Daily water intake in litres, glasses and fl oz from body weight, exercise and climate, compared with EFSA and US adequate intakes.
The estimate comes to about 2.75 L of fluid a day, roughly 11 glasses of 250 mL: 2.45 L for your body weight, 0.3 L for 30 minutes of exercise.
30–35 mL per kg per day is the clinical rule of thumb; the Nutrients 2020 review cites 30 mL/kg for older adults.
0.6 L/h is the midpoint of the ACSM 2007 suggestion of 0.4–0.8 L/h; heavy sweaters in heat need more.
IPCS reference values rise from about 2 L/day in normal conditions to 2.8–3.4 L/day at 32 °C (WHO/SDE/WSH/03.02, Table 1); hot adds 1.0 L, warm 0.5 L.
Glasses are rounded up. EFSA and NASEM figures count water from food too (about 20–30 % of the total), so they are context, not the same quantity.
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.
Checked against: Python 3.8 decimal: 70 × 35 / 1000 + 0.6 × 30/60 = 2.75 L; 2.75 / 0.25 = 11 glasses
Checked against: Python 3.8 decimal: 180 × 0.45359237 × 35 / 1000 + 0.6 + 1.0 = 4.457632 L = 150.73 US fl oz
Checked against: Python 3.8 decimal: 60 × 30 / 1000 = 1.8 L; 1.8 / 0.25 = 7.2, rounded up to 8
Checked against: Python 3.8 decimal: 55 × 35 / 1000 + 0.6 × 1.5 + 0.5 = 3.325 L
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.
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.
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.
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.
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.
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).
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
4 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
Body mass index (BMI) from height and weight in metric or imperial units, with the WHO adult category and the healthy weight range for your height.
Calories burned by an activity from its MET value in the Compendium of Physical Activities, your body weight and the time spent.
Daily protein intake in grams from body weight, goal and training level, using published g/kg ranges, with a per-meal split.
Basal metabolic rate (BMR) by Mifflin-St Jeor, Harris-Benedict or Katch-McArdle, and daily calorie needs (TDEE) for five activity levels.
Daily calorie target for losing, maintaining or gaining weight, with macros in grams of protein, carbohydrate and fat per day and per meal.
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