Water intake calculator: how much water per day

Adequate Intake for total water for men, as published by the IOMworked example

3.7 L

Of which, typically from drinks: 3 LAbout 80% of the total
And from food: 700 mLThe part the eight-glasses rule forgot
"Eight glasses" for comparison: 1.89 L1.11 L below even the beverage share

The IOM set this as an Adequate Intake from what healthy, adequately hydrated people were observed to consume — a population reference point rather than a daily quota, and it counts the water in food, coffee and tea. Thirst, and anything your clinician has told you about fluid, both outrank a table.

Which Adequate Intake row applies

The IOM published one figure for each, from observed intakes.

Pregnant or breastfeeding?

The same report sets 3 L in pregnancy and 3.8 L during lactation. Nothing you enter here leaves your browser.

Body weight (optional)Only used for the 30 mL/kg convention, which is a comparison here rather than the answer.Daily calories (optional)For the older 1 to 1.5 mL per kcal convention — the line Valtin identified as the origin of the eight-glasses rule.

Where "eight glasses a day" actually came from

Valtin went looking for the evidence in 2002 and found none. His best explanation: a 1989 National Research Council recommendation of about 1 mL of water per kcal of energy expenditure, which at 2,000 kcal a day comes to roughly 2 L. The next sentence of that report noted most of the quantity is already contained in prepared foods. Drop that sentence and you have a drinking-water target nobody wrote.

The published figure and the conventions people quote instead
Figure	Amount	What it means	Where it comes from
IOM Adequate Intake, total water	3.7 L	Includes water from food	IOM 2004
The same AI, beverages only	3 L	About 80% of the total	IOM 2004
"Eight glasses a day"	1.89 L	64 US fl oz of drinking water	No published source (Valtin 2002)

IOM 2004 for the Adequate Intake and the beverage share; NRC 1989 for the per-calorie convention; Masot 2020 for the 30 mL/kg convention used with older adults, which that review notes works badly at very low and very high body weights; Valtin 2002 for the 8 x 8 rule. Rows marked convention are field practice, not published requirements.

The per-calorie convention needs an energy figure, and the TDEE calculator estimates one. If you are here because a training plan asked for a fluid target, the protein calculator covers the other number those plans usually specify.

The eight-glasses rule is about 1.9 litres of drinking water and nobody published it. The IOM set an Adequate Intake for total water of 3.7 litres a day for men and 2.7 for women, food included, and this calculator shows that figure beside the two conventions people quote instead.

Adequate Intake for total water — total water 3.7 L/day (men) and 2.7 L/day (women), about 80% of it from beverages; 3.0 L/day in pregnancy and 3.8 L/day during lactation. Verified 2026-07-30 against Institute of Medicine, DRI for Water, Potassium, Sodium, Chloride and Sulfate (2004) (An Adequate Intake set from what healthy, adequately hydrated people were observed to consume - not a requirement, and not a daily target to hit. It covers water from food as well as drink.)
Water per calorie expendedconvention — 1 to 1.5 mL of water per kcal of energy expenditure. Verified 2026-07-30 against National Research Council, Recommended Dietary Allowances, 10th edition (1989), Water and Electrolytes (A practical convention for adults under average conditions, raised toward 1.5 mL/kcal to cover variation in activity, sweating and solute load.)
Water per kilogram of body weightconvention — 30 mL of fluid per kg body weight per day, with a floor of about 1.5 L. Verified 2026-07-30 against Masot et al., Nutrients 2020 (critical review of fluid-intake standards for adults over 65) (A weight-based clinical convention used mainly for older adults. The review that documents it notes it fails for very thin or very heavy people and prefers the guideline route instead.)
The eight-glasses rule, examined — no scientific evidence was found for 8 x 8 fl oz per day, which is about 1.9 L of drinking water. Verified 2026-07-30 against Valtin, Am J Physiol Regul Integr Comp Physiol 2002 (The review found no evidence for the rule as a universal instruction. It does not argue that drinking water is unimportant, and it excludes people with specific medical conditions from its scope.)

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Frequently asked questions

How much water should I drink a day?

The IOM set an Adequate Intake for total water of 3.7 litres a day for men and 2.7 for women, and roughly 20 percent of that arrives in food. That leaves about 3.0 and 2.2 litres from drinks, which is more than the eight-glasses rule and rests on an actual document.

Where did the eight glasses a day rule come from?

Valtin traced it in 2002 to a 1989 recommendation of 1 mL of water per kcal of energy expenditure, whose very next sentence noted that most of that water is already contained in food. That sentence got dropped, and the remainder became a rule nobody had written.

Does coffee or tea count toward water intake?

The IOM figure is total water from all beverages and food, so it does count them. The report treats caffeinated drinks as contributing to daily total water intake, which is why this tool shows a total-water figure rather than a plain-water target.

Should I drink water based on my body weight?

The 30 mL per kg convention exists and is mostly used in clinical settings for older adults. The review that documents it notes it works badly for very thin or very heavy people and prefers the guideline route, so it appears here as a comparison line rather than as the answer.

Do pregnancy and breastfeeding change the figure?

Yes. The same IOM report sets total water at 3.0 litres a day in pregnancy and 3.8 litres during lactation, and both options are in the tool. Those are Adequate Intakes for a population rather than personal prescriptions, and thirst and clinical advice both outrank a table.

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The next number

No body metric means much on its own. Here is what this one feeds into.

TDEE calculatorThe older per-calorie convention needs an energy figure, and the TDEE page is where that number comes from.Macro calculatorCarbohydrate is stored with water, which is part of why the first weeks of any change in eating move the scale oddly.

Related calculators

TDEE calculatorMetabolism Protein calculatorNutrition targets Macro calculatorNutrition targets

Every formula on this page is listed with its source on the methodology page, and changes to any reference value are logged in the data changelog. Canonical URL: https://myhealthmywellness.com/water-intake-calculator