Every month, tens of thousands of people ask Google:

“How much water should I drink?”

And Google points them to stories that go something like this:

“Most people are dehydrated—and they don’t know it.”

They list headaches, constipation, bad breath, and other dehydration dangers. And they encourage you to guzzle water, lest you dry up like a sad raisin.

In reality, however, the answer to “how much water should I drink?” is incredibly short and simple:

If you’re thirsty, drink something. If you’re not thirsty, don’t worry about it.

Sooo… why doesn’t this article end right here?

Because there are a few exceptions.

Plus, I’m guessing you might not believe that staying hydrated is so simple.

So let’s walk through why do you need water and how much water you should drink together.

Why do you need water?

You might’ve already heard:

Your body is more than 60 percent water.

It uses that fluid for some obvious things—blood, sweat, tears—and some less obvious things: regulating body temperature, helping your body make hormones, and stopping your brain from smashing into your skull when you’re doing burpees.

It’s true that chronic dehydration can raise your risk for a host of problems no one wants to have: kidney stones, urinary tract infections, and negative cognitive and physical performance.

But there’s a difference between chronic dehydration (being mildly dehydrated a lot of the time) and acute dehydration (which is more severe, and requires timely intervention).

How much water do you need?

Humans need about 3 liters (101 ounces) of fluid per day, though the exact amount will vary from person to person.

Depending on someone’s diet, about 34 ounces (1 liter) of that will probably come from food, especially if they’re eating watery foods like veggies, fruit, prepared oatmeal, or yogurt.

That leaves about 2 liters (67 ounces) to get from beverages.

So the old “drink 8 cups of water a day”—which adds up to 64 ounces—is actually a pretty good general rule.

However:

How much water you need will depend on a range of factors, like age, weight, health status, and activity level, to name a few. If you’re small and sedentary, you might need less than 3 liters. If you’re in a larger body and also exercise in a hot humid environment, you’ll need more.

That’s why thirst is probably a much better gauge than forcing yourself to guzzle a predetermined volume.

Unless, however, you’re an athlete, elderly, or pregnant.

How much water should athletes and exercisers drink?

Though your thirst mechanism works well during rest, it doesn’t function as well during activity.

We’ve known this since the 1930s when researchers asked a man and a dog to walk just under 20 miles (32 km) in the 104F (40C) heat of Boulder City, Nevada. Both the man and the dog could drink whenever they wanted. But only the dog finished well hydrated. The man, on the other hand, lost about 6 pounds (3 kg) of his body mass through water loss.

Research shows that, when people doing intense exercise rely on thirst alone, they tend to under consume fluid, replacing only about half of what they lose.

People can lose 1-2 percent of their body weight very quickly when they’re exercising intensely in a hot, humid environment. That’s enough to raise heart rate, body temperature, and your perception of effort. For aerobic efforts, like cycling, it’ll also slow you down.

In addition to the liter of fluid you get from food, consume at least 3 liters (101 ounces) of fluid on the days that you exercise.

That breaks down as follows:

• 1 liter (34 ounces) before and during exercise
• 1 liter (34 ounces) after exercise
• 1 liter (34 ounces) throughout the day, with roughly 1-2 cups of water with each meal

During long-duration and/or intense exercise, of course, you lose more than just water. This includes sodium, potassium, and other electrolytes—and it’s important to replace those, too. Otherwise you risk hyponatremia (sodium deficiency).

So, instead of plain water, consume an electrolyte beverage (such as a sports drink) for exercise lasting more than an hour in intense heat and/or humidity or more than two hours in any condition.

How much water should you drink during pregnancy?

When you’re growing a human, your blood volume increases, which boosts your overall fluid needs.

As a general rule of thumb, you’ll want to consume roughly 1 liter (34 ounces) more during pregnancy than you consumed before.

If your urine is pretty dark, you’re probably underdrinking. If it’s light to clear, you’re doing great.

How much water should older people drink?

When elderly folks are admitted to the hospital, they’re often dehydrated.

That’s probably because, as we get older, our thirst mechanisms don’t work like they used to. Neither do our kidneys. Certain medications can increase urine output, too. Plus, our bodies don’t seem to hold as much fluid.

All of that increases our risk of becoming dehydrated.

If you’re 65 or older:

• Drink an additional 8 to 16 ounces (.25 to .5 liters) of fluid—over and above your level of thirst.
• Consume whatever beverage you enjoy—an electrolyte drink, iced tea, or even diet cola.
• Monitor your urine and drink up if it’s bright yellow or darker.
• Know the symptoms of dehydration: dark urine, fatigue, dizziness, headaches, dry mouth.

How much should I drink to lose weight?

A few years back, researchers did a series of studies that found drinking water could increase calorie burning. 

However, the researchers predicted an extra 2 liters (67 ounces) of water might only boost energy expenditure by about 96 Calories.

For context, that’s about the number of calories in a medium banana.

Maybe you’re thinking: Doesn’t water at least dull the appetite, helping people to eat less?

It might.

One study found that downing half a liter (16 ounces) of water before meals helped people eat less and consequently lose weight.

You can also take small sips of water between bites, which will help you eat more slowly.

How to calculate your water needs

How do you make sure you’re hydrated?

You have two options.

Option #1: Drink when you’re thirsty

This super-simple option works for most people, including:

✓ People who live in cool-to-moderate climates

✓ People younger than 65

✓ Non-athletes

Option #2: Monitor your urine

Some people occasionally suffer from acute dehydration.

Like when lots of stuff is coming out of both ends due to food poisoning or an infection. Or when exercising intensely in a hot climate.

As long as they replace what they lost, it’s no big deal.

How do you know if you’ve replaced what you’ve lost?

The answer: Check your toilet.

The more dehydrated you are, the greater your urine osmolality (saltiness).

Luckily, you can also assess osmolality through color: The greater the osmolality, the darker your urine.

If you compare your urine color to what’s shown on the chart below you’ll get an answer that’s almost as accurate as an expensive hydration test that your physician or trainer would run in the office.

This option works best for:

✓ People who live in hot, dry environments and worry about dehydration

✓ People whose jobs make on-demand drinking difficult (such as a healthcare worker who wears a mask for a 12-hour shift)

✓ People who exercise moderately in hot and/or dry environments

✓ People who are pregnant

✓ People age 65 and older