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Daily Water Intake Calculator (by weight, activity & climate)
Work out how much water to drink each day from your body weight, exercise and climate — not the outdated "8 glasses a day" rule. The bodyweight method uses about 33 mL of water per kilogram (roughly 30–35 mL/kg, close to the "half your weight in pounds, in ounces" shortcut), then adds around 12 mL for every minute of exercise and about 15% more in hot, humid weather. For example, a 70 kg person doing 45 minutes of exercise on a temperate day gets a base of 2,310 mL plus 540 mL for activity — about 2.85 L, or roughly 96 fl oz. Enter your weight in kilograms or pounds, your daily exercise minutes and your climate, and the calculator shows your daily target in mL, litres and fluid ounces, broken down into base, activity and climate. Remember this is beverage water — NASEM total-water figures (~3.7 L men / 2.7 L women) also count the ~20% you get from food. General hydration guidance, not medical advice: the real day-to-day check is pale-straw urine and drinking to thirst.
Base = weight_kg × 33 (≈ 30–35 mL/kg)
- 70 kg + 45 min exercise, temperate → ≈ 2.85 L (~96 fl oz)
- Hot & humid adds +15%; activity adds ≈ 12 mL/min
Frequently asked questions
How much water should I drink a day?
It depends on your body weight, activity and climate — not a fixed 8 glasses. Start from about 33 mL per kilogram of body weight, add roughly 12 mL per minute of exercise, and add about 15% in hot, humid weather. A 70 kg person doing 45 minutes of exercise on a mild day needs around 2.85 L (about 96 fl oz) of beverage water.
Is 8 glasses of water a day the right amount?
Not really — the "8 glasses a day" rule is not evidence-based. Eight 8-oz glasses is about 1.9 L, which is only a rough average. Your actual need scales with weight, activity and climate, so a larger or very active person in the heat needs more, while a smaller, sedentary person may need less.