Daily Water Intake Calculator
Daily water needs depend primarily on body weight, activity level, and environmental conditions. The National Academies of Sciences, Engineering, and Medicine set Adequate Intake (AI) values for total water from all sources at 3.7 L/day for adult men and 2.7 L/day for adult women. As a practical formula, 35 mL per kilogram of body weight provides a baseline from beverages, with an additional allowance for physical activity. This calculator applies that weight-based formula and adds an activity bonus: sedentary adults add 0 mL, lightly active adults add 350 mL, moderately active adults add 600 mL, and highly active individuals (athletes or manual workers) add 1,000 mL. Results are shown in milliliters, liters, and US fluid ounces.
Water intake formula
Water (mL) = weight (kg) * 35 + activity bonus (mL)
Water (L) = Water (mL) / 1000
Water (fl oz) = Water (mL) / 29.5735
Activity bonus: sedentary = 0, lightly active = 350 mL, moderately active = 600 mL, highly active = 1,000 mL.
Hydration tips
- About 20% of daily water intake typically comes from food, so total fluid requirements are higher than beverage intake alone.
- Urine color is a practical hydration indicator: pale yellow indicates good hydration; dark yellow or amber indicates dehydration.
- Thirst is not always a reliable early signal of dehydration, especially in older adults and children.
- Caffeinated beverages such as coffee and tea do contribute to daily fluid intake despite mild diuretic effects.
- Increase intake during illness, heat exposure, travel on aircraft, and at high altitude.
Daily water intake calculator: frequently asked questions
How much water should I drink per day?
The National Academies of Sciences, Engineering, and Medicine set Adequate Intake (AI) at 3.7 liters (125 fl oz) per day for men and 2.7 liters (91 fl oz) for women, from all beverages and food. Approximately 20% of intake comes from food; the remaining 80% comes from drinks.
Does body weight affect water needs?
Yes. A common rule of thumb is 30-35 mL of water per kilogram of body weight per day. A 70 kg adult would target approximately 2,100-2,450 mL per day from beverages alone. This calculator uses 35 mL/kg as its base factor.
How does exercise increase water needs?
During physical activity, sweat losses can range from 0.5 to 2 liters per hour depending on intensity, heat, and individual variation. The American College of Sports Medicine recommends drinking to prevent more than 2% body weight loss from sweat. This calculator adds a fixed volume based on your selected activity level.
Does the 8 glasses per day rule have a scientific basis?
The popular advice to drink 8 glasses (64 fl oz, about 1.9 L) per day is a rough approximation with no single authoritative source. The National Academies AI values are higher and vary by sex, body size, activity, and climate.
Should I drink more water in hot weather?
Yes. Heat and humidity increase sweat rates, raising fluid needs significantly. This calculator does not adjust for ambient temperature; add an extra 500-1,000 mL per day in hot conditions or if you work outdoors.
Official sources
- National Academies of Sciences, Engineering, and Medicine: Dietary Reference Intakes (DRI).
- CDC Nutrition: Water and Healthier Drinks.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.