# Water intake calculator

> Daily water intake in litres, glasses and fl oz from body weight, exercise and climate, compared with EFSA and US adequate intakes.

Interactive version: https://www.calcopenly.com/health/water-intake-calculator
Subject: Health and fitness calculators

The estimate adds three parts: a baseline of 35 mL per kg of body weight (30 mL/kg is often used for adults over 65), 0.6 L for each hour of exercise, which is the midpoint of the American College of Sports Medicine's 0.4–0.8 L per hour, and 0.5 or 1.0 L for a warm or hot climate. The total is shown in litres, 250 mL glasses and US fluid ounces.

People use it to set a daily drinking goal. A 70 kg adult who exercises for 30 minutes in a temperate climate gets 2.45 + 0.3 = 2.75 L, about 11 glasses. The European Food Safety Authority's adequate intake is 2.5 L for men and 2.0 L for women, and the US National Academies' is 3.7 L and 2.7 L; both count water from food as well as drinks.

These are population figures for healthy adults. Sweat rates vary widely, and heart failure, kidney disease and some medicines call for a fluid limit set by a clinician.

## Inputs

- **Weight**
- **Exercise per day**
- **Climate** (options: Temperate, Warm, Hot (around 32 °C))
- **Sex (for the reference intakes)**: Only changes the EFSA and US comparison bars, not the estimate. (options: Male, Female)
- **Baseline per kilogram**: 30–35 mL/kg/day is the usual clinical estimate; 30 is typical for adults over about 65.

## Results

- Daily fluid (L/day) — main result
- Glasses of 250 mL
- US fluid ounces (fl oz/day)
- From body weight (L)
- For exercise (L)
- For climate (L)

## Formula

$$
V = \frac{w \times k}{1000} + 0.6 \times \frac{t_{\text{exercise}}}{60} + V_{\text{climate}}\quad(\text{L/day})
$$

## Worked examples

### 70 kg, 30 min exercise, temperate

- Weight: 70 kg
- Exercise per day: 30 min
- Climate: Temperate
- Sex (for the reference intakes): Male
- **Daily fluid: 2.75 L/day**
- **Glasses of 250 mL: 11**
- **From body weight: 2.45 L**
- **For exercise: 0.30 L**
- Checked against: Python 3.8 decimal: 70 × 35 / 1000 + 0.6 × 30/60 = 2.75 L; 2.75 / 0.25 = 11 glasses

### 180 lb, 60 min, hot climate

- Weight: 180 lb
- Exercise per day: 60 min
- Climate: Hot (around 32 °C)
- Sex (for the reference intakes): Male
- **Daily fluid: 4.46 L/day**
- **US fluid ounces: 151 fl oz/day**
- Checked against: Python 3.8 decimal: 180 × 0.45359237 × 35 / 1000 + 0.6 + 1.0 = 4.457632 L = 150.73 US fl oz

### Edge: no exercise, 30 mL/kg, 60 kg

- Weight: 60 kg
- Exercise per day: 0 min
- Climate: Temperate
- Sex (for the reference intakes): Male
- Baseline per kilogram: 30 mL/kg
- **Daily fluid: 1.80 L/day**
- **For exercise: 0.00 L**
- **For climate: 0.00 L**
- **Glasses of 250 mL: 8**
- Checked against: Python 3.8 decimal: 60 × 30 / 1000 = 1.8 L; 1.8 / 0.25 = 7.2, rounded up to 8

### 55 kg, 90 min, warm climate

- Weight: 55 kg
- Exercise per day: 90 min
- Climate: Warm
- Sex (for the reference intakes): Male
- **Daily fluid: 3.32 L/day**
- Checked against: Python 3.8 decimal: 55 × 35 / 1000 + 0.6 × 1.5 + 0.5 = 3.325 L

## Questions

### How much water should you drink a day?

Reference values count total water from drinks and food. EFSA sets adequate intakes of 2.5 L a day for men and 2.0 L for women; the US National Academies set 3.7 L and 2.7 L, of which about 3.0 L and 2.2 L typically come from drinks. These are population averages for healthy adults in temperate conditions, and needs rise with heat, exercise, pregnancy and breastfeeding.

### Is the 8 glasses of water a day rule true?

It is a rule of thumb, not a guideline. Eight 8 oz glasses make 1.9 L, and a 2002 review by Heinz Valtin in the American Journal of Physiology found no scientific evidence behind the rule. The figure falls between EFSA's adequate intakes of total water for women (2.0 L) and men (2.5 L), which already include the 20–30 % that usually comes from food.

### How much water should you drink during exercise?

The American College of Sports Medicine's 2007 position stand gives about 0.4 to 0.8 L per hour as a starting point, adjusted to sweat rate, with the aim of keeping body weight loss under 2 %. Weighing before and after a session shows the loss: each kilogram lost is about 1 litre of sweat. Drinking well beyond sweat losses risks exercise-associated hyponatraemia.

### Do coffee and tea count toward water intake?

Yes. The US National Academies' 2005 reference intakes for water concluded that caffeinated drinks contribute to total water intake much as non-caffeinated drinks do, because moderate caffeine does not cause a net loss of body water over the day. Food counts too: it supplies about 20 % of total water intake in the US diet.

### How accurate is the water intake calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “70 kg, 30 min exercise, temperate” is checked against Python 3.8 decimal: 70 × 35 / 1000 + 0.6 × 30/60 = 2.75 L; 2.75 / 0.25 = 11 glasses.

### Where does the method come from?

Masot O. et al. Fluid intake recommendation considering the physiological adaptations of adults over 65 years: a critical review. Nutrients 2020;12(11):3383 (30 mL/kg/day standard); American College of Sports Medicine. Exercise and fluid replacement (position stand). Med Sci Sports Exerc 2007;39(2):377–390; Howard G, Bartram J. Domestic water quantity, service level and health. WHO/SDE/WSH/03.02 (2003), Table 1 (IPCS 1994 reference intakes); EFSA Panel on Dietetic Products. 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. National Academies Press, 2005 (AI 3.7 L men, 2.7 L women).

## Sources

- [Masot O. et al. Fluid intake recommendation considering the physiological adaptations of adults over 65 years: a critical review. Nutrients 2020;12(11):3383 (30 mL/kg/day standard)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7694182/)
- [American College of Sports Medicine. Exercise and fluid replacement (position stand). Med Sci Sports Exerc 2007;39(2):377–390](https://pubmed.ncbi.nlm.nih.gov/17277604/)
- [Howard G, Bartram J. Domestic water quantity, service level and health. WHO/SDE/WSH/03.02 (2003), Table 1 (IPCS 1994 reference intakes)](https://iris.who.int/handle/10665/67884)
- [EFSA Panel on Dietetic Products. Scientific opinion on dietary reference values for water. EFSA Journal 2010;8(3):1459](https://doi.org/10.2903/j.efsa.2010.1459)
- [Institute of Medicine. Dietary Reference Intakes for water, potassium, sodium, chloride, and sulfate. National Academies Press, 2005 (AI 3.7 L men, 2.7 L women)](https://doi.org/10.17226/10925)

_Note: medical information, not professional advice._