Water Intake Calculator
Work out how much water to drink a day from the National Academies Adequate Intake, with extra for exercise from your own sweat rate and for hot weather, in litres, cups or fluid ounces.
How to Use
- Choose Daily intake, or Sweat test to measure your sweat rate from a training session first.
- Pick male or female: the National Academies Adequate Intake for adults 19 and over is 3.7 L a day for men and 2.7 L for women, from drinks and food together.
- If you exercise, enter the minutes and your sweat rate in litres per hour; add any extra you drink on hot days.
- Choose litres, US cups or US fluid ounces for the results.
- Read the total, the amount to drink, the part that comes from food and the extra, and see the glass and cups on the chart.
Worked Example
A woman on a rest day. The Adequate Intake of total water for women 19 and over is 2.7 L. About 80 % comes from drinks: 2.7 × 0.8 = 2.16 L to drink, which is 2.16 ÷ 0.2366 = 9.1 US cups. The other 0.54 L comes from food.
The same woman on a training day. She runs for 60 minutes and has measured a sweat rate of 1.0 L an hour, so she adds 1.0 × 60 ÷ 60 = 1.0 L: 3.16 L to drink, 3.7 L of total water.
The common mistake: treating the whole figure as drinking water. 3.7 L is men’s total water from drinks and food. Drinking all of it on top of a normal diet means about 0.74 L more than the intake describes; the drinks part is 3.7 × 0.8 = 2.96 L, or 100 US fl oz.
Show Work
Formulas
Where the Numbers Come From
The figures come from the National Academies report Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, released in 2004 for the United States and Canada. The panel found that hydration, judged by blood concentration (serum osmolality), stayed the same across a wide range of intakes, so instead of a requirement it set an Adequate Intake equal to the median total water intake in the US NHANES III survey: 3.7 L a day for men and 2.7 L for women aged 19 to 30, carried through to older adults.
The report stresses that these are not targets everyone must hit: healthy people meet their needs over a wide range of intakes, mostly guided by thirst and by drinking with meals, and people who are physically active or in hot environments need more. It also notes that roughly 80 % of total water comes from drinks and 20 % from food, and that the kidneys can excrete at most about 0.7 L an hour.
About This Tool
This calculator starts from the National Academies Adequate Intake of total water for adults, splits it into drinks and food, and adds two things only you can know: your sweat losses from exercise, from a sweat rate you can measure in the Sweat test mode, and any extra you drink in hot weather. Results come in litres, US cups or US fluid ounces. It does not cover children, pregnancy or breastfeeding, which have their own intakes in the same report.
Everything runs in your browser; nothing you enter is sent anywhere. For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health, especially if you have heart or kidney disease or have been told to limit fluids.
Related tools: Calorie Burn Estimator, TDEE Calculator, and BMI Calculator.
Frequently Asked Questions
How much water should I drink a day?
The National Academies set an Adequate Intake of total water of 3.7 L a day for men and 2.7 L for women aged 19 and over, counting drinks and the water in food. About 80 % usually comes from drinks: 2.96 L (12.5 US cups) for men and 2.16 L (9.1 cups) for women.
Does the water in food count?
Yes. The Adequate Intake is for total water, and food usually supplies about a fifth of it. In the US survey data behind the figures, drinks gave 19- to 30-year-old men about 3.0 L (about 13 cups) and women about 2.2 L (about 9 cups) a day, roughly 81 % of their total water.
How much extra do I need when I exercise?
As much as you sweat, which varies a lot from person to person, so measure it. Weigh yourself before and after a session: losing 0.8 kg in an hour while drinking 0.5 L means about 0.8 + 0.5 = 1.3 L of sweat, a rate of 1.3 L per hour. The Sweat test mode does this sum.
Do coffee and tea count?
The National Academies report says the diuretic effect of drinks containing caffeine or alcohol seen in some studies may be short-lived, and that such drinks do contribute to total water intake. A 2.7 L day can include them.
Can you drink too much water?
The report sets no upper limit for water, but notes that healthy kidneys can excrete at most about 0.7 L an hour. Drinking much faster than that, for example 2 L in an hour, outpaces what the kidneys can clear.
How do I use the Water Intake Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
A daily baseline
A woman with no exercise needs about 2.16 L of drinks (9.1 cups) on top of the water in her food.
Training days
Add a 60-minute run at a measured 1.0 L per hour: a woman’s drinks go up to 3.16 L.
Hot, long sessions
A man training 90 minutes at 1.2 L per hour with 0.5 L extra for the heat needs about 5.26 L of drinks that day.
Measuring sweat in pounds
Going from 165 lb to 163 lb in 75 minutes while drinking 16 fl oz is a sweat rate of about 1.10 L (37 fl oz) an hour.
Cups and ounces
Switch units: a man’s 2.96 L of drinks is 100 US fl oz, or 12.5 cups of 8 fl oz.
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