How much water should you drink a day?

There is no single number, and the famous eight glasses a day has no study behind it. For total water, from drinks and food together, the European Food Safety Authority (EFSA) sets adequate intakes of 2.0 liters a day for women and 2.5 liters for men. The US National Academies put them at about 2.7 and 3.7 liters. In the US data, food supplies about a fifth of that, so the part you drink is smaller, and for most healthy people thirst and pale urine are a good enough guide.

What the official figures say#

Who	Women	Men	What it counts
EFSA (Europe)	2.0 liters a day	2.5 liters a day	Total water: drinks of all kinds plus the water in food
US National Academies	About 2.7 liters (91 ounces)	About 3.7 liters (125 ounces)	Total water: drinks plus the water in food
NHS (UK)	6 to 8 cups or glasses	6 to 8 cups or glasses	Drinks only, offered as a guide
ICMR-NIN (India)	No figure	No figure	"Adequate quantities of safe water"

None of these is a quota to measure out. The European and US figures are built from what adequately hydrated people were observed to take in, and both assume moderate temperatures and moderate activity; the US report says its figure covers the expected needs of healthy, sedentary people in temperate climates, and that it offers no rule of thumb in glasses because water comes from many sources. EFSA adds about 300 mL a day in pregnancy and about 700 mL while breastfeeding.

India's guidelines give no figure at all and put the weight somewhere else: drink safe water, boil it when its safety is in doubt, and do not treat soft drinks as a substitute for it.

Where the eight glasses came from#

A physiologist who went looking for the source of "drink at least eight glasses a day" in 2002 found no scientific support for it, and concluded that surveys of healthy adults suggested such large amounts were not needed, at least for healthy, mostly sedentary adults in a temperate climate. His best guess at the source was a 1945 suggestion from the US Food and Nutrition Board of 1 milliliter of water for each calorie of food. Its next sentence, that most of this quantity is contained in prepared foods, seems to have been overlooked.

He was careful to add that more fluid is called for in hard physical work, exercise and hot weather, and to prevent or treat some conditions, kidney stones among them. The UK still speaks in glasses, six to eight a day, but only as a guide, and it counts water, lower-fat milk, and tea and coffee.

Food, coffee and tea count#

About 80% of the water people take in comes from drinks and about 20% from food, by the US report's figures. Soup, fruit and vegetables all contribute; eating for steady energy covers the food side.

Coffee and tea count too. The US panel found that caffeine's diuretic effect is passing and saw no convincing evidence that it adds up to a water deficit, so caffeinated drinks count toward the day as much as other drinks. In a trial with 50 men who habitually drank coffee, four cups a day for three days kept them as well hydrated as the same amount of water. The NHS's advice on dehydration is more cautious: it suggests water and says caffeinated drinks and alcohol can both dehydrate you. When you are already short of water, plain water is the safer choice.

What the guides do not want as your water is sugar. The NHS limits fruit juice and smoothies to one small glass, 150 mL, a day. The US guidelines say to avoid sugar-sweetened drinks, energy drinks included, and to choose water, still or sparkling. India's say soft drinks are no substitute for water.

What a little dehydration does to mood and energy#

Being a little short of water does seem to make people feel worse. In a lab study of 25 young women, losing 1.36% of body mass in fluid, through exercise without overheating, brought lower mood, more fatigue, tasks that felt harder, poorer concentration and headaches, though most measures of thinking were unaffected. A similar study of 26 young men, at 1.59%, found more fatigue and tension and some slips in vigilance and working memory. And when 30 people who usually drank little raised their water to 2.5 liters a day for three days, they felt less tired and less muddled, while 22 heavy drinkers cut to 1 liter felt less calm, less content and less vigorous.

These are small, short studies, and all three were funded by Danone Research; Danone is one of the world's largest sellers of packaged water. They point the same way, and the NHS's own advice on tiredness notes that it is sometimes simply mild dehydration. They are not a reason to drink by the clock.

Simple guides: thirst and color#

For most healthy people, the US panel concluded, thirst is a good enough guide: the great majority meet their needs by drinking when thirsty and with meals. The NHS adds a check you can see: aim for urine that is pale yellow. Its signs of dehydration are thirst, dark yellow and strong-smelling urine, peeing less often than usual, a dry mouth, headache, dizziness and tiredness. Babies, children and older adults are more at risk. Get medical help the same day for unusual tiredness, dizziness on standing that does not pass, peeing much less than normal, or fast breathing or a racing heart, and emergency help for confusion, difficulty breathing, or skin or lips that turn pale, blue or blotchy.

When you need more#

• Heat and hard exercise. The US report notes that very active people continually exposed to hot weather often need six liters or more a day in total. Exercise for energy and mood covers the moving part.
• Illness. Vomiting, diarrhea, heavy sweating and a high temperature all raise the risk of dehydration, by the NHS's list.
• Pregnancy and breastfeeding. EFSA's extra 300 and 700 mL a day.
• Some medicines. The NHS lists diuretics, which make you pee more, among the risks. Do not change a medicine on your own; ask the doctor who prescribed it.

When to be careful#

Some people are told to drink less, not more. People with heart failure may be given a daily fluid limit by their doctor or nurse, and the British Heart Foundation's advice is never to change it on your own, and to check with the care team when the weather turns hot. People on dialysis for kidney failure set a daily limit with their care team, because dialysis can remove only so much fluid at a time. If you have a heart or kidney condition, follow your doctor's figure, not any number on this page.

Too much water is also dangerous, though rarely. Drinking far more than the body loses can drive the sodium in the blood dangerously low, a condition called hyponatremia; its symptoms include confusion, headache and nausea, and a severe drop can be a life-threatening emergency. The US panel calls dangerous overdrinking highly unusual, seen in some athletes in strenuous events and some people with certain psychiatric conditions. In a study of 488 runners at the finish of the 2002 Boston Marathon, 13% had hyponatremia and 0.6% had it critically; it went with gaining weight during the race, a sign of drinking more than was lost, with drinking more than 3 liters, and with racing for more than four hours.

Water and the urge#

Water will not stop urges, and nothing here should suggest it does. What it can do is remove one avoidable reason to feel flat, and a flat, tired evening is one of the inside cues the habit feeds on; why tiredness makes the urge louder explains how. A glass of water is also step four of what to do when an urge hits, not because water fixes anything, but because it is something small to do with your hands that is not the phone.

Questions people ask#

Does coffee count toward your daily water?#

Yes. The US National Academies concluded that caffeinated drinks count as much as others, and in a trial of 50 habitual coffee drinkers, four cups a day hydrated as well as water. If you are already dehydrated, the NHS suggests water instead.

Is sparkling water as good as still water?#

For hydration, yes. The US guidelines name water, "still or sparkling," as the drink to choose.

Does drinking more water give you more energy?#

Only if you were short of it. In small studies, people who usually drank little felt less tired after drinking more for a few days, and a 2002 review found no evidence that healthy adults in a temperate climate need large amounts.

Water is the simplest of the basics, which is the reason to get it out of the way. Drink when you are thirsty and with meals, keep your urine pale, drink more in heat, exercise and illness, and follow your doctor if you have a heart or kidney condition. Then put your attention where it matters more: stopping completely for thirty days, and seeing what a month of steadier days does to the cues.