Even if you don’t want to lug a hulking bottle around with you all day, you know that hydration is important. Drinking enough water is the most basic healthy habit anyone can have, yet it’s so easy not to do it.

Not drinking enough water impacts the brain and body quicker than you think. “Dehydration symptoms start with as little as 1% body water loss,” says Heather Colleran, PhD, RDN, CSSD, a sports dietitian and visiting assistant professor at the University of North Carolina Greensboro. While you won’t exactly drop dead by losing 1% of your body’s water content, Dr. Colleran says you could start experiencing brain fog, have trouble concentrating, and start feeling irritable.

For the sake of your body (and mood), it’s important to know how much water to drink a day and whether that amount changes during the summer, if you’re working out, or what you’re hydrating with. Let’s dive in.

Why hydration is important

You know drinking water is important, but what does that water actually do for your body? Sports dietitian and Fueling Intentionally founder Hector Martinez, RD, CSSD, explains that the brain and body can’t function properly if you’re not drinking enough water. “Mild dehydration can affect attention, memory, and how hard effort feels,” he says.

If you consistently don’t drink enough water, Martinez says that this can lead to feeling more tired, less sharp, and less tolerant of exercise or heat. “It may also contribute to headaches, constipation, kidney stones, and a higher risk of heat illness,” he says. “The effects are usually gradual, which is why people often do not notice the pattern right away.”

Part of the reason dehydration affects us so significantly has to do with our body composition. “The average adult is about 60% body water, making staying hydrated important for nearly all bodily functions, including regulating body temperature, lubricating joints and tissues, transporting nutrients and oxygen, and removing waste through urine and sweat,” says Dr. Colleran.

And severe dehydration is really bad for your body. “In severe cases, dehydration can cause rapid heartbeat, sunken eyes, confusion, fainting, and death,” says Dr. Colleran. This happens because when the body doesn’t get enough water, there is a loss of blood volume, the kidneys can’t function properly, and there is an electrolyte imbalance. This dangerous domino effect leads to shock and organ failure.

How much water to drink a day

Fortunately, all of that is easily avoidable by drinking enough water. When it comes to how much water to drink a day, Brandon Willingham, PhD, an assistant professor of exercise science at the University of South Florida, says the standard guideline is 3.7 liters for men and 2.7 liters for women. For men, this comes out to 15.5 8-ounce glasses of water a day—a heck of a lot more than the standard eight glasses a day that’s often preached.

This is the ideal amount of water to drink a day because it reflects population-based guidance for maintaining normal hydration over time. “The body loses water every day through urine, sweat, breathing, and stool, so intake has to keep pace with those losses,” says Martinez. “That is why no single number works perfectly for everyone.”

While drinking 15.5 glasses of water a day is the standard advice for men, Dr. Willingham says that the exact amount someone should get depends on body size, activity level, and how much they’re sweating. “Water intake goal is based on what’s being lost on any given day so you can come out net neutral. We’re trying to match intake with outtake,” Dr. Willingham says, adding that there’s no real benefit to being overhydrated, but, as explained earlier, there are definite risks to becoming dehydrated.