Water Intake Calculator: How Much Water Should I Drink a Day?

Most healthy adults need about 9 to 13 cups (2.2 to 3.0 liters) of fluids a day from drinks. A simple personal rule: drink about half your body weight in ounces, then add about 12 oz for every 30 minutes of exercise. The calculator above does that math for your weight, exercise, climate and pregnancy, in oz, cups or liters, or as 5 quick questions in quiz mode.

Below: the official numbers, a chart by weight, our formula and why other calculators disagree.

How much water should I drink a day?

About 9 cups (2.2 liters) of drinks a day if you're a woman and about 13 cups (3.0 liters) if you're a man, according to the National Academies' Dietary Reference Intakes for water. These are adequate intakes for healthy adults: averages that keep most people well hydrated, not minimums.

The numbers you see quoted most, 2.7 liters for women and 3.7 liters for men, are something else: total water. In its 2004 report announcement, the National Academies explained that about 80 percent of that total comes from drinks and about 20 percent from food. Many sites present the total as what you should drink, which overshoots the drinks figure by about a quarter.

These are group averages. A 120 lb woman who walks to work and a 230 lb man at a desk need different amounts, and that's what the calculator is for.

How much water should I drink for my weight? (chart from 100 to 300 lbs)

Drink about half your body weight in ounces: if you weigh 190 lbs, aim for about 95 oz (2.8 liters) a day, or about 107 oz with 30 minutes of moderate exercise. Find your weight in the left column of the chart. Each row has its own link (for example, #190-lbs), and every figure comes from the same calculation as the tool.

From 270 lb up, the calculator holds the base at 135 oz (about 4.0 liters). A rule that grows with every pound keeps climbing well past the National Academies' reference for men, so we cap it there. At the other end, nobody gets less than 50 oz (about 1.5 liters), so a 90 lb adult still gets 50 oz.

How we calculate your water intake (and why calculators disagree)

This calculator starts from your weight: it multiplies your body weight in pounds by 0.5 to get the ounces to drink in a day, so 190 lb gives 95 oz. It then adds about 12 oz for every 30 minutes of moderate exercise (8 oz for light, 16 oz for intense), 16 oz for hot or humid weather or high altitude, 8 oz in pregnancy and 30 oz while breastfeeding. The base stays between 50 and 135 oz. The pregnancy and breastfeeding additions follow the National Academies' reference intakes from drinks: about 1 cup more in pregnancy (10 cups against 9) and about 0.9 liters more while breastfeeding (3.1 liters against 2.2). The exercise and heat additions are common rules of thumb, not study results. Your total water, including food, is the drink figure divided by 0.8, because about a fifth of your water comes from food. It's an estimate for healthy adults: it can't see your kidneys, your medicines or how much you sweat.

The formula in one line: daily water to drink (oz) = body weight (lb) × 0.5, plus 12 oz per 30 minutes of moderate exercise, plus extra for heat, altitude, pregnancy or breastfeeding.

Same person, four methods

Ask four websites what a 190 lb man should drink and you get four answers:

We chose half your body weight because it's simple, personal and lands near the official reference for most people. At 150 lb it gives 75 oz, right at the women's reference of about 74 oz from drinks. At 190 lb it gives 95 oz, a little under the men's 101 oz. At 250 lb it gives 125 oz, above both. The two-thirds rule runs above the reference for nearly everyone.

And "8 glasses a day"? In a 2002 review in the American Journal of Physiology, Heinz Valtin, a physiologist at Dartmouth Medical School, found no scientific evidence for it as a fixed rule for healthy adults. He suggested it may go back to a 1945 recommendation from the Food and Nutrition Board of the National Research Council: about 1 milliliter of water for every calorie of food, or roughly 2.5 liters a day. The next sentence, that most of this is already in prepared food, tends to get left out.

How much extra water do you need when you exercise or it's hot?

A common rule of thumb is about 12 oz (355 ml) extra for every 30 minutes of moderate exercise, less for an easy walk and more for hard training or a hot day. The calculator adds it minute by minute, so 45 minutes of moderate exercise adds 18 oz. Hot or humid weather and high altitude add another 16 oz.

Treat these as a starting point. In its 2007 position stand on exercise and fluid replacement, the American College of Sports Medicine says sweat rates and the salt in sweat vary so much between people that a custom plan beats any single rule.

Our suggestion: weigh yourself right before and after an hour of your usual workout. The weight you lost is mostly sweat, so it shows whether 12 oz is close for you. Long sessions also cost you electrolytes, so above 2 hours of intense or 3 hours of any exercise the calculator tells you to talk to a sports dietitian.

If your exercise is walking, the steps to miles converter turns a step count into minutes, which you can type in here.

What counts toward your daily water intake (coffee, tea and food)?

Every non-alcoholic drink counts, coffee and tea included. The National Academies counted caffeinated drinks toward total water when they set the 2004 reference intakes. So your morning coffee is part of your goal, not a debt you have to pay back.

Food covers the rest: about 20 percent of the water you take in, according to the National Academies, mostly from fruit, vegetables, soup and dairy. Watermelon, cucumber, oranges, strawberries and lettuce are mostly water. That's why the result card shows two figures. For 190 lb with 30 minutes of exercise, 107 oz to drink means about 134 oz (4.0 liters) of total water once food is included.

Plain water is still the best base. Juice, soda and sweet tea count as fluid, but they bring sugar along. We leave alcohol out of your count altogether.

How do you know if you're drinking enough water?

Pale urine and rarely feeling thirsty are the two simplest signs you're drinking enough. Mayo Clinic names both: if your urine is colorless or light yellow and you seldom feel thirsty, your intake is probably fine, whatever a formula says.

Vitamins, some medicines and beets can change the color, so read it with how you feel. Other signs you're behind:

• You feel thirsty or have a dry mouth
• You get a headache or feel tired in the afternoon
• You feel dizzy when you stand up

Get medical help if someone is confused, can't pee for many hours or is too dizzy to stand.

Can you drink too much water?

Yes. Drinking a lot of water in a short time can push the sodium in your blood dangerously low, which Cleveland Clinic describes as water intoxication, or hyponatremia. Milder signs are nausea, vomiting and bloating. Confusion and seizures mean it's an emergency.

Healthy kidneys clear about 0.8 to 1.0 liters of water an hour, according to Medical News Today. So a sensible rule of thumb, not an official guideline, is to drink no more than about 1 liter (34 oz) in an hour. The calculator shows that line under every result, and the tracker warns you if you log more than that within 60 minutes.

The people most at risk are endurance athletes who drink only plain water for hours, and people with kidney or heart problems. If your doctor has given you a fluid limit, for example for kidney disease or heart failure, follow it and not this calculator. Tick the fluid limit box and the tool won't show a number.

Turn your water goal into a daily habit

Start with one glass of water right after you wake up, before coffee, and make it the first step of your morning routine. After a night without a drink, it puts your first 8 oz on the board before the day gets busy, and every glass after it is easier to remember.

Then tie the rest to things you already do. That's habit stacking: a glass after you brush your teeth, one with every meal, one when you sit down at your desk. A fixed wake-up time makes the first glass automatic, and the sleep cycle calculator helps you pick one. If you fast, water still counts during your fasting window, and the intermittent fasting timer keeps track of the window itself.

The tracker on this page counts today's glasses. For every day after that, one way to do it is Greatr: add "Drink a glass of water" as the first habit in your morning routine, tap Play and tick it off. Every habit earns XP, and Greatr counts every day you show up, so the streak gives you a reason to fill the glass.

Tonight, put a full glass next to your coffee maker. Drink it tomorrow before the first cup.