Water Intake Calculator
How much water should you drink a day?
Your needs depend on weight, activity, climate, and exercise. The U.S. National Academies (NASEM) recommend about 3.7 L for men and 2.7 L for women as total water including food, of which roughly 20% comes from food. So the water you actually need to drink is less than that. This calculator gives a drinking-water target tailored to your weight and lifestyle.
The formula
Drinking water (ml) = (weight[kg] × activity factor + exercise bonus) × climate factor. Activity factors are 30 (sedentary), 35 (moderate), 40 (active) ml/kg, exercise adds about 355 ml per 30 minutes, and hot climates multiply by 1.1–1.2 for sweat loss.
Activity & climate adjustments
| Factor | Effect |
|---|---|
| Sedentary (little exercise) | 30 ml per kg |
| Moderate (exercise 3–4×/week) | 35 ml per kg |
| Active (exercise most days) | 40 ml per kg |
| Temperate | ×1.0 (no change) |
| Hot | ×1.1 |
| Very hot | ×1.2 |
Source: U.S. Institute of Medicine (IOM/NASEM, 2004) dietary reference intakes for water.
Is the '8×8' rule true?
The 8×8 rule ('drink eight 8-oz glasses a day') has little scientific basis (Valtin, 2002). Needs vary by person, and water from coffee, tea, and food all counts. The most reliable cues are thirst and urine color — pale yellow means you're well hydrated.
FAQ
Do coffee and tea count toward hydration?
Yes. Caffeinated drinks still hydrate you for the most part. Caffeine's diuretic effect is small at everyday amounts, so coffee and tea count toward your daily fluids. Just watch sugary drinks, which add calories too.
Can drinking too much water be dangerous?
Rarely, yes. Drinking a very large amount in a short time can dilute blood sodium and cause hyponatremia. Spreading your target across the day keeps it safe. There's no need to force large amounts when you're not thirsty.
How much extra water do I need when exercising?
A good baseline is about 350 ml (1.5 cups) extra per 30 minutes of exercise. If you sweat heavily or it's hot, you may need 0.5–1 L per hour. This calculator's exercise and climate options add that automatically.
How do I know if I'm dehydrated?
The easiest cue is urine color: pale straw means you're fine, while dark yellow or brown means drink more. Thirst, dry mouth, headache, and fatigue can also signal it. Logging workouts in the Planfit app makes it easy to match your hydration to how much you train.