Water intake calculator
A body-weight-based estimate with adjustments for exercise and climate. The published reference values are shown alongside, because they answer the same question differently.
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Result
These are estimates from population-level equations, not a measurement of you and not medical advice. Individual results vary widely. Talk to a doctor or a registered dietitian before making significant changes to how you eat or train.
How this is calculated
Body-weight-based fluid estimatedefault
baseline (L) = body weight (kg) × 32.5 mL / 1000
exercise (L) = hours × 0.5
total = (baseline + exercise) × climate factor
climate: temperate ×1.0 · hot ×1.15 · very hot ×1.3
- Source
- Reference values: Institute of Medicine, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate, 2005 (3.7 L/day men, 2.7 L/day women, total water); EFSA Journal 8(3), 2010, 1459 (2.5 L and 2.0 L). Sweat replacement: Sawka MN et al., ACSM position stand, Medicine and Science in Sports and Exercise 39(2), 2007, pp. 377–390.
- Assumes
- An average sweat rate. The ACSM stand declines to give a single figure because real rates range from about 0.3 to 2.4 L per hour.
- Accuracy
- The least precise calculator on this site, and honestly so. The climate multipliers are directional rather than derived — no published equation maps ambient temperature to intake for the general public. Weighing yourself before and after a session is the only way to know your own sweat rate.
Daily fluid by body weight and exercise
Litres per day from drinks, in a temperate climate. The Institute of Medicine report behind these reference values notes that most healthy people meet their needs by letting thirst guide them.
| Body weight | None | 30 min | 60 min | 90 min |
|---|---|---|---|---|
| 50 kg | 1.6 L | 1.9 L | 2.1 L | 2.4 L |
| 60 kg | 2 L | 2.2 L | 2.5 L | 2.7 L |
| 70 kg | 2.3 L | 2.5 L | 2.8 L | 3 L |
| 80 kg | 2.6 L | 2.9 L | 3.1 L | 3.4 L |
| 90 kg | 2.9 L | 3.2 L | 3.4 L | 3.7 L |
| 100 kg | 3.3 L | 3.5 L | 3.8 L | 4 L |
Questions
- Is eight glasses a day correct?
- It has no identifiable source in the scientific literature. Nobody has been able to trace it to a study. It is not harmful advice, but it is not evidence either.
- Do I need to hit this number exactly?
- No. The Institute of Medicine report behind these reference values states plainly that the great majority of healthy people meet their needs by letting thirst guide them. These are population reference values, not targets.
- Does coffee count?
- Yes. The diuretic effect of caffeine at normal intakes is smaller than the fluid volume of the drink, so caffeinated drinks contribute net positively to hydration. So does food, at roughly 20% of total water intake.
- Why do the IOM and EFSA figures differ?
- They used different methods and different reference populations. The IOM values are based on median intakes from US survey data; EFSA derived theirs from observed intakes and desirable urine osmolality in European populations. Both are shown rather than picking one.
- Can you drink too much water?
- Yes. Drinking far beyond need, particularly during endurance events, can dilute blood sodium — exercise-associated hyponatraemia — which is dangerous. It is rare, but it is the reason "drink as much as possible" is bad advice.