How this calculator works
Here is how the number is worked out:
- A baseline for your body. Your weight times 34 ml per kg (women) or 36 ml per kg (men). These are the rates behind the European Food Safety Authority's adequate intakes of 2.0 litres a day for women and 2.5 litres for men. "Prefer not to say" uses the midpoint, 35 ml per kg.
- Sensible limits. The baseline never drops below 80% of those adequate intakes, and never goes above the 95th percentile of intakes EFSA reports (3.1 litres for women, 4.0 litres for men), because per-kilo rates overestimate needs at high body weights. From age 65 the floor is the full adequate intake, so older adults get at least 1.6 litres (women) or 2.0 litres (men) of drinks a day, the minimum in the ESPEN guideline for older adults.
- Pregnancy and breastfeeding. EFSA adds about 300 ml a day in pregnancy and about 700 ml while breastfeeding.
- Drinks, not food. The figures above are total water, from drinks and food. Drinks supply about 70–80% of it (EFSA) or about 80% (US National Academies), so your drinks goal is 80% of the total.
- Activity and heat. Sweat is replaced by drinking, so it is added to the drinks goal in full: 150 to 1,000 ml depending on how active you are, based on the low end of the 0.5–2 litres an hour that people sweat during exercise (American College of Sports Medicine), and 250 to 750 ml for warm to very hot weather. The heat amounts are our own estimate, not a guideline value.
What it doesn't cover
- Children under 14: EFSA sets fixed values by age group instead.
- Fever, vomiting or diarrhoea, or long workouts in the heat: you may need more, and sometimes electrolytes too.
- Heart, kidney or liver conditions, or any fluid limit from your doctor: follow your doctor's advice instead.
Your thirst and the colour of your urine are still good daily checks. Read more in how much water you really need each day.
Sources
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010. doi:10.2903/j.efsa.2010.1459
- Volkert D, et al. ESPEN guideline on clinical nutrition and hydration in geriatrics. Clinical Nutrition, 2019. doi:10.1016/j.clnu.2018.05.024
- Sawka MN, et al. American College of Sports Medicine position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 2007. doi:10.1249/mss.0b013e31802ca597
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2005. doi:10.17226/10925