How to use it
- Choose metric or US units and your sex; women can choose pregnancy or breastfeeding.
- Enter your weight and how many minutes you exercise on a typical day.
- Compare the EFSA reference with the weight-based estimate, and add the sweat range for exercise days.
Where the numbers come from
EFSA reference (2010). The EFSA Panel on Dietetic Products, Nutrition and Allergies set adequate intakes of total water – drinks plus the water in food – for adults in a temperate climate with moderate activity:
| Group | Water a day |
|---|---|
| Women | 2.0 L |
| Men | 2.5 L |
| Pregnancy | + 0.3 L (2.3 L) |
| Breastfeeding | + 0.7 L (2.7 L) |
Rule of thumb. 30 to 35 ml for each kilogram of body weight. This is a common practical estimate, not a reference value, and is shown only for comparison.
Exercise. The American College of Sports Medicine position stand on fluid replacement (2007) reports sweat rates of roughly 0.5 to 2.0 litres an hour. The calculator multiplies your exercise time by both ends of that range.
US units: 1 US fl oz = 29.5735 ml, 1 lb = 0.45359237 kg.
Worked example
A man weighing 80 kg who trains for an hour: EFSA reference 2.5 L; rule of thumb 80 × 30 = 2.4 L to 80 × 35 = 2.8 L; on training days he may lose another 0.5 to 2 L in sweat.
Good to know
- Thirst, the colour of urine (pale yellow is a good sign) and how you feel are practical guides for healthy people.
- Hot weather, altitude, fever, vomiting or diarrhoea raise needs.
- People with heart or kidney disease, or who take diuretics, may have been told to limit fluids – follow your doctor’s advice rather than any calculator.