How it is calculated
- 35 ml per kg of body weight as a base.
- Plus 350 ml for every 30 minutes of exercise.
- Plus 0.5 litres in hot or humid weather.
This is a common rule of thumb, not a medical prescription. For comparison, the European Food Safety Authority (EFSA, 2010) sets adequate total water intake at 2.0 litres a day for women and 2.5 litres for men, from drinks and food combined.
People with kidney, heart or liver conditions, and pregnant or breastfeeding women, should follow their doctor's advice on fluids.