The FoodGPT water calculator: how it works and where it gets it wrong
The water calculator does not use a formula. It looks up a row in Table 7 of the Russian guideline MR 2.3.1.0253-21 by sex, age and activity group, and an adult gets between 1.0 and 2.2 litres of water and drinks a day. Hot weather, a workout, pregnancy and medication are not part of that table.
What you enter and what you get
The form takes sex, an age from 18 to 100 and one of four activity levels. The answer is one figure in litres, such as "1.3-1.4". It is a range, not a point, because that is how the source prints it.
The activity levels in the table are named after jobs, because in the document they are groups of work intensity, not sport. An evening workout does not enter the choice.
| Activity level (factor) | Men, L a day | Women, L a day |
|---|---|---|
| 1.4: desk work, office | 1.3-1.4 | 1.0-1.1 |
| 1.6: light work, such as a shop assistant, driver, nurse | 1.5-1.6 | 1.2-1.3 |
| 1.9: mechanic, machine operator, gardener | 1.8-1.9 | 1.4-1.5 |
| 2.2: heavy physical labour | 2.1-2.2 | 1.7-1.8 |
| From 65, one row (factor 1.7) | 1.4-1.5 | 1.1-1.2 |
From age 65 the document gives a single figure for moderate activity, so the activity selector no longer changes the result. The calculation runs in your browser.
Where the numbers in the table come from
Table 7 is published in MR 2.3.1.0253-21, the 2021 physiological requirement norms of Rospotrebnadzor, Russia's consumer protection agency. The table title reads "Recommended levels of intake of water and drinks for maintaining the body's water balance (BMI 20-25 kg/m²)". So the figures are meant for people with a body mass index between 20 and 25.
The "Water" section of the text says that under normal conditions an adult's requirement is 1.0 to 1.5 ml per kilocalorie, and that in a temperate continental climate with moderate exertion 1 ml per kilocalorie can be recommended. It also gives the split: about 60% comes from drinking water and fluids in dishes and drinks (tea, coffee, fruit compotes and drinks, soups), and about 40% from solid food.
The table agrees with those numbers. Take the energy requirement for the same group from the MR tables, multiply by 1 ml per kcal and take 60%. For men aged 18-29 at factor 1.4 that is 2,400 kcal, or 1.44 L, and the table says 1.3-1.4. For women aged 18-29 at the same factor, 1,900 kcal gives 1.14 L against 1.0-1.1. Across all eight rows for ages 18-64, in all three age subgroups, the difference is no more than 0.1 L. This is an arithmetic check, not wording from the document, but it shows what the table is built from: the "drinking" share of total water.
The European Food Safety Authority (EFSA) gives different numbers because it answers a different question. EFSA counts all water, including the moisture in food: 2.0 L a day for women and 2.5 L for men, at moderate temperature and moderate activity (factor 1.6). Sixty percent of those is 1.2 and 1.5 L, and the MR table for factor 1.6 says 1.2-1.3 and 1.5-1.6. The two systems do not contradict each other. They measure different things.
Where the calculator gets it wrong
It cannot see weather or exercise. The table is built "for optimal microclimate parameters", in the MR's own words. EFSA says its values hold only for moderate temperature and moderate activity, and that water losses in extreme conditions can reach about 8 litres a day. In heat, on a long workout or with a fever the calculator's figure will be too low, and by how much depends on the person. How to estimate your own losses by weighing yourself, and when electrolytes matter, is covered in the article on water and electrolytes in training.
It knows nothing about pregnancy or breastfeeding. The MR states that in pregnancy the need for fluid rises by about 300 ml a day on average, and by 700 ml while breastfeeding. EFSA gives the same numbers. The calculator has no field for it, so a woman in either state has to adjust the result herself and discuss it with her doctor.
After 65 it gives less than for younger adults, which is debatable. The MR table gives 1.4-1.5 L for men from 65 and 1.1-1.2 L for women. EFSA keeps the same values for the elderly as for adults and explains that per unit of energy their water need is higher because the kidneys concentrate urine less well. More on older people and water is in the article How much water you need and where "two litres" came from.
It is built for a BMI of 20-25. For people outside that range the table says nothing. Weight is not an input, so the calculator will not warn you that the figure is less reliable for you.
It does not calculate for under-18s. The children's Table 8 in the document is laid out so that the columns by sex read ambiguously, and the calculator is for adults only. A paediatrician sets the amount for a child.
The line between "drinks" and food is blurry. The table is titled water and drinks, while the MR text places soups in the group of fluids that supply about 60%. In practice the difference is a bowl of soup, but if you count fluid in litres, decide in advance how to log soup and keep to the rule.
It knows nothing about illness or medication. The 2021 European Society of Cardiology guidelines say that in severe heart failure or hyponatraemia a fluid restriction of 1.5-2 L a day may be considered (class IIb, meaning "may be considered"). For such a person the calculator's 2.2 L is above what was prescribed. With heart or kidney disease the doctor's instruction applies, not the table. The same goes for medicines that change how the body excretes fluid, such as diuretics.
What the result means in practice
The figure answers one question: how much water and drinks on an ordinary day, when it is not hot and exertion is moderate. For a 30-year-old office worker it shows 1.0-1.1 L, which surprises many people, because "two litres" feels like the norm. The difference is that "two litres" is often retold as water on top of everything you eat, while in the MR table about 40% of fluid arrives with solid food. Where the number two came from and what was lost in retelling is described in the separate article.
A workable way to use the number:
- treat it as a reference for an ordinary day, not a target to drink at any cost;
- add fluid in heat, during exercise, and with vomiting, diarrhoea or fever, guided by thirst;
- count tea, coffee and milk: the MR includes drinks in the table, and whether coffee dehydrates is covered in the coffee article;
- do not drink past thirst to meet a figure: EFSA notes the kidneys excrete no more than about a litre an hour, and excess fluid leads to hyponatraemia with brain swelling.
Caffeine is often counted alongside water. There is a second calculator for that, described in The FoodGPT caffeine calculator: how it works and where it gets it wrong.
When to see a doctor
- A doctor has restricted your fluids, or you live with heart or kidney failure or cirrhosis, or take diuretics. The calculator's figure does not apply to you.
- Pregnancy and breastfeeding: the amount and routine are discussed with a doctor.
- Constant strong thirst together with a large volume of urine.
- Vomiting or diarrhoea where you cannot keep fluids down, especially in a child or an older person.
- Headache, nausea or confusion after a lot of water drunk in a short time: this is an emergency, call an ambulance.
The calculator is on the page Water intake calculator. It computes only what is in the table, which is enough to get a reference for an ordinary day.
Frequently asked questions
- How much water should an adult drink according to Rospotrebnadzor?
- Table 7 of MR 2.3.1.0253-21 gives adults aged 18-64 from 1.0-1.1 to 1.7-1.8 L of water and drinks a day for women and from 1.3-1.4 to 2.1-2.2 L for men, depending on how heavy their work is. From age 65 it is 1.1-1.2 L for women and 1.4-1.5 L for men.
- Why does the calculator show less than two litres?
- The table covers water and drinks, and about 40% of daily fluid comes with solid food according to the MR text. EFSA, which counts all water including food, gives 2.0 L for women and 2.5 L for men; 60% of those is 1.2 and 1.5 L.
- Does the calculator account for heat, workouts and pregnancy?
- No. The table is built for an optimal microclimate. The MR separately states an extra 300 ml a day in pregnancy and 700 ml when breastfeeding, but the calculator does not include it, nor workouts or heat.
- Do tea, coffee and soup count towards the figure?
- Tea, coffee, milk and other drinks count. In the MR text soup belongs to the fluids that supply about 60% of water, while the calculator page treats it as extra. Pick one way of logging it and keep to it.
- Can you drink too much water?
- Yes. According to EFSA the kidneys excrete up to about a litre of water an hour. If you drink faster, blood sodium is diluted and hyponatraemia with brain swelling can follow. Headache, nausea and confusion after a large volume call for an ambulance.
Read next
- How much water to drink per day and where the two liters came from — Everyone knows the "two liters a day" rule, but almost no one knows where this figure came from.
- Water and electrolytes during training: when plain water is not enough — Plain water is enough for almost everyone, almost all the time.
- The FoodGPT caffeine calculator: how it works and where it gets it wrong — The caffeine calculator adds up milligrams from a table of average values per drink and compares the total with the EFSA level: up to 400 mg a day for an adult, up to 200 mg in pregnancy and breastfeeding, and 3 mg per kilogram of body weight for a child or teenager.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.