Do you really need eight glasses of water a day?

Few health rules are as widely repeated as “drink eight glasses of water a day.” It is simple and easy to remember. It is also not based on much. When the kidney physiologist Heinz Valtin went looking for the scientific basis in 2002, he could not find any studies supporting it for healthy adults in a mild climate.

That does not mean water is unimportant. It means the rule is too rigid, and for most people, getting enough is easier than it sounds.

How much you actually need

The most cited figures come from the US National Academies. They set an adequate intake of about 2.7 liters of total water a day for women and 3.7 liters for men. The word “total” matters. That figure includes water from every drink and from food.

About a fifth of the water we take in usually comes from food. Fruit, vegetables, soup, yogurt and even cooked grains are mostly water. That leaves roughly 2.2 liters, or about nine cups, from drinks for women and about 3 liters, or thirteen cups, for men. These are averages for healthy, moderately active adults, not personal targets.

Coffee and tea count

Caffeine has a mild diuretic effect, which led to the belief that coffee dehydrates you. In practice, regular coffee and tea drinkers develop a tolerance, and the fluid in the cup more than makes up for any extra trips to the bathroom. Coffee, tea, milk and sparkling water all count toward your daily intake.

Plain water is still the best default because it has no sugar or calories. Sugary drinks hydrate you too, but they bring other costs.

The signs worth watching

For most healthy people, your body is a good guide. Two signals are worth paying attention to.

Thirst is the first. It kicks in well before you are meaningfully dehydrated, so drinking when you are thirsty keeps most people in a healthy range.

The color of your urine is the second. Pale yellow, like straw, generally means you are well hydrated. Dark yellow or amber suggests you need more fluid. Completely clear urine all day may mean you are drinking more than you need, which is usually harmless but not necessary.

When you need more

Some situations raise your needs above the everyday baseline:

• Hot or humid weather, or high altitude.
• Exercise, especially long or sweaty sessions.
• Illness with fever, vomiting or diarrhea.
• Pregnancy and breastfeeding.

Older adults should pay extra attention, because the sense of thirst tends to weaken with age. Keeping a glass or bottle within reach and drinking with each meal helps.

Can you drink too much?

Yes, though it is uncommon. Drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatremia. It occurs most often in endurance athletes who drink far more than they sweat out. For everyday life, drinking to thirst avoids both extremes.

Some medical conditions, including heart failure and certain kidney diseases, require limiting fluids. If you have been given a fluid target by your doctor, follow it instead of general advice.

Your small step this week

Skip the counting. Instead, keep a glass or bottle of water where you can see it during the day, drink when you feel thirsty, and check the color of your urine once in the afternoon. If it is dark, have a glass of water with your next meal.