Water Intake Calculator (daily target by weight)
Enter your weight and activity level to see a realistic daily hydration target. Uses 35 mL/kg as the baseline, scaled up for activity. Compares to bottles, cups, and ounces.
How it works
Where '35 mL per kg' comes from
The 35 mL/kg/day baseline comes from clinical hydration guidelines for healthy adults: roughly 1 mL of fluid per kcal of energy intake, applied to a typical maintenance diet. For a 70 kg adult, that's 2.45 L/day. Activity adds 15-30% on top to replace sweat losses.
This number is a population-level guideline, not a hard rule. The famous '8 glasses a day' is approximate and convenient — it works out to about 2 L, close to the 70 kg baseline. Your personal need varies with body size, climate, exercise, and what you eat (food contributes 20-30% of total water intake).
What counts as water?
All fluids count toward hydration: water, tea, coffee, milk, soup, juice. The old myth that caffeine 'dehydrates' is overstated — moderate coffee/tea consumption is net hydrating. Even alcohol contributes some water, though the diuretic effect makes it net negative at higher doses.
Solid food contributes 20-30% of daily water intake. Fruits (watermelon ~92%, apple ~85%), vegetables (cucumber ~96%, lettuce ~95%), and broth-based dishes are major contributors. The 'pure water' target is therefore lower than total fluid; this calculator gives the total fluid target.
When the formula breaks
Hot climates and intense exercise can push needs to 4-6 L/day or more. Endurance athletes track sweat rate (weigh yourself before and after a workout; lost weight ≈ lost water in kg, plus what you drank).
Some medical conditions (kidney disease, heart failure, certain medications) require restricted fluid intake. Pregnancy and breastfeeding require more (300-700 mL extra/day). The calculator's number is for healthy adults at moderate climate; consult a doctor for medical adjustments.
Frequently asked questions
›Is 35 mL per kg accurate for everyone?
It's a population-level baseline. Big people, hot climates, and intense exercise push it higher. Older adults and those with certain conditions may need less.
›Do tea and coffee count?
Yes. Moderate caffeine doesn't significantly dehydrate. The total fluid target includes all drinks.
›Can I drink too much water?
Yes — water intoxication (hyponatremia) can be life-threatening if you drink very large volumes in a short time, especially during endurance exercise. Healthy adults rarely reach this from normal hydration.
›What about food water?
Food provides 20-30% of total water intake. The calculator's number is total fluid target, so you don't need to drink the full liters as plain water — soup, fruit, etc. count.
›Why does activity raise the number by only 15-30%?
Casual activity replaces small sweat losses. Endurance training in heat can require much more — track your own sweat rate for personalized targets.
›Is this safe for kids or pregnant people?
It's an adult guideline. Kids' needs vary with age (small children need more per kg). Pregnant and breastfeeding people need 300-700 mL/day more. Use the calculator as a starting point and consult a clinician.
›Does coffee count toward my target?
Yes. Caffeinated beverages are net hydrating in normal amounts.
›Does the data leave my browser?
No. Calculation runs locally.
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