important to avoid alcohol in hot weather because it can interfere with your body’s ability to regulate core temperature, says Feltheimer.

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Cohen notes that coffee is only problematic in excess. “Anything above four cups of coffee starts to become a diuretic,” she says.

Add water-rich foods to your diet

Eating foods with a high water content is a great way to stay hydrated in the heat, says Karpinos. She recommends fruits and vegetables like watermelon, cantaloupe, berries, peppers, lettuce and cucumbers. “You can also add mint leaves or fresh orange slices to flavor the water, which can make it easier to drink.”

Cohen also recommends smoothies, chia seeds, and flax seeds due to their high fiber content. “Chia seeds are kind of the star of the show,” she says. “They increase up to three times their amount with water, so they really retain hydration well.”

Hydrate before you feel thirsty

One of the biggest mistakes people make is waiting until they feel thirsty to drink water, says Karpinos. This is because by the time you feel thirsty, you have already lost fluids and are on your way to dehydration.

Begdache adds that it’s also a good idea to drink water before bed and first thing in the morning, as you lose water while you sleep through breathing and sweating.

Pay special attention to children and the elderly

Because children are smaller than adults, their water reserves are also smaller and they are more likely to become dehydrated in hot weather, says Feltheimer. The amount of water children need varies—toddlers need about 32 ounces a day, while preteens need about 60 ounces, says Karpinos. “Watch children closely for facial flushing, fussiness, or less urination, which could be a sign of dehydration,” she says.

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Elderly people are also more likely to become dehydrated. This is because the body’s water content decreases with age. One 2023 Study published in International Journal of Clinical and Laboratory Medicine analyzed the hydration status of almost 1,000 older adults. The researchers found that 31% of participants were dehydrated. Diabetes and chronic kidney failure were more common in the dehydrated group, as was the likelihood of falling.

Athletes must exercise extra caution

It’s particularly important for athletes to have a hydration plan in the heat, says Karpinos, the Vanderbilt University track and field team physician. “The goal is to start an exercise session well hydrated, maintain hydration during exercise and then correct excessive losses,” he says.

The amount of water to add varies depending on the athlete’s gender, weight and the amount of sweat, as well as the intensity of the workout and the outside temperature, she says. But in general, athletes should drink an additional 12 ounces of water two hours before exercising and about 8 ounces of water every 15 to 20 minutes during a workout, says Karpinos.

Athletes should also check for signs of dehydration when they finish exercising. “If athletes lose weight quickly during an exercise session, it could be a sign of dehydration,” says Karpinos. “In this case, we recommend drinking about 20 ounces of fluids for every pound lost during the exercise session to correct dehydration.”

Be aware of dehydration

Early signs of dehydration include feeling thirsty, dry lips and mouth, not sweating enough, and dark urine, says Karpinos. Feltheimer adds that body cramps, lethargy and inability to urinate “are signs that your body is trying to conserve whatever fluid it has.”

Being dehydrated can have a greater impact than many people realize. Even just a 2% drop in hydration can cause mental confusion, headachesand dizziness, says Cohen. A 2019 study found that dehydration can also have a negative effect on short-term memory, attention, energy and mood.

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Drinking plenty of water is also important in the long term. One 2023 Study published in Lancet found that chronic dehydration can lead to premature aging, chronic diseases (including heart failure, diabetes and dementia) and premature death.

“There is nothing that can function in the body without hydration, right down to the cellular level,” says Cohen. “The most important thing we can do to treat and prevent chronic diseases is to learn to hydrate properly.”

Don’t overhydrate

Some people – in an attempt to stay hydrated on hot summer days – overdo it. Cohen says he regularly sees patients with lab results that are very low in sodium and chloride because they drink a lot of water. “I see this more often than not. [you’d] think,” she says. Overhydration, she says, can lead to many of the same symptoms as dehydration, like muscle cramps and fatigue.

While a little extra water is nothing to worry about, drinking too much can lead to hyponatremia, a condition caused by a lack of sodium in the blood that can cause seizures. Cohen notes that overhydration at this level is exceptionally rare, but it’s still something to be aware of.