How Much Water Should You Drink Per Day?

The popular advice to drink eight 8-ounce glasses of water per day has no strong scientific basis — it was a rough estimate from 1945. Your actual needs depend on your weight, activity level, climate, and diet, and the numbers can vary by a factor of two or more.

Weight-based estimates

A common formula is 30–35 ml of water per kilogram of body weight per day (roughly 0.5 oz per pound). A 70 kg (154 lb) person would need about 2.1–2.5 liters from all sources. This is a reasonable starting point before adjusting for exercise, climate, and diet.

Exercise and heat add to the baseline

Sweat losses during exercise can reach 0.5–2 liters per hour depending on intensity and temperature. A simple rule: add 400–600 ml per 30 minutes of moderate exercise. In hot weather, your baseline also rises even without exercise. Our water intake calculator folds these adjustments in automatically.

Food contributes roughly 20%

On an average Western diet, food provides about 20% of total water intake — fruits, vegetables, soups, and coffee all count. The calculator's output is a net liquid target. If you eat plenty of water-rich foods, you may need less from drinks alone. A simple gauge: pale yellow urine means good hydration; dark yellow means drink more.