How much water to drink a day

Daily water intake for adults

How much water should you drink a day?

Those figures come from the Institute of Medicine's 2005 report, now the National Academy of Medicine, and doctors still reference them. It helps to keep two numbers apart, because many articles blur them. Total water includes everything, food too. Fluids are only what you drink.

Daily adequate intake for healthy adults (1 cup = 8 ounces, about 237 ml)

The math is simple. About 20% of your water comes from food, especially fruits, vegetables and soups. Take 20% off 3.7 liters and you get about 3 liters, or 13 cups. The same step takes 2.7 liters down to about 2.2 liters, or 9 cups. So eight glasses is a little under the target for women and well under it for men.

Don't read 13 cups as 13 glasses of plain water. Harvard Health puts it at about 4 to 6 cups of plain water for most people, with the rest coming from other drinks and food. Your coffee, milk and soup all count.

These targets also carry less precision than they appear to. A 2018 review in Nutrients points out that adequate intakes are based on survey data, and that there's no widespread consensus on what different groups need. In the large survey it reviewed, intakes among men ran from 1.7 to 7.9 liters a day. The same review found the body starts a mild defense of its water when total intake falls below about 1.8 liters a day.

What changes your water needs

What raises how much water you need?

The baseline numbers assume a moderately active adult in a temperate climate. Change either of those and your needs shift.

Exercise is the most obvious factor. Skin water losses range from about 0.3 liters an hour to about 2 liters an hour, depending on intensity and conditions. The American College of Sports Medicine recommends customizing your fluids because sweat rates vary so much between people, and suggests estimating yours with a scale. Weigh yourself before and after a workout. A pound of body weight lost is about 16 ounces of water to replace. The goal during exercise is to avoid losing more than 2% of your body weight.

Heat and humidity increase sweat even when you're not exercising. If you've stepped outside on a muggy summer day and felt damp right away, you know how fast your body tries to cool itself. The same review puts normal adult intake at about 2.5 to 3 liters a day, rising to about 6 liters with extreme heat and activity. So hot conditions can double your needs, though an ordinary warm day won't.

Altitude matters too. At higher elevations you breathe faster and lose more water through respiration, and many people urinate more as the body adjusts. If you're traveling to the mountains, plan to drink more for the first few days.

Illness counts as well. Vomiting, diarrhea and other losses of body fluid increase what you need, so sip more when you're sick and call your doctor if you can't keep fluids down.

Pregnancy and breastfeeding raise fluid needs. Common guidance is about 10 cups a day during pregnancy. For breastfeeding, WebMD says about 12 cups and the National Academy of Medicine figure is about 13. Treat those as starting points, and ask your healthcare provider for a more personal target.

Drinking water for weight loss

Does drinking more water help you lose weight?

Three studies get quoted most often here. Each one is smaller or weaker than the headlines suggest.

The diet study. A secondary analysis of a weight loss trial followed 173 overweight premenopausal women who drank less than 1 liter of water a day at the start. Over 12 months, those who increased their drinking water lost more weight and body fat, independent of diet group, food intake and activity. That's an association in women already dieting, not proof that water caused the loss.

The pre-meal trial. This is the stronger evidence, because people were randomly assigned. Adults who drank 500 ml of water 30 minutes before main meals lost 1.3 kg more over 12 weeks than a group asked to imagine their stomachs were full. After adjusting for age, gender, ethnicity and deprivation, the gap was 1.2 kg and no longer statistically significant. The authors called it preliminary. Water takes up room in your stomach, which is a plausible reason it might help you eat a bit less.

The metabolism claim. A study of 14 healthy, normal-weight people found that 500 ml of water raised metabolic rate by 30%, peaking at 30 to 40 minutes. The 30% sounds big, but the total was about 100 kJ, or about 24 calories, and about 40% of that came from warming the water. A later randomized crossover study then found no increase in energy use after room-temperature water, and only a 4.5% rise over 60 minutes with water chilled to 3 C. Its authors said the results cast doubt on water as a tool for obesity. We wouldn't count on this one.

Where water helps most is simple arithmetic. It has no calories, so it saves you whatever the drink it replaces would have added. For nutrition strategies that keep you full longer, see our complete guide to the best foods for weight loss.

Signs you're drinking enough water

How can you tell if you're drinking enough?

Forget counting cups for a moment. Your body gives you useful feedback, with a few limits worth knowing.

Urine color is the simplest check. Harvard's Nutrition Source calls the amount and color of urine a rough estimate of hydration: it darkens as it gets more concentrated, and small volumes of darker urine may point to dehydration. First thing in the morning, darker urine is normal, since you haven't had fluids for hours. By midday it should lighten. If it's completely clear all day, you're likely drinking more than you need.

Energy and focus can suffer too. The same source lists fatigue, confusion or short-term memory loss and mood changes as symptoms that may occur with as little as a 2% water deficit. If you hit an afternoon slump, a glass of water is a cheap thing to try before more coffee. Slumps have plenty of other causes, though, so don't assume dehydration is the reason.

Thirst is a reliable signal for most healthy adults, but it fades with age. Older adults may not feel thirsty even when they need fluids, and thirst also falls short in some athletes and people who are ill.

Coffee, tea & other drinks that count

Do coffee & tea count toward your water?

The old belief that coffee dehydrates you doesn't hold up well. Harvard's Nutrition Source says more than 180 mg of caffeine a day, about two cups of brewed coffee, may increase urination in the short term in some people but won't necessarily lead to dehydration. Harvard Health calls the idea that caffeinated drinks are dehydrating a myth.

Your limit is about caffeine, not water. The FDA's 400 mg a day figure is for most healthy adults, and WebMD suggests about 2 to 4 cups of coffee a day, less if it makes you anxious or jittery.

That said, not all drinks are equal. Plain water has no calories, sugar or additives. Unsweetened coffee and tea work well too. Sugary sodas and juice hydrate you, but they bring calories that can work against your weight goals.

A modeling study of 15,765 Spanish adults estimated what swapping one daily drink for water would do over about four years. Replacing one sugar-sweetened soda a day was linked to about 15% lower odds of developing obesity. It wasn't linked to a significant difference in average weight change. That extra benefit showed up only for beer, at about 328 g. So the swap looks worthwhile, but it's a model built on food questionnaires, not a trial.

Hydration for workouts

How much should you drink during a workout?

If you exercise regularly, your hydration plan matters more than it does for someone who sits at a desk all day. Start your workout already hydrated. Drink a glass or two in the hour beforehand, spaced out rather than chugged all at once.

For longer sessions, hot environments or heavy sweating (you know who you are, the one whose shirt is soaked after 20 minutes), electrolytes help replace the sodium lost in sweat. The ACSM position stand notes that drinks with electrolytes and carbohydrates can beat water alone under certain circumstances. We cover options in our guide to water supplements that make staying hydrated more enjoyable.

For how to structure your training, see our complete exercise guide for weight loss.

Drinking too much water

Can you drink too much water?

Harvard's Nutrition Source says there's no upper limit set for water because the body can usually get rid of extra through urine or sweat. The trouble comes when a large amount goes in over a short time. Symptoms include nausea, confusion, muscle spasms and seizures, and severe cases are an emergency.

It's mostly seen in people with kidney problems, under extreme heat stress or during prolonged strenuous exercise. Marathon runners and triathletes are at risk because they can drink large amounts of plain water while losing sodium in sweat. Your kidneys' maximum output is about 1 liter an hour, so forcing fluids well past that adds up quickly.

For everyday life, overhydration isn't a major concern. The fix is plain: drink to thirst, add electrolytes for long or hot efforts, and don't buy into the idea that more water is always better.

Easy ways to drink more water

How can you make drinking water easier?

Knowing how much to drink is one thing. Doing it is another. A few habits make it easier.

• Keep water visible. A bottle on your desk, a glass by your bed, a pitcher in the fridge. The easier it is to reach, the more you'll drink. Some people like a bottle with time markers to pace the day.
• Pair it with routines. A glass when you wake up, one with each meal, one before your workout. Attaching water to what you already do means less to remember.
• Add flavor. Lemon, lime, cucumber, mint or berries can make water more appealing without adding sugar or significant calories.
• Eat your water. Fruits, vegetables and soups count toward your total. Our complete meal prep guide can help you keep more of them on hand.

Food is the other 20%, and it's where meals come in. Meals built around vegetables, lean proteins and whole grains tend to hold more water than processed snack foods. Our Build A Meal Plan lets you pick meals one by one, from $8.99 to $13.99 each, if the real problem is getting a balanced dinner on the table. It won't hydrate you on its own, and some of our meals are high in sodium, so check the Nutrition Info page if you're watching that. You still need the water.

For a grab-and-go option alongside a glass of water or unsweetened tea, our Protein Peanut Butter & Jelly is $2.99, with 250 calories and 11 to 12 grams of protein.

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