Free Calculator
Water Intake Calculator
How much water do you really need? Enter your age, sex, weight, height, activity and climate. The calculator uses an equation from a 2022 study in Science that measured the water use of more than 5,600 people. It checks the result against official guidelines, then turns it into a drinking target and a simple schedule for your day.
- Runs in your browser. Nothing is sent or stored
- Based on published research and guidelines
- Last updated: September 2026
Your daily water needs
Drink about daily target
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Total water from all sources
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A simple drinking schedule
| Time | Drink | Running total |
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This is an estimate for healthy people. Thirst, urine colour and how you feel are useful day-to-day guides. If you have heart, kidney or liver disease, or have been told to limit fluids, follow your doctor’s advice instead.
How today is going
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Still to go
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What Is a Water Intake Calculator?
A water intake calculator estimates how much water your body uses in a day, and how much of that you should drink. Your body loses water all the time through urine, sweat, breathing and skin. What you drink and eat replaces it.
Most calculators multiply your weight by a fixed number. That misses the things that change water needs the most: activity, heat, sex and age. This calculator uses an equation from a large 2022 study that measured real water use in 5,604 people from 26 countries, aged 8 days to 96 years. It also shows the official guideline for your age and sex, so you can see both.
What about 8 glasses a day?
Eight 250 mL glasses is 2 litres. It is a reasonable rough guide for some people, but it was never based on strong evidence. The 2022 study found daily water use ranged from about 1 litre to 6 litres a day between people.
How to Use the Water Intake Calculator
- Enter your sex and age. Both change water use, as the study found.
- Enter your weight and height. Use kg or lb, and cm or inches.
- Pick pregnancy or breastfeeding if either applies. Both raise water needs.
- Choose your activity level, the usual temperature and humidity where you live, and your altitude.
- Set your wake-up and bedtime so the calculator can spread your water across the day.
- Read your results. You get a daily drinking target, your total water need from all sources, the official guideline for comparison, a height-based check and a drinking schedule.
- Use the second tab during the day. Enter what you have drunk to see how much is left.
How the Water Intake Calculator Calculates Your Result
Step 1: Total water need (the main result)
The calculator uses the water turnover equation from Yamada and colleagues, published in Science in 2022. Water turnover is the amount of water your body uses and replaces each day. The researchers measured it with isotope-labelled water, the most accurate method available.
Water (mL/day) = 1,076 × activity level + 14.34 × weight (kg) + 374.9 × sex (male = 1) + 5.823 × humidity (%) + 1,070 × athlete (1 or 0) + 0.4726 × altitude (m) + 24.78 × age − 0.3529 × age² + 1.865 × temperature² − 19.66 × temperature (°C) − 713.1
The full equation also includes a term for how developed a country is. The calculator sets this to “high”, which fits the US, UK, Canada, Australia and most of Europe.
Step 2: How much of that to drink
Not all water comes from drinks. Food provides some, and your body makes a small amount when it burns food. The US National Academies estimate that about 80% of total water comes from drinks. So the calculator’s drinking target is 80% of your total.
Step 3: Two cross-checks
- Official guideline: the US National Academies Adequate Intake for your age and sex. Pregnancy and breastfeeding are included.
- Body-size check: 1,500 mL for each square metre of body surface area. Body surface area is worked out from your height and weight using the Mosteller formula. Doctors use this rule to estimate basic fluid needs at rest.
Daily Water Intake Recommendation
Two major bodies set guidance for adults. Both figures are total water, from drinks and food together.
| US National Academies | European Food Safety Authority | |
|---|---|---|
| Adult women | 2.7 L a day | 2.0 L a day |
| Adult men | 3.7 L a day | 2.5 L a day |
| Pregnancy | 3.0 L a day | +0.3 L a day |
| Breastfeeding | 3.8 L a day | +0.6 to 0.7 L a day |
The US figures are higher because they describe what well-hydrated Americans actually drank and ate in surveys. The European figures are closer to the minimum needed. Both agree that most healthy people can use thirst as a daily guide, and that heat and activity raise needs.
Water Intake Calculator by Weight
Heavier bodies use more water. In the equation, each extra kilogram adds about 14 mL a day. The table shows total water need by weight for a 35-year-old with average activity, in a mild climate, at sea level.
| Weight | Men: total | Men: drink | Women: total | Women: drink |
|---|---|---|---|---|
| 50 kg (110 lb) | 3.1 L | 2.5 L | 2.7 L | 2.2 L |
| 60 kg (132 lb) | 3.3 L | 2.6 L | 2.9 L | 2.3 L |
| 70 kg (154 lb) | 3.4 L | 2.7 L | 3.0 L | 2.4 L |
| 80 kg (176 lb) | 3.5 L | 2.8 L | 3.2 L | 2.5 L |
| 90 kg (198 lb) | 3.7 L | 2.9 L | 3.3 L | 2.6 L |
| 100 kg (220 lb) | 3.8 L | 3.1 L | 3.5 L | 2.8 L |
Many websites use a simple “35 mL per kg” rule. It over-rewards weight. A 100 kg person would need twice as much as a 50 kg person, but real measurements show a much smaller gap, as the table above shows.
Water Intake Calculator by Age
Water use rises into early adulthood, stays fairly steady to around age 50, then slowly falls. The table uses a 70 kg man and a 60 kg woman with average activity in a mild climate.
| Age | Man, 70 kg: total | Woman, 60 kg: total |
|---|---|---|
| 18 | 3.3 L | 2.8 L |
| 25 | 3.4 L | 2.8 L |
| 35 | 3.4 L | 2.9 L |
| 45 | 3.4 L | 2.8 L |
| 55 | 3.3 L | 2.7 L |
| 65 | 3.1 L | 2.6 L |
| 75 | 2.8 L | 2.3 L |
| 85 | 2.5 L | 2.0 L |
For children and teens, these are the US National Academies’ total water guidelines:
| Age | Boys | Girls |
|---|---|---|
| 1–3 years | 1.3 L a day | |
| 4–8 years | 1.7 L a day | |
| 9–13 years | 2.4 L | 2.1 L |
| 14–18 years | 3.3 L | 2.3 L |
Older adults: thirst gets weaker with age, so people over 65 can become dehydrated without feeling thirsty. Drinking at set times, such as with each meal, helps.
Water Intake Calculator by Height
Height affects water needs mainly through body size. Taller people have more body surface, which loses more water through the skin and lungs. The research equation does not use height directly. That is why the calculator adds a separate height-based check: 1,500 mL for each square metre of body surface area.
| Height | Weight at a healthy BMI of 22 | Body surface area | Body-size check |
|---|---|---|---|
| 150 cm (4’11”) | 49.5 kg | 1.44 m² | 2.2 L |
| 160 cm (5’3″) | 56.3 kg | 1.58 m² | 2.4 L |
| 170 cm (5’7″) | 63.6 kg | 1.73 m² | 2.6 L |
| 180 cm (5’11”) | 71.3 kg | 1.89 m² | 2.8 L |
| 190 cm (6’3″) | 79.4 kg | 2.05 m² | 3.1 L |
This check describes basic needs at rest. It does not include exercise or heat, so it is usually a little lower than the main result.
Water Intake Calculator for Men
Men generally need more water than women of the same weight. In the 2022 study, being male added about 375 mL a day, even after weight was accounted for. This is mostly because men usually have more muscle, which holds and uses more water.
- US guideline: 3.7 L total a day, or about 3.0 L from drinks.
- A typical 70–80 kg man with average activity in a mild climate: about 3.4–3.5 L total.
- Active men in hot weather can need 5 L or more. The National Academies notes needs of 6 L or more in very active people in constant heat.
Water Intake Calculator for Women
- US guideline: 2.7 L total a day, or about 2.2 L from drinks.
- A typical 60–70 kg woman with average activity in a mild climate: about 2.9–3.0 L total.
- Pregnancy raises the US guideline to 3.0 L total. Breastfeeding raises it to 3.8 L, because breast milk is mostly water.
The calculator adds the extra for pregnancy or breastfeeding using the difference between the US guidelines, which is 0.3 L and 1.1 L. If you have pregnancy sickness, swelling or a condition that affects fluid, ask your midwife or doctor.
Factors That Affect Your Water Needs
The 2022 study ranked what matters most. Physical activity came first, then sex, then where you live, then age.
| Factor | Effect |
|---|---|
| Activity | The biggest factor. Going from sedentary to active adds about 0.5 L a day. Athletes in training use roughly 1 L more on top of that. |
| Heat | A hot climate (28°C) adds about 0.8 L a day compared with a mild one (15°C). |
| Humidity | Moving from dry to humid air adds about 0.25 L a day. |
| Altitude | Each 1,000 m adds nearly 0.5 L a day, because you breathe more and lose more water. |
| Body size | About 14 mL for each extra kilogram. |
| Sex and age | Men need more. Needs fall slowly after about 50. |
| Pregnancy and breastfeeding | Raise needs, especially breastfeeding. |
| Illness | Fever, vomiting and diarrhoea increase losses. Some heart, kidney and liver conditions require less fluid, not more. |
| Diet | Fruit, vegetables and soups add water. Salty or high-protein diets raise needs a little. |
For exercise
Weigh yourself before and after a workout. Each kilogram lost is about 1 litre of sweat. Sports medicine guidance suggests drinking about 1.5 L for each kilogram lost to fully replace it over the next few hours.
Water Intake Calculation Example
Example 1: a 35-year-old woman, 65 kg, 165 cm, average activity, mild climate
- Total water need from the equation: about 2.95 L a day.
- Drinking target (80%): about 2.36 L, which is roughly 9 to 10 glasses of 250 mL, or 80 US fl oz.
- US guideline for women: 2.7 L total. The body-size check (1.73 m² × 1.5 L) gives about 2.6 L.
Example 2: a 28-year-old man, 82 kg, active, hot climate (28°C)
- Total water need: about 4.6 L a day.
- Drinking target: about 3.7 L, or about 15 glasses.
- That is well above the US guideline of 3.7 L total. Activity and heat together make the difference.
Water Intake Calculator FAQs
How much water should I drink a day?
For most adults, about 2 to 3 litres of drinks a day, plus the water in food. US guidelines are 2.7 L total for women and 3.7 L total for men, with about 80% from drinks. Activity, heat and body size can raise this a lot. Use the calculator for your own estimate.
How do I calculate water intake by body weight?
A quick rule is about 30 to 35 mL per kg, but it overestimates for heavier people. A research-based equation adds about 14 mL per kg on top of other factors, including activity, sex, age and climate. This calculator uses that equation.
Do tea and coffee count towards water intake?
Yes. The US National Academies counts caffeinated drinks towards daily water intake. At usual amounts, their mild effect on urine output does not cancel out the fluid they provide.
Can you drink too much water?
Yes. Drinking very large amounts in a short time can dilute the sodium in your blood (hyponatraemia), which can be dangerous. It is most common in long endurance events. Spread your water through the day, and drink to thirst during long exercise rather than forcing fluids.
How can I tell if I am drinking enough?
Pale straw-coloured urine and rarely feeling thirsty are good signs. Dark urine, headaches, tiredness and passing little urine can mean you need more. Some vitamins and medicines change urine colour, so it is a guide, not a test.
Do I need extra water when taking supplements like shilajit or creatine?
There is no evidence that either needs extra water. Shilajit resin is usually dissolved in a glass of warm water, which simply counts towards your day. Research does not show that creatine causes dehydration. Just aim for your normal daily target. See our creatine intake calculator and shilajit dosage guide.
Does this calculator store my information?
No. Everything runs in your browser. Nothing is sent, saved or shared.
Sources and further reading
- Yamada Y, Zhang X, Henderson MET, et al. Variation in human water turnover associated with environmental and lifestyle factors. Science. 2022;378(6622):909–915. PubMed 36423296
- National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. 2004. Summary: nationalacademies.org
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal. 2010;8(3):1459. efsa.europa.eu
- University of Wisconsin–Madison news release. Stop counting cups. There’s an ocean of difference in our water needs. 2022. eurekalert.org
- American College of Sports Medicine. Exercise and fluid replacement: position stand. Med Sci Sports Exerc. 2007;39(2):377–390. PubMed 17277604
Reviewed and updated: September 2026.