You may have heard of the 8×8 rule. It states that you should
drink eight 8-ounce glasses of water per day.
That’s half a gallon of water (about 2 liters).
This claim has become somewhat of an accepted wisdom and is
very easy to remember. But is there truth to this advice or is
it just a myth?
The Origin of the 8×8 Rule
The source of the 8×8 rule has not been confirmed (1).
One theory suggests it may have originated in 1945, when one
research organization released a report stating that the
average person needs to consume 1 ml of water per calorie
of food they consume.
For someone eating a diet of 2,000 calories per day, this adds
up to 2,000 ml (roughly 64 oz), or eight 8-oz glasses.
But the report also declared that much of this water could be
obtained from foods you consume.
Another probable origin of the 8×8 rule is the work of a
nutritionist named Dr. Frederick Stare. He coauthored a book
published in 1974 that recommended drinking six to eight
glasses of water per day.
The book also pointed out that fruits and vegetables, as well
as other beverages, are high in water.
However, this part of the story seems to have been neglected
when information from this book spread to the public,
researchers and health organizations.
Bottom Line: It’s unknown where the
recommendation to drink eight 8-oz glasses of water per day
comes from originally, but a couple of theories exist.
What Studies Say About the 8×8 Rule
One article from 2002 examined the scientific evidence behind
the 8×8 rule (1).
It reviewed dozens of studies, surveys and articles, finding
absolutely no scientific evidence suggesting that you need to
drink eight 8-oz glasses of water per day for adequate water
However, it must be noted that this finding is limited to
healthy, albeit mostly sedentary adults living in a mild
While there are certainly circumstances in which water needs
increase, healthy men and women generally don’t need to be
consuming water in such large quantities.
On the other hand, not drinking enough water can cause mild
dehydration, defined as the loss of 1–2% of body weight due to
fluid loss. In this state, you may experience fatigue, headache
and impaired mood (2, 3).
But in order to stay hydrated and avoid mild dehydration, you
don’t need to rigorously follow the 8×8 rule. Luckily, you have
a built-in instinct called thirst.
For this reason, most people don’t need to worry about their
water intake — thirst will tell you when you need water.
Bottom Line: There is no scientific evidence
to support the 8×8 rule. Water intake varies by individual
and you should let thirst guide your intake.
You Can Stay Hydrated From More Than Water
It’s not just plain water that supplies your body with water.
Other beverages, like milk and fruit juice, count as well.
These beverages only become significant diuretics when you
consume them in large amounts. Diuretics are substances that
increase fluid loss by making you pee more often.
A lot of the foods you eat also contain significant amounts of
How much water you get from food depends on the amount of
water-rich foods you eat. Fruits and vegetables are
particularly rich in water, and foods like meat, fish and
eggs also have a relatively high water content.
Lastly, some amount of water is produced within your body when
you metabolize nutrients. This is referred to as metabolic
In the US, the proportion of water people get from food intake
is estimated at around 20%, much lower than in some European
People who get a low amount of water from foods need to drink
more than those who eat more water-rich foods (11).
Bottom Line: Besides water, other foods and
beverages you ingest also contribute to your overall daily
intake of fluids and help keep you hydrated. Some water is
also created within your body through metabolism.
Drinking Enough Water Has Some Health Benefits
You need to be drinking enough water to stay optimally
Generally speaking, that means replacing the water you lose
through breath, sweat, urine and feces.
Drinking enough water may
offer health benefits, including:
Weight loss: Drinking enough water may help
burn more calories, reducing appetite if consumed before
a meal and lowering the risk of long-term weight gain
(12, 13, 14).
Better physical performance: Modest
dehydration may impair physical performance. Losing only 2%
of your body’s water content during exercise may increase
fatigue and reduce motivation (11, 15, 16).
Reduced severity of headaches: For those
prone to headaches, drinking additional water may reduce the
intensity and duration of episodes. In dehydrated
individuals, water may help relieve headache symptoms
Constipation relief and prevention: In
people who are dehydrated, drinking enough water may help
relieve constipation (11, 19).
Decreased risk of kidney stones: Although
more research is needed, there is some evidence that
increasing water consumption may help prevent recurrence in
people with a tendency to form kidney stones (20, 21).
Bottom Line: Staying hydrated may aid in
weight loss, help maximize physical performance, relieve
constipation and more.
How Much Water Should You Drink Each Day?
There is no single answer to this question.
Adequate intake (AI) of water in the US is considered to be 91
ounces (2.7 liters) per day for women and 125 ounces (3.7
liters) per day for men (22).
Note that this is the total intake of water from all sources,
not just pure water.
While this may certainly be used as a guideline, there are a
number of factors, both inside your body and in your
environment, that influence your need for water.
Body size, composition and activity level vary greatly from
person to person. If you’re an athlete, live in a hot climate
or are currently breastfeeding, your water requirements
Taking all this into account, it’s clear that water needs are
Drinking eight glasses of water per day may be more than enough
for some people, but it may be too little for others.
If you want to keep things simple, just listen to your body and
let thirst be your guide.
Drink water when you’re feeling thirsty. Stop when you’re not
thirsty anymore. Make up for fluid loss by drinking more during
hot weather and exercise.
However, keep in mind that this does not apply to everyone.
Some elderly people, for example, may need to consciously
remind themselves to drink water even if they’re not thirsty.
Read this for a more detailed overview of
how much water you should drink per day.