Water Intake Calculator
Water Intake Calculator
Work out how much water to drink a day from your body weight, or from the National Academies guidance for men, women, pregnancy and breastfeeding, with extra for exercise and hot weather. The page gives the answer in liters, ounces, cups and bottles, spreads it across your day, and sets it beside the other common rules so you can see how they compare.
Your figure against the common rules
Spread across the day
Body weight rule: half your weight in pounds as fluid ounces, which is 32.6 mL per kg and sits in the middle of the 30 to 35 mL per kg clinical rule of thumb. National Academies figures are the Adequate Intakes for adults: total water of 3.7 L for men and 2.7 L for women, about 80 percent of it from drinks, so 3.0 L and 2.2 L to drink; 2.3 L in pregnancy and 3.1 L while breastfeeding. Exercise adds 0.4 to 0.8 L an hour, the range in the ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada joint position; hot weather adds a rough 0.5 L. Cups are US cups of 8 fl oz. People with heart, kidney or liver conditions may be told to limit fluids and should follow their doctor. This is general information, not medical advice. The CSV, image and printed report contain the values you entered, nothing preloaded.
How to use this tool
Enter your weight in pounds or kilograms. With My body weight selected, the page uses the half your weight in ounces rule; choose a National Academies option instead for the figure for adult men, women, pregnancy or breastfeeding, which does not depend on weight. Add the minutes you exercise on a typical day and whether the weather is hot or humid, and the total updates as you type.
The answer appears in liters, fluid ounces, US cups and 500 mL bottles. The first chart sets your figure beside every common rule so you can see how far apart they are, the second spreads it across your waking day, and the table turns it into glasses, cans and bottles of the sizes you actually use.
How the figure is worked out
The body weight route halves your weight in pounds and reads the result as fluid ounces: 180 lb gives 90 fl oz, which is 2.66 L. In metric terms that is 32.6 mL per kilogram, the middle of the 30 to 35 mL per kg rule of thumb used in clinical nutrition, so the two weight rules agree. The National Academies route uses their Adequate Intakes, set from what healthy people actually drink: 3.7 L of total water a day for men and 2.7 L for women, about 80 percent of it from drinks.
Exercise is added at 0.6 L an hour, the middle of the 0.4 to 0.8 L an hour that sports nutrition bodies suggest for most people, and hot or humid weather adds a rough 0.5 L. Sweat rates vary more than any formula can capture, from 0.3 to 2.4 L an hour, so the exercise figure is a starting point you can refine by weighing yourself.
Water by body weight
| Weight | Half weight, fl oz | Liters | 30 to 35 mL per kg | With 60 min exercise |
|---|---|---|---|---|
| 100 lb | 50 | 1.5 | 1.4 to 1.6 L | 2.1 L, 70 oz |
| 120 lb | 60 | 1.8 | 1.6 to 1.9 L | 2.4 L, 80 oz |
| 140 lb | 70 | 2.1 | 1.9 to 2.2 L | 2.7 L, 90 oz |
| 160 lb | 80 | 2.4 | 2.2 to 2.5 L | 3.0 L, 100 oz |
| 180 lb | 90 | 2.7 | 2.4 to 2.9 L | 3.3 L, 110 oz |
| 200 lb | 100 | 3.0 | 2.7 to 3.2 L | 3.6 L, 120 oz |
| 220 lb | 110 | 3.3 | 3.0 to 3.5 L | 3.9 L, 130 oz |
| 240 lb | 120 | 3.5 | 3.3 to 3.8 L | 4.1 L, 140 oz |
| 260 lb | 130 | 3.8 | 3.5 to 4.1 L | 4.4 L, 150 oz |
| 280 lb | 140 | 4.1 | 3.8 to 4.4 L | 4.7 L, 160 oz |
| 300 lb | 150 | 4.4 | 4.1 to 4.8 L | 5.0 L, 170 oz |
Water to drink a day. Exercise is added at 0.6 L an hour, the middle of the 0.4 to 0.8 L range.
The National Academies figures
| Group | Total water | From drinks | Cups to drink | Fluid ounces |
|---|---|---|---|---|
| Adult man | 3.7 L | 3.0 L | about 13 | 101 |
| Adult woman | 2.7 L | 2.2 L | about 9 | 74 |
| Pregnant | 3.0 L | 2.3 L | about 10 | 78 |
| Breastfeeding | 3.8 L | 3.1 L | about 13 | 105 |
National Academies Adequate Intakes for adults 19 and over. About 20 percent of total water usually comes from food.
These are Adequate Intakes rather than strict requirements: the panel found that most healthy people meet their needs through normal drinking and by letting thirst guide them. They are averages for people in temperate climates, and the panel noted that very active people who are continually exposed to hot weather often need six liters or more of total water a day.
A worked example you can check on paper
Take someone who weighs 180 lb. Half of 180 is 90, so the body weight rule gives 90 fl oz. A US fluid ounce is 29.57 mL, so 90 fl oz is 2.66 L, or a little over five 500 mL bottles. In kilograms, 180 lb is 81.6 kg, and 30 to 35 mL per kg gives 2.4 to 2.9 L, with the 2.66 L sitting in the middle.
On a day with an hour of exercise, add 0.6 L for 3.26 L. If that day is hot, add another 0.5 L for about 3.8 L, or 127 fl oz. That is close to the 3.7 L total water the National Academies give for an adult man before any exercise, which shows how much an active day in the heat adds.
Signs you are drinking about the right amount
Urine color. Pale straw yellow suggests you are well hydrated. Dark yellow suggests you need more; completely clear urine all day can mean you are drinking more than you need.
Thirst. For most healthy adults thirst is a reliable signal. Older adults are the exception, as thirst weakens with age, so a routine of regular drinks helps.
Body weight around exercise. Losing more than about 2 percent of body weight in a session means you are falling behind; gaining weight during a long event means you are drinking too much.
Mistakes that catch people out
Forgetting that food counts. Fruit, vegetables, soup and dairy supply about a fifth of daily water. The figures to drink already allow for it.
Thinking coffee and tea do not count. For regular drinkers the mild diuretic effect of caffeine is small, and these drinks add to the total.
Forcing water well past thirst. More is not better. Drinking liters on top of need only sends you to the bathroom, and in extreme cases lowers blood sodium.
Using the same figure every day. A rest day and a long run in summer can differ by two liters or more.
Ignoring medical advice. People with heart failure, kidney or liver disease may be told to limit fluids, and that advice comes first.
Popular calculations
Each opens the calculator with its own address, and every value can be changed once it is open.
By weight in pounds
By weight in kilograms
Men, women, pregnancy and breastfeeding
With exercise
Hot weather
How this was checked
The engine was tested in Node against hand calculations: 180 lb gives exactly 90 fl oz, 2.6616 L; 80 kg gives 2.608 L by the half weight rule, matching the same sum done by hand; the 30 to 35 mL per kg band brackets the half weight figure at every weight; and the National Academies options return 3.0, 2.2, 2.3 and 3.1 L to drink before exercise and heat are added. Exercise was checked at 0, 30, 60 and 120 minutes against 0.6 L an hour.
Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.
Common questions
How much water should I drink a day?
The National Academies suggest about 3.7 L of total water a day for men and 2.7 L for women. About a fifth usually comes from food, which leaves roughly 3.0 L, or 13 cups, to drink for men and 2.2 L, or 9 cups, for women. A common rule of thumb based on weight is half your body weight in pounds as fluid ounces.
How much water should I drink based on my weight?
Take half your weight in pounds and drink that many fluid ounces. At 150 lb that is 75 oz, about 2.2 L; at 200 lb it is 100 oz, about 3.0 L. It works out to 32.6 mL per kg, the middle of the 30 to 35 mL per kg range used in clinical nutrition.
Does coffee count towards my water intake?
Yes. The National Academies panel concluded that caffeinated drinks such as coffee and tea contribute to daily water intake much as other drinks do. Milk, juice, soup and water rich fruit and vegetables all count too.
Can you drink too much water?
Yes, though it is rare. Healthy kidneys can clear roughly a liter an hour, and drinking far more than that for hours can dilute the sodium in the blood, a condition called hyponatremia. It is seen mostly in long endurance events, where people drink more than they sweat.
How much water should I drink when I exercise?
Sports nutrition guidance suggests 0.4 to 0.8 L an hour during exercise for most people, adjusted for sweat rate. Weigh yourself before and after a session: each kilogram lost is about a liter of sweat, and drinking about one and a half times that afterwards makes it up.
Is eight glasses of water a day enough?
Eight 8 oz glasses is about 1.9 L, a little below the National Academies figures of 2.2 L to drink for women and 3.0 L for men, though food and other drinks make up much of the gap. The rule has no clear scientific origin, and a 1945 recommendation of 2.5 L a day noted that most of it comes from food.
Related Calculators and Reference
- Protein Intake Calculator: the other daily target people ask about most, by weight and goal.
- TDEE Calculator: daily calories from your size and activity.
- Oz to mL Converter: for bottle and glass sizes marked in the other unit.
- mL to Cups Converter: for turning a liter target into cups.
- Pace Calculator: to work out how long a run will be, and so how much to drink.
- Heat Index Calculator: when heat and humidity raise the risk and the need to drink.
- Caffeine Half Life Calculator: how much caffeine remains from your coffee and tea.
- National Academies report on water intake: the Adequate Intakes for men and women used here.