How much water to drink a day: the real figure based on your weight, training and heat
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The 8 glasses of water a day do not come from any study. What EFSA and the ACSM recommend, how to calculate what you lose when you train and when drinking too much becomes a real risk.
"Drink eight glasses of water a day" is probably the most repeated health advice in the world. And it does not come from any study. In 2002, physiologist Heinz Valtin published a review in the American Journal of Physiology looking for its origin and for evidence to support it. His conclusion was that he found no scientific study supporting that figure for healthy adults in a temperate climate, and that caffeinated drinks also count towards the total.
That does not mean water does not matter. It means the right amount depends on you: your size, how much you train, how hot it is and what you eat. In this guide you will see what official bodies recommend, how to calculate your figure, how much to drink when you train, how to tell whether you are well hydrated without doing sums, and when drinking too much becomes a problem.
What official bodies recommend
The reference in Europe is the 2010 opinion of the European Food Safety Authority (EFSA) on reference values for water. It sets an adequate intake of total water, meaning what you drink plus what comes in food:
| Group | Total water per day | Of which, roughly, drunk |
|---|---|---|
| Adult women | 2.0 litres | About 1.6 litres |
| Adult men | 2.5 litres | About 2.0 litres |
| Pregnancy | 2.3 litres (+300 ml) | About 1.8 litres |
| Breastfeeding | 2.6-2.7 litres (+600-700 ml) | About 2.1 litres |
EFSA assumes that around 20% of total water comes with food: fruit, vegetables, soups, dairy and cooked rice all contain a lot. That is why the part you need to drink is lower than the total figure. The US National Academies, in their 2005 report, give somewhat higher figures for total water (3.7 litres for men and 2.7 for women) because they start from what the healthy population of that country consumes on average, not from a minimum.
Important: these figures are for healthy adults with light activity in a temperate climate. If you train, if it is hot or if you sweat a lot, your needs go up, sometimes considerably.
The 8-glasses myth and why your figure is different
The largest study ever done on how much water the human body actually turns over was published in Science in 2022. Yamada and colleagues measured the water turnover of 5,604 people from 23 countries, from eight-day-old babies to 96-year-olds, using isotopes that track the water entering and leaving the body. Turnover depended on age, body size and composition, physical activity, being an athlete, pregnancy, socioeconomic status and climate: temperature, humidity, altitude and latitude.
With so many variables, a single figure for everyone makes no sense. A large person training in summer may need twice as much water as a small, sedentary person in winter. On top of that, part of the water the body turns over is not drunk at all: it comes from food and from metabolism itself.
How to calculate your figure from your weight
A widely used rule of thumb for adults is 30 to 35 ml of total water per kilo of body weight on a non-training day. It is not an exact scientific rule, but it gives results consistent with EFSA's figures and works as a starting point:
| Body weight | Total water per day | Approximate drinking (80%) |
|---|---|---|
| 55 kg | 1.7-1.9 litres | 1.3-1.5 litres |
| 65 kg | 2.0-2.3 litres | 1.6-1.8 litres |
| 75 kg | 2.3-2.6 litres | 1.8-2.1 litres |
| 85 kg | 2.6-3.0 litres | 2.0-2.4 litres |
| 95 kg | 2.9-3.3 litres | 2.3-2.7 litres |
On top of that base, add what you lose in training and what the heat demands. And adjust using your body's signals, which are more reliable than any formula.
Do coffee, tea and milk count?
Yes. Maughan and colleagues (2016) gave 72 men one litre of 13 different drinks and measured how much urine they produced over the next four hours. Milk and oral rehydration solutions retained more fluid than water, and coffee and tea did not differ significantly from water. The idea that coffee "dehydrates" does not hold up for regular drinkers in normal amounts.
How much water to drink if you train
The reference document is the American College of Sports Medicine's position stand on exercise and fluid replacement, by Sawka and colleagues (2007). Its central goal is to avoid losing more than 2% of body weight through dehydration during exercise, because beyond that point performance starts to suffer. Its guidelines:
- Before: start hydrated. If needed, drink about 5-7 ml per kilo at least four hours beforehand; for a 70 kg person, about 350-500 ml.
- During: drink enough not to lose more than 2% of your weight, without overdoing it. Because sweating varies enormously between people, they recommend individual plans.
- After: replace what you lost. Since part of what you drink is passed as urine, it is worth drinking somewhat more than you lost, in the order of 1.5 litres per kilo.
How to calculate how much you sweat
This is the method the ACSM recommends, and you only need a scale:
- Weigh yourself without clothes just before training.
- Train for an hour and note down what you drink during the session.
- Dry off the sweat and weigh yourself again without clothes.
- Your sweat loss per hour is the weight difference plus what you drank. Each kilo lost is roughly one litre.
If you weigh 75 kg and lose 0.8 kg in an hour without drinking, your sweat rate is around 0.8 litres per hour. 2% of your weight is 1.5 kg: in a one-hour session you are far from that limit, but on a long summer run you are not.
| Situation | What to do |
|---|---|
| One-hour strength session, temperate climate | Water according to thirst; no planning needed |
| Cardio lasting over an hour | Drink during the session so you do not lose more than 2% of your weight |
| Heat or very heavy sweating | Water with electrolytes (sodium) and weigh yourself before and after |
| Efforts lasting over two hours | Drink with carbohydrates and sodium, without exceeding what you sweat |
Does dehydration affect strength?
Less than people think. Cheuvront and Kenefick (2014) reviewed the literature in Comprehensive Physiology and found a fairly clear threshold of 2% dehydration for endurance performance to worsen. In strength and power the effect was small, though potentially relevant, and with no defined threshold. In practice: in the gym, drinking to thirst is usually enough; in long sports and in the heat, hydration does make a difference.
How to tell if you are well hydrated without doing sums
The body has a very precise system for regulating water, and thirst is its main signal. Urine colour is also a useful indicator. Armstrong and colleagues (1994) found that urine colour correlates well with its specific gravity and osmolality, two laboratory measures of hydration status, and concluded that it is valid for use outside the lab.
| Colour | What it usually indicates | What to do |
|---|---|---|
| Very pale or straw yellow | Good hydration | Keep it up |
| Deep yellow | Borderline hydration | Drink a bit more through the day |
| Amber or dark | Probable dehydration | Drink and keep an eye on it, especially before training |
| Clear all the time | You may be drinking more than you need | No need to push further |
Note: some supplements, such as B vitamins, turn urine bright yellow even when you are well hydrated.
Water and weight loss
Water does not burn fat, but it can help you eat less. Dennis and colleagues (2010) split 48 overweight adults aged 55 to 75 into two groups following the same low-calorie diet: one drank 500 ml of water before each meal and the other did not. Over 12 weeks, the water group lost about 2 kilos more, and in the test meal at the start they ate less when they drank water beforehand. The effect is modest and was tested in older people, but it is a cheap strategy. What really moves the scale is still a calorie deficit.
What if I take creatine?
This is one of the most searched questions. Creatine makes muscle retain more water inside its cells, which is why many guides advise drinking "lots of water". You do not need extraordinary amounts: follow the same signals as always, thirst and urine colour, and adjust if you train in the heat. All the details are in the complete creatine guide.
When drinking too much is dangerous
Drinking too much water in a short time can dilute the sodium in your blood and cause hyponatraemia, a condition that can be serious. It is not a theoretical risk. Almond and colleagues studied 488 runners from the 2002 Boston Marathon and published the results in the New England Journal of Medicine in 2005: 13% finished with hyponatraemia and 0.6% with critical hyponatraemia. The associated factors were gaining weight during the race, which is the sign of having drunk more than you sweated, drinking more than 3 litres, taking longer than four hours and having an extreme body mass index.
The rule for long events is simple: do not drink more than you sweat. If you finish heavier than you started, you drank too much. In everyday life, a healthy adult who drinks to thirst has a very low risk.
Situations that change your needs
- Heat and humidity: sweating can multiply. Drink more and add sodium if you sweat a lot.
- Altitude: you lose more water through breathing.
- Older age: the sense of thirst decreases, so it is worth drinking more regularly without waiting to feel thirsty.
- Pregnancy and breastfeeding: needs go up, as the EFSA table shows.
- Illness: fever, vomiting or diarrhoea increase losses. Some kidney or heart conditions require limiting fluids: in that case, your doctor is in charge.
Conclusion
The 8 glasses a day have no scientific basis, but they are not far off for many people either. EFSA recommends about 2 litres of total water for women and 2.5 for men, of which around 80% is drunk. If you train, weigh yourself before and after to know how much you sweat, avoid losing more than 2% of your weight and replace a bit more than you lost. Be guided by thirst and urine colour, and do not drink more than you sweat during long efforts. And if you also want to get your food right around training, see what to eat before training and the guide to post-workout nutrition.
If you want to turn this into a specific routine, the Progrevia routine generator builds one for free in under a minute, adapted to your goal, level and equipment.
- Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? American Journal of Physiology. Regulatory, Integrative and Comparative Physiology. 2002;283(5):R993-R1004.
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific Opinion on Dietary Reference Values for water. EFSA Journal. 2010;8(3):1459.
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press; 2005.
- 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.
- Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. The American Journal of Clinical Nutrition. 2016;103(3):717-723.
- American College of Sports Medicine, Sawka MN, Burke LM, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007;39(2):377-390.
- Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Comprehensive Physiology. 2014;4(1):257-285.
- Armstrong LE, Maresh CM, Castellani JW, et al. Urinary indices of hydration status. International Journal of Sport Nutrition. 1994;4(3):265-279.
- Dennis EA, Dengo AL, Comber DL, et al. Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults. Obesity. 2010;18(2):300-307.
- Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. The New England Journal of Medicine. 2005;352(15):1550-1556.
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Frequently asked questions
How much water should you drink a day?
EFSA recommends about 2 litres of total water a day for women and 2.5 for men, counting what comes in food. Since around 20% comes with meals, the part you drink is around 1.6 and 2 litres respectively. If you train or it is hot, you need more.
Is it true you should drink 8 glasses of water a day?
There are no studies supporting that figure. Valtin's review (2002) found no scientific evidence for healthy adults in a temperate climate. The right amount depends on your size, activity, climate and what you eat.
How do I calculate how much water to drink based on my weight?
A rule of thumb is 30 to 35 ml of total water per kilo on a non-training day: about 2.3-2.6 litres for 75 kg. Add what you lose in training, which you can calculate by weighing yourself before and after the session.
Does coffee count as water?
Yes. In Maughan's study (2016), coffee and tea did not differ from water in how much fluid the body retained, and milk retained even more. For regular drinkers in normal amounts, coffee does not dehydrate.
Can you drink too much water?
Yes. Drinking much more than you sweat, especially during long efforts, can cause hyponatraemia. In the 2002 Boston Marathon, 13% of the runners studied developed it, associated with gaining weight during the race and drinking more than 3 litres.
Informational content based on the available scientific evidence: it does not replace advice from a healthcare professional. If you have an injury or a medical condition, or are pregnant, check with one before changing your training or diet.
