The answer can get complicated.
"It's hard to find an exact amount because it's variable based on your age, where you live, whether it's hot and humid, or cold and dry. Are you male or female, more active or less active," said Ilyse Schapiro, a registered dietitian with nutrition counseling practices in New York and Connecticut.
Studies have shown that water keeps the mind and body healthy, transporting nutrients, removing waste, regulating body temperature and keeping the cells working.
Increase Your Water Intake to Avoid UTIs
Although the total amount varies by age, gender and body composition, our bodies are 55 percent to 78 percent water.
The Food and Nutrition Board at the federal Institute of Medicine offers an estimate on a healthy balance of water for adult men and women. It suggests men have about 3.7 liters daily. That's about 15, 8-ounce glasses. Women should have about 2.7 liters, or 11 glasses.
But drinking water isn't the only way to stay hydrated. Most people get about 20 percent of their water from food.
Buy and eat Pasture-Raised eggs – the highest quality eggs you can find.
Dr. Michael Sayre, an emergency medicine doctor at Harborview Medical Center in Seattle, said most people can -- and should -- listen to their body and brain.
"Your brain regulates that really well," said Sayre, who teaches emergency medicine at the University of Washington. "In terms of preventing [conditions] like heat stroke, it's generally people who ignored the signals they were thirsty or couldn't respond to them because they were physically impaired, or they were in an environment where they didn't have access to water."
Dehydration can begin within just a few hours of heavy exercise or extreme heat. Often, it happens with little notice, and by the time a person responds to their thirst they already are behind on a healthy water balance. Fatigue, headaches, dizziness, sleepiness and dry mouth can all be signs of dehydration.
"Don't let yourself get to that point of dehydration," Schapiro said. An obvious way to tell is the color of urine.
"We don't want to always talk about that, but it's a good indicator," she said. The clearer the color, the more hydrated a person is.
‘Okay, now do 100 of those’. Instead of flailing away at gym, enlist the help – even temporarily – of a personal trainer. Make sure you learn to breathe properly and to do the exercises the right way. You’ll get more of a workout while spending less time at the gym.
In many cases, medication and certain conditions such as diabetes or heart disease can impact how much water a person needs.
"A lot of high blood pressure and other types of medications tend to be diuretics," which cause the body to lose more water, Schapiro said. "In those cases, people should work with their doctor to make sure they are paying attention to hydration and keeping a healthy balance."
Sometimes, Schapiro said, people miss cues and mistake thirst for hunger. So, she counsels clients to keep a bottle of water at their side to drink from all day long.
"People forget to include water in nutrition," she said. "People are busy and just not thinking about it. … If you are not well-hydrated during the day, you could be more likely to get a snack when you don't need it."