Most healthy men need about 125 fluid ounces of water a day and most women about 91 fluid ounces (3.7 and 2.7 liters), counting the water in food. Drinks make up most of that: about 13 cups a day for men and 9 cups for women. You'll need more if you exercise, sweat a lot or live somewhere hot.
These numbers come from the National Academies of Sciences, Engineering, and Medicine. They're averages, and your own number may be higher or lower. The right amount for you depends on how much you sweat, what you eat and where you live.
The official guideline
The National Academies set an "adequate intake" for total water. That includes plain water, everything else you drink, and the water in your food:
| Total water a day | From drinks (about 80%) | |
|---|---|---|
| Men | About 125 fl oz (3.7 liters) | About 101 fl oz, or 13 cups (3.0 liters) |
| Women | About 91 fl oz (2.7 liters) | About 74 fl oz, or 9 cups (2.2 liters) |
Food supplies about 20% of your daily water. Fruits and vegetables are mostly water. Cucumber, lettuce, watermelon and strawberries are over 90% water, so if you eat a lot of produce, you're getting more water than you might think. They're low in calories too, so we put watermelon and strawberries on our Low-Calorie Snacks list.
What about eight glasses a day?
The "8 × 8" rule, eight 8-ounce glasses a day (about 1.9 liters), is easy to remember, but no study ever set it as a requirement. A review of the research found no scientific basis for it, and noted that most healthy people get enough fluid from drinks and food by drinking when they're thirsty. Eight glasses is a fine minimum for many people.
What changes how much you need
- Exercise. Sweat loss during a workout varies a lot from person to person, from about 10 to 80 fluid ounces an hour (0.3 to 2.4 liters).
- Heat and humidity. You sweat more in hot weather, even when you're not exercising.
- Body size. The bigger you are, the more water your body uses and loses.
- Diet. Diets high in protein, salt or fiber raise how much water you need. Eating lots of fruit and vegetables gives you more water from food.
- Altitude and illness. High altitude, fever, vomiting and diarrhea all make you lose more water.
Hydration for workouts
According to the American College of Sports Medicine, losing more than about 2% of your body weight in sweat can hurt both endurance and strength. If you weigh 176 lb (80 kg), that's about 3.5 lb (1.6 kg). You can stay well clear of that with a few simple habits:
- Before: drink about 14 to 20 fluid ounces (400 to 600 milliliters) in the few hours before you train, so you start hydrated.
- During: sip throughout long or sweaty sessions. For most gym workouts under an hour, drinking when you're thirsty is enough.
- After: weigh yourself before and after a hard workout. For every pound you lost, drink about 23 fluid ounces over the next few hours (1.5 liters per kilogram).
When electrolytes help
Sweat contains sodium as well as water. If you train for more than about an hour, sweat heavily or work out in the heat, a drink or snack with some sodium helps your body hold on to the water you drink. For a typical 45-minute lifting session, plain water and your normal meals are enough.
Signs you're well hydrated
Most health organizations point to two simple guides:
- Thirst. For healthy adults at rest, thirst is a reliable signal. If you drink when you're thirsty and with meals, you'll stay in a good range most of the time.
- Urine color. Pale yellow means you're well hydrated in most cases. Dark yellow or amber means you could use more water. Some vitamins, especially B vitamins, can turn urine bright yellow no matter how much you drink.
Headaches, tiredness, dizziness and a dry mouth can also be signs you're low on fluid, though they can have lots of other causes.
Does coffee count?
Yes. Caffeine makes you pee a little more, but a study that compared moderate coffee drinking with the same amount of water found no difference in hydration. Tea, milk, sparkling water and other drinks count toward your daily fluid too. Sugary drinks count as water, but they add calories, so it's worth tracking them along with your food.
Can you drink too much water?
It's rare in everyday life, but it can happen. Drinking far more than your body loses, especially during long endurance events, can dilute the sodium in your blood to a dangerous level. That's called hyponatremia. Drinking to thirst, and getting some sodium during very long events, keeps the risk low.
What a well-hydrated day can look like
Spread over a day, 100 fluid ounces (about 3 liters) is easier than it sounds:
| When | Drink | Running total |
|---|---|---|
| Wake up | 16 fl oz water (0.5 L) | 16 fl oz |
| Breakfast | 12 fl oz coffee (0.35 L) | 28 fl oz |
| Lunch | 16 fl oz water (0.5 L) | 44 fl oz |
| Workout | 24 fl oz water (0.7 L) | 68 fl oz |
| Dinner | 16 fl oz water (0.5 L) | 84 fl oz |
| Evening | 16 fl oz tea or water (0.5 L) | 100 fl oz |
Water and the scale
Your weight can swing 2 to 4 lb (1 to 2 kg) from one day to the next from water alone, after a salty meal, a big carb day or a hard workout. Your weekly average is a better guide than any single weigh-in. For more on adjusting your calories based on the trend, see How to calculate your macros.
Easy ways to drink more
- Keep a water bottle where you can see it: on your desk, in your car and in your gym bag.
- Drink a glass of water with every meal and snack.
- Have a glass of water before your morning coffee.
- Eat more fruit and vegetables, which count toward your total.
- Track what you drink for a week to see where you land.
Common questions
How much water do I need if I work out every day?
Start with the daily guideline and add about what you lose in training. Weigh yourself before and after a few workouts to see your own sweat rate. Each pound you lose is about 16 fluid ounces of water (each kilogram is about a liter).
Is sparkling water as hydrating as still water?
Yes. The bubbles don't change how well it hydrates you. Unsweetened flavored sparkling water counts too.
Does water help with weight loss?
Water has no calories, so swapping it in for sugary drinks cuts calories right away. You may also find that a glass of water before meals helps you feel full sooner.
Do people with health conditions need different amounts?
Often, yes. Kidney, heart and liver conditions, and some medications, change how much fluid you need. If that applies to you, go with the amount your doctor recommends.
Sources
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005).
- American College of Sports Medicine. Position stand: exercise and fluid replacement. Medicine and Science in Sports and Exercise (2007).
- Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 × 8"? American Journal of Physiology (2002).
- Killer S.C. et al. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLOS ONE (2014).
