CalculatorsBody and fitnessWater intake calculator
Water intake calculator
Start from the official intake figures, then add for your exercise and your weather. No 8 glasses rule, and no weight-based guess.
2.5litres of drinks a day
- In US cups and fluid ounces
- 10 cups83 fl oz
- All water, with what's in food
- 3.0 LFood brings about a fifth
- The European figure
- 2.0 LEFSA, all water, before exercise and heat
- Added for exercise and heat
- 0.3 LA rough allowance, not a measurement
See the working
National Academies, from drinks: 2.2 LExercise: 30 min × 0.5 L an hour = 0.25 LTotal = 2.45 LSweat varies a lot between people. Pale yellow urine and not feeling thirsty are the everyday check. If you've been told to limit fluids, for your heart or kidneys, follow that instead.
Where the number comes from
The US National Academies looked at how much healthy people actually drink and set adequate intakes: 3.7 litres of water a day for men and 2.7 for women, from everything. Food brings about a fifth of that, which leaves 3.0 and 2.2 litres from drinks.
The European food safety authority sets lower figures, 2.5 and 2.0 litres from all sources. Both are fair, so we show both. Coffee and tea count.
Exercise and heat
People lose anywhere from about half a litre to two litres of sweat in an hour of exercise, depending on size, heat and effort. We add half a litre an hour, and half a litre for a hot climate. These are rough allowances of ours, not official figures.
If you want your own number, weigh yourself before and after an hour of training. Each kilo lost is about a litre of sweat, less whatever you drank.
Questions
Do I really need 8 glasses a day?
There's no good evidence for that exact rule. It's roughly in the right range for many people, once you count other drinks.
Does coffee count?
Yes. Caffeine has a mild effect on urine, but coffee and tea still add to your water overall.
Can you drink too much?
Yes, though it's rare. Drinking well over a litre an hour for hours, as some endurance athletes do, can dilute your blood sodium to a dangerous level.
Who shouldn't follow this?
Anyone told to limit fluids, often for heart failure or kidney disease. Follow what your doctor told you.
Sources
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2005.
- EFSA Panel. Scientific opinion on dietary reference values for water. EFSA Journal, 2010.
Written by LifeFrom from the sources above, and not yet reviewed by a clinician. An estimate for your own understanding: it doesn't diagnose or treat anything, and it doesn't replace your doctor.
