Water Intake Calculator – How much water should you drink per day?

Calculating optimal daily water intake

Our water calculator will help you easily estimate how much water you need per day including how much of it you will need to drink in the form of fluids (pure water or beverages). The outputs of our water intake calculator are in liters, milliliters, cups (equivalent to a standard glass), and ounces of water. Of course, these are estimates based on population averages so consulting your physician or nutritionist is always recommended before making changes to your water consumption or exercise routine.

Calculating your recommended total daily water intake and thus optimal hydration requires the estimation of your total daily energy expenditure (TDEE) which measures how much energy (in kcal, kilocalories) you expend during a typical day. Unlike other tools that estimate daily water intake by weight (in kg/litre), this tool employs a rule for water needs in relation to one’s energy requirements expressed in ml/kcal. This method is superior since energy requirements are strongly evidence-based in each age and gender group and take into account body size, body weight, and activity level. These are crucial determinants of the energy needs that must be met by dietary intake.

Those same determinants are also applicable to water utilization and balance which provides an argument for pegging water/fluid intake recommendations to the better studied energy recommendations. The extent to which water intake requirements are determined by energy needs is understudied but in the clinical setting it has long been practice to supply 1 ml per kcal administered by tube to patients unable to take in food or fluids ^[1]. Since using TDEE avoids the compromise inherent in other Adequate Water Intake estimations, this daily water intake calculator bases its output on the link between estimated energy needs and fluids intake needs.

Sources of water in the human body

Water input in the human body comes from three main sources: water and other beverages, food, and as a side-effect of metabolic processes. Since water and beverages are only a part of the input, our calculator will output both your total water intake recommendation as well as how much of it you need to get through drinking fluids.

You should be careful to not confuse how much water you need with how much water you need to drink. The latter is usually only around 4/5 of the total, thus you actually need to drink slightly less water than your total daily needs. Say you need a total of 70 ounces per day. This would mean that you need to drink just 56 oz of water a day while the rest will enter your body through food and metabolic processes. That equals about 7 cups a day which is roughly equivalent to 7 standard water glasses a day (see oz to cups conversion). Our water content of selected foods table below can be useful in determining which foods help you stay hydrated.

Why do we need to drink so frequently?

Our bodies constantly lose water as part of the metabolic processes. Water is ejected from the body in the form of urine, as result of gastrointestinal processes, as part of respiration, as well as through sweating and other insensible outputs. Therefore we need to frequently replenish the fluids in our system for it to function optimally (see the benefits of optimal hydration).

The human body is always trying to maintain homeostasis and since sweating varies considerably depending on physical activity and how hot the environment is it can result in dramatically different water drinking needs. Living in a climate which is too hot or too cold, or being pregnant are also important factors. Some adjustments in the amount of water may be needed if you are subject to such influences as explained in the adjustments section.

General water drinking recommendations

Below we present the general recommended amount of water intake based on recommendations from the EFSA and IOM. EFSA stands for European Food Safety Authority and IOM stands for the U.S. Institute of Medicine, data is based on reference 3. These are population-wide adequate intake estimations and are thus less preferred than the personalized calculations from our water intake calculator above.

In the table, the calculated water intake is in L/day (liters per day a.k.a. litres per day). For reference, one liter is approximately equal to four standard water glasses.

It should be noted that, in general, our bodies are fairly good at estimating how much water one needs on a daily basis and this happens via the mechanism of thirst. If you are thirsty, you should certainly drink water no matter what any kind of calculation or chart is telling you about how much water per day you need to drink.

Adjustments for climate, excessive sweating & pregnancy

If you are a woman and you are pregnant, you will require more water per day and you will require even more water if you are lactating. Our water drinking calculator performs these adjustments based on a compromise between tables provided by the European Food Safety Authority (EFSA) and the U.S. Institute of Medicine (IOM) ^[3].

The results of this water calculator assume most of your day is spent in a moderate climate. If you live in a hot climate and spend your days mostly outside or in a non-climatized building, then you will need to adjust your water intake upwards, but we are not aware of good research estimating by how much depending on temperature, exposure to sun, etc.

The results of the hydration calculator also assume you are not subject to vigorous physical activity in a hot environment, in which cases losses in excess of 3L per hour are possible ^[4] and you need to adjust your fluid intake accordingly to compensate for that. Cold climates have less of an effect, but extremely cold climates may result in increased energy needs to compensate for the heat loss and thus you may need a higher water intake per day.

Benefits of staying optimally hydrated

One of the reasons to use a hydration calculator is to maintain a healthy life, but scientific studies also link adequate water intake to benefits for the treatment of health conditions as well as mental state improvement. Water comprises between 75% of the body weight (in infants) and 55% (in the elderly) and is essential for cellular homeostasis and life so it is no wonder water is so important to one’s health. Amazingly, a recent large sample-size study found that in a free-living population from Germany, Spain, and Greece only approximately 60% of participants were properly hydrated while about 20% were hyperhydrated and 20% dehydrated on average over a seven-day period ^[5].

Kidney health

The kidneys function more efficiently in the presence of an abundant water supply. If the kidneys economize on water, this results in producing a more concentrated urine which comes at a greater cost in energy and more wear on their tissues. This is especially likely to occur when the kidneys are under stress, for example when the diet contains excessive amounts of salt or toxic substances that need to be eliminated. Consequently, drinking enough water helps protect this vital organ ^[1].

There is strong evidence that recurrence of kidney stones is much less likely when your hydration status is good due to the increased urine volume ^[1]. So, there is your first reason for using a daily water intake calculator to estimate how much water you need to drink per day.

Sports performance

During challenging athletic events, it is not uncommon for athletes to lose 6–10% of body weight in sweat loss, thus leading to dehydration if fluids have not been adequately replenished. Decrements in physical performance in athletes have been observed under much lower levels of dehydration: as little as 2% Under relatively mild levels of dehydration, individuals engaging in rigorous physical activity will experience decrements in performance related to reduced endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort. Rehydration can reverse these deficits, and also reduce oxidative stress induced by exercise and dehydration. Hypohydration appears to have a more significant impact on high-intensity and endurance activity such as tennis and long-distance running than on anaerobic activities such as weight lifting or on shorter-duration activities, such as rowing ^[1].

Children are especially prone to voluntary dehydration so child athletes and children doing sports in hot climates should start well-hydrated every day to avoid any potential issues.

Cognitive abilities

A body of studies examined by Popkin et. al ^[1] indicates that low to moderate dehydration may negatively affect cognitive performance. Rather than indicating that the effects of hydration or water ingestion on cognition are contradictory, many of the studies differ significantly in methodology and in measurement of cognitive behaviors. Most studies in which dehydration was induced combined heat and exercise, thus it is difficult to disentangle the effects of dehydration on cognitive performance in temperate conditions from the effects of heat and exercise. In practice, relatively little is known about the mechanism of mild dehydration’s effects on mental performance. It has been proposed that mild dehydration acts as a physiological stressor which competes with and draws attention from cognitive processes. However, research on this hypothesis is limited and merits further exploration.

A more recent study by Riebl S. K. and Davy B. M. ^[2] came to the conclusion that even though traditionally a 2% or more body water deficit was thought to produce cognitive performance decrements more recent literature suggests that even mild dehydration – a body water loss of 1–2% – can impair cognitive performance. Counseling clients about their health and wellbeing should include conveying the importance of water for normal body functioning, as well as its effects on physical and cognitive performance. ^[7] It would therefore seem that getting close to your optimal hydration level with the help of a hydration calculator can have benefits for your mental power.

Headache prevention and treatment

A 2012 randomized control trial by Spigt et. al ^[9] suggests that considering the observed positive subjective effects, it seems reasonable to recommend headache patients to try this non-invasive intervention for a short period of time to see whether they experience improvement. The study is critiqued for low statistical power and biases due to partial unblinding of participants by Price A. and Burls A. (2015) ^[10], so the jury appears to still be out on this one.

Type II Diabetes

Epidemiological research has demonstrated that low daily total water intake is associated with increased diagnosis of hyperglycemia and a study confirms that 3 days of low total water intake in people with type 2 diabetes mellitus acutely impairs blood glucose response during an oral glucose tolerance test via cortisol but not RAAS-mediated glucose regulation, where RAAS stands for renin-angiotensin-aldosterone system ^[8].

Constipation and water intake

Constipation, characterized by slow gastrointestinal transit, small, hard stools, and difficulty in passing stool, has a number of causes including medication use, inadequate fiber intake, poor diet, and illness. Inadequate fluid consumption is touted as a common culprit in constipation and increasing fluid intake is a frequently recommended treatment. However, the evidence suggests that increasing fluids is only of usefulness in individuals in a hypohydrated state and is of little utility in properly hydrated people. In young children with chronic constipation, increasing daily water intake by 50% did not affect constipation scores. ^[1] If you are having difficulty with constipation, a physician may calculate your water intake (how much water you should drink per day) and may advise you to increase the amount to a level providing adequate hydration.

Heart rate and blood pressure

Water intake acutely reduces heart rate and increases blood pressure in people with normal or increased blood pressure. The effects of water intake on the pressor effect and heart rate occur within 15–20 minutes of drinking water and can last for up to 60 minutes. Water ingestion is also beneficial in preventing vasovagal reaction with syncope in blood donors at high risk for post-donation syncope ^[1]. Some strong reasons to consider using a water intake calculator and to adhere to the estimate as best as one can.

Bronchopulmonary disorders

There is strong evidence from meta analyses of randomized controlled trials that exercise related asthma is linked with low fluid intake. Thus, increasing water intake per day has a beneficial effect on the condition ^[1].

Skin and skin conditions

There is no evidence to support the notion that drinking more water will improve the skin complexion, remove wrinkles, acne, or help with other skin conditions. Increasing the amount of water drank in a day will improve the skin thickness and density in persons with low initial water intake, but if you are already drinking adequate amounts of water every day, increasing that will not be beneficial for your skin ^[1].

All-risk mortality

A prospective study in 2017 found that “there was no survival advantage in association with higher total or plain water intake in men or women in this national cohort” ^[6] so even though proper water intake appears to have certain benefits they may not go as far as increasing your lifespan. However, it is unclear what size of effect the study was powered to exclude. While all-risk mortality is an important measurement, quality of life and achievements should be taken into account when considering the usefulness of knowing how much water you need to drink and of abiding to those recommendations.

 Weight loss by drinking more water?

A 2014 non-controlled study ^[11] of 50 female overweight women established that drinking water three times a day 30 min before breakfast, lunch, and dinner, to a total of 1.5L above the recommended daily amount, resulted in a decrease in body weight, BMI, sum of skinfold thickness, and appetite scores of the participants over an 8-week period and establishes the role of drinking excessive water in weight reduction, body fat reduction, and appetite suppression of participants. Thus, water drinking induced increase in sympathetic activity is an important and unrecognized component of daily energy expenditure and hence weight loss. It seems water has the potential to be a cost free intervention useful as adjunctive treatment in overweight and obese individuals and using a water drinking calculator such as ours to estimate the recommended amount would be an essential part of such an effort and may result in lost weight.

A systematic review of the impact of water intake on energy intake and weight status by Daniels & Popkin ^[12] finds reasonable agreement among a number of single meal studies that replacing water with sugar-sweetened beverages will increase energy intake by about 8%, possibly leading to weight gains. There is sparse data on replacing sugar-sweetened beverages with water and/or milk and juices, but the available studies point to modest improvements in reducing energy intake as well as reducing risk of being overweight. Most of the studies are observational and there is a distinct lack of randomized controlled trials and long-term studies of the weight loss effects of replacing other beverages with water. Still, it appears that the available literature suggests some positive effects on weight-loss from drinking more water before eating and for replacing sugar-sweetened drinks with water. Note that the recommended daily water intake still remains the same, and our hydration calculator does not differentiate between water from sweet drinks and other sources.

 Water content in foods

Below is a reference table for the water content in selected foods, based on Altman P. “Blood and Other Body Fluids” (1961), Federation of American Societies for Experimental Biology as cited in Popkin B. M., D’Anci K. E., Rosenberg I. H. (2010) “Water, Hydration and Health” ^[1].

Using this table can help you determine which foods to include in your diet so you stay better hydrated and closer to the recommended water intake per day as estimated using our daily water intake calculator.