How we work out your daily target
We start from reference values. Bulgaria's national nutrition norms and the European Food Safety Authority (EFSA) set an adequate intake of 2.0 L of water a day for women and 2.5 L for men. That includes water from drinks and from food, and it assumes mild weather and moderate activity.
Bigger bodies need more water, so we allow 30 ml per kilogram. The nutrition societies of Germany, Austria and Switzerland recommend 35 ml per kilogram for adults up to 50. That figure, however, includes about 330 ml a day of water the body makes itself as it breaks down food. Without it, about 30 ml per kilogram remains. At 67 kg that gives 2.0 L and at 83 kg 2.5 L, which matches the values for women and men.
About a fifth of our water comes from food such as fruit, vegetables, soup and yoghurt. EFSA assumes food provides 20–30% of water and drinks 70–80%. The calculator compares drinks only, so the target is 80% of the total. The European Society for Clinical Nutrition and Metabolism (ESPEN) uses the same assumption in its guideline for older adults.
The target never drops below 1.6 L from drinks, which is the drinks share of the reference value for women. Before exercise it never goes above 3.2 L, because for heavy bodies the per-kilogram rule overestimates needs. In a study of 5,604 people, 50 kg of extra body weight meant only about 0.7 L more water turnover a day. And 95% of men in the European data take in up to 4 L of water a day in total. With 80% from drinks, that makes 3.2 L.
For exercise we add 0.5 L per hour. The American College of Sports Medicine gives 0.4–0.8 L an hour as a possible starting point for marathon runners, more for bigger, faster runners in the heat and less for smaller, slower runners in cool weather. Sweat rates vary a lot, so it advises everyone to tailor the amount and not to drink more than they lose in sweat.
The guide is for mild weather. In the heat you need more: in the large study, people at around 30 °C turned over about a litre more water a day than at 0–10 °C.
The formula in short
- Weight × 30 ml = your daily water from drinks and food, but no less than 2.0 L and no more than 4.0 L.
- 80% of that comes from drinks.
- We add 0.5 L for every hour of exercise.
- We round to the nearest 0.1 L.
Example: 80 kg and 30 minutes of exercise a day. 80 × 30 ml = 2.4 L. 80% of that is 1.92 L, plus 0.25 L for exercise, or about 2.2 L from drinks a day.
How is water linked to your gut?
With constipation, water makes most sense together with fibre. According to the American Gastroenterological Association, there is no evidence that drinking more water on its own relieves constipation unless you are not drinking enough.
So far there is only one trial on this. In it, 117 people with chronic constipation ate about 25 g of fibre a day for two months. Half drank 2 L of mineral water a day and the rest drank as much as they liked, on average about 1.1 L. Bowel movements became more frequent in both groups, but more so in the 2-litre group, who also used fewer laxatives.
That is why the British Dietetic Association advises drinking more fluid when you take a fibre supplement, where appropriate. In trials, high-mineral water helped more people with constipation than low-mineral water. Some of these waters are high in sodium, so check the label if you watch your salt.
If you want to see what the microbes in your gut are doing, the gut microbiome test shows your microbes' potential for gut motility and for making short-chain fatty acids when they ferment fibre. Its food plan also has a “Drinks” group: for 9 drinks, including coffee, black and green tea and kombucha, you see how often to have them in each phase. This is information about your microbes, not a diagnosis.
When to ask a doctor how much to drink
- if you have a heart or kidney condition, because doctors sometimes advise less fluid;
- if you take medicines that affect body water, such as water tablets (diuretics);
- if you are pregnant or breastfeeding: the reference values are then 2.3 L and 2.7 L a day, from drinks and food together;
- if you train for long periods in the heat or are preparing for a marathon;
- if you feel thirsty all the time even though you drink a lot;
- if you have a fever, vomiting or diarrhoea that does not go away.
This calculator gives a guide, not medical advice. It is meant for healthy adults in mild weather and does not replace a conversation with your doctor.
Sources (12)
- [1]Bulgarian Ministry of Health. Ordinance No. 1 of 22 January 2018 on the physiological nutrition norms for the population (State Gazette No. 11/2018), Art. 10 and Annex 9: adequate intake of water.
- [2]EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA), 2010. Scientific Opinion on Dietary Reference Values for water. EFSA Journal 8(3):1459.
- [3]German, Austrian and Swiss Nutrition Societies (DGE, ÖGE, SGE). D-A-CH reference values: water (derived 2000).
- [4]Volkert D, Beck AM, Cederholm T et al., 2022. ESPEN practical guideline: Clinical nutrition and hydration in geriatrics. Clinical Nutrition.
- [5]American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER et al., 2007. Exercise and fluid replacement (position stand). Medicine & Science in Sports & Exercise.
- [6]Yamada Y, Zhang X, Henderson MET et al., 2022. Variation in human water turnover associated with environmental and lifestyle factors. Science.
- [7]Killer SC, Blannin AK, Jeukendrup AE, 2014. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One.
- [8]NHS. Water, drinks and your health (reviewed 17 May 2023).
- [9]NHS. Thirst (reviewed 9 November 2023).
- [10]Bharucha AE, Pemberton JH, Locke GR 3rd, 2013. American Gastroenterological Association technical review on constipation. Gastroenterology.
- [11]Anti M, Pignataro G, Armuzzi A et al., 1998. Water supplementation enhances the effect of high-fiber diet on stool frequency and laxative consumption in adult patients with functional constipation. Hepato-Gastroenterology.
- [12]Dimidi E, van der Schoot A, Barrett K et al., 2025. British Dietetic Association Guidelines for the Dietary Management of Chronic Constipation in Adults. Journal of Human Nutrition and Dietetics.
