How Much Water Should You Drink a Day?

The U.S. National Academies set an adequate daily intake of about 3.7 liters (125 ounces) of total water for men and 2.7 liters (91 ounces) for women. That total includes water from food as well as drinks, and about a fifth usually comes from food. Most healthy people meet their needs by drinking when they are thirsty and with meals, so the figure is a general guide rather than a target you must hit every day.

What the recommendations actually say

The National Academies figures are "adequate intakes" - the typical amount that healthy, well-hydrated people consume. They are averages, not minimums. The panel deliberately did not set a "glasses a day" rule, because water comes from so many sources.

The NHS gives a simpler rule of thumb for the UK: about 6 to 8 cups or glasses of fluid a day, with more in hot weather, during long periods of activity, when ill, or while pregnant or breastfeeding.

Does coffee or tea count?

Yes. Water, milk, tea and coffee all count toward your fluid intake. Caffeine can increase urination in the short term, but moderate amounts do not make these drinks dehydrating overall. Sugary drinks and fruit juice count too, but they add sugar and calories, so plain water is the better default. If you drink a lot of coffee, our caffeine guide covers daily limits.

Why your needs may be higher or lower

• Heat and humidity. You sweat more, so you lose more water.
• Physical activity. Long or intense exercise raises needs, especially outdoors.
• Illness. Fever, vomiting and diarrhea all increase fluid loss.
• Pregnancy and breastfeeding. Both raise fluid needs.
• Body size. Larger people generally need more.
• Health conditions and medicines. Some heart, kidney and liver conditions, and some medicines, mean you may be told to limit fluids. If a doctor has given you a fluid limit, follow that rather than general advice.

Why staying hydrated matters

Water helps your body keep a normal temperature, cushions joints, protects the spinal cord and helps remove waste. According to the CDC, not drinking enough can lead to unclear thinking, mood changes, overheating, constipation and kidney stones. Many people notice that even mild dehydration leaves them tired or headachy.

Some popular claims go too far. Drinking extra water will not "flush toxins" beyond what healthy kidneys already do, and there is no good evidence that it erases wrinkles. The real benefit is simpler: your body works best when it is not short of fluid.

How to tell if you are drinking enough

For most healthy people, a few simple signals are more useful than counting ounces:

• Urine color. The NHS suggests drinking enough that your urine is a clear, pale yellow. Dark yellow usually means you need more. Some vitamins and foods can change the color.
• Thirst. Thirst is a normal, reliable cue for most healthy adults. Older adults can feel less thirsty, so regular drinks with meals matter more with age.
• How you feel. Dry mouth, headache, tiredness and dizziness can all point to low fluid intake.

Can you drink too much water?

Yes, though it is rare. Drinking far more than your kidneys can clear in a short time can dilute the sodium in your blood, a condition called hyponatremia. It is most often seen during very long endurance events when people drink large amounts without replacing salt, and in people with kidney problems. Symptoms include nausea, headache, confusion and swelling, and it needs urgent care. During long events, drink to thirst rather than forcing fluids.

Simple habits that help

1. Pair water with things you already do. A glass with each meal and one after you wake up covers a good share of the day.
2. Keep a bottle in sight. On your desk, in your bag or in the car.
3. Eat water-rich foods. Fruit, vegetables, soups and yogurt all add water.
4. Drink before, during and after exercise, more so in the heat.
5. Swap one sugary drink a day for water, or flavor water with lemon, lime or cucumber.
6. Use reminders if you forget. A nudge every couple of hours works for many people with busy days.

Logging water with FitRest

FitRest has a water log on iPhone and Apple Watch. You set a daily goal, log drinks as you have them and see your progress through the day. Entries are saved to Apple Health. You can turn on water reminders, add a quick-add water widget to your Home Screen to log a drink with one tap, or log from your wrist and keep a Water complication on your watch face. Learn more on the health logs page.

Sources

• National Academies - Report Sets Dietary Intake Levels for Water, Salt, and Potassium
• Harvard T.H. Chan School of Public Health - The Nutrition Source: Water
• CDC - About Water and Healthier Drinks
• NHS - Water, drinks and hydration