How we calculate it
- Base: about 33 ml per kg of body weight a day (typical range 30–35 ml/kg).
- Exercise: about 350 ml extra for every 30 minutes of sweaty activity.
- Hot weather: about half a litre extra a day.
- Pregnancy and breastfeeding: about 300 ml extra when pregnant and 700 ml when breastfeeding, following the European Food Safety Authority (EFSA).
For comparison, EFSA considers an adequate total water intake to be about 2 litres a day for women and 2.5 litres for men, from drinks and food together, in a mild climate with moderate activity.
Worked example
The base is about 33 ml per kg of body weight (between 30 and 35 ml), plus about 0.35 litres for every half hour of exercise, 0.5 litres in hot weather, 0.3 litres when pregnant and 0.7 litres when breastfeeding. For example, someone weighing 70 kg who exercises for half an hour: 70 × 0.033 = 2.31 + 0.35 = about 2.7 litres a day.
All fluids count
According to the European Food Safety Authority (EFSA), about 20% of your water comes from food (fruit, vegetables, soup) and the rest from all drinks: water, juice, milk, and even tea and coffee in moderate amounts. The simplest sign you are drinking enough: pale yellow urine.
Signs you need to drink more
- Dark urine (it should be pale yellow).
- Thirst, headache, dizziness or unexplained tiredness.
- Dry mouth and lips.
Frequently Asked Questions
Do tea, coffee and juice count?
Yes, all fluids count, as does food with lots of water such as vegetables, fruit and soup. Water is still best, and it’s wise to limit drinks high in sugar or caffeine.
Can you drink too much water?
Rarely, but yes. Drinking very large amounts in a short time (litres in a row) can lower blood sodium. Spread your drinking through the day, and let thirst and urine colour guide you.
How should I drink during Ramadan?
Spread it between iftar and suhoor instead of drinking it all at once, for example a glass or two every hour, and cut down on salty food and high-caffeine drinks.
What if I have kidney or heart disease?
Then this calculator is not for you: some kidney and heart conditions need less fluid, not more, so follow the amount your doctor gives you.