Daily Water Intake Calculator
Calculate your recommended daily water intake based on body weight and activity level.
How to use this tool
- Enter body weight and activity level in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your water per day and the full breakdown beneath it.
A common guideline is 35 ml of water per kg of body weight per day, adjusted upward for high activity. This calculator gives a practical daily target.
⚠ This tool is for general information and education only and is not medical advice. Consult a qualified healthcare professional before making health decisions.
Formula
Base water (ml) = body weight (kg) × 35
Adjustment: +500 ml if active; −200 ml if sedentary; no adjustment if moderate.
Glasses = round( total ml ÷ 250 )
How it works
The calculator starts from a baseline of 35 ml per kilogram of body weight per day, a commonly used general recommendation for adults. An activity-level adjustment adds 500 ml for active individuals (to account for sweat losses) or subtracts 200 ml for sedentary individuals (who have lower overall fluid needs). The glass count is derived by dividing the total by 250 ml, a standard glass size. Actual hydration needs vary with climate, health status, and dietary fluid from food.
Worked example
- Body weight = 70 kg, Activity level = moderate.
- Base intake = 70 × 35 = 2450 ml.
- No adjustment for moderate activity level.
- Total = 2450 ml = 2.45 L.
- Glasses = round(2450 ÷ 250) = round(9.8) = 10 glasses.
2.45 L per day (10 glasses of 250 ml)
Common mistakes to avoid
- Treating the output as a total fluid target rather than a water target — coffee, tea, and food moisture count toward total fluid intake but vary in hydrating efficiency.
- Entering weight in pounds instead of kilograms — the formula multiplies weight by 35 ml/kg; using pounds gives a recommendation 2.2x too high.
- Ignoring heat and sweat: the active/sedentary adjustment does not account for hot climates or prolonged outdoor work, where needs can be substantially higher.
Key terms
- 35 ml/kg rule
- A general guideline recommending approximately 35 millilitres of water per kilogram of body weight daily for healthy adults.
- Fluid balance
- The equilibrium between water intake (drinks, food, metabolic water) and water loss (urine, sweat, respiration, faeces).
- Hyponatraemia
- Dangerously low blood sodium caused by drinking excessive water, a risk in endurance athletes who over-hydrate.
- Thirst mechanism
- The physiological signal driven by rising blood osmolality that prompts drinking; generally reliable in healthy adults but can lag behind actual needs during exercise.
- Dietary water
- Water contained in food (fruits, vegetables, soups); contributes roughly 20–30% of total daily water intake and is not captured by this calculator.
Frequently asked questions
- How much water should I drink per day?
- The NHS recommends 6–8 cups (1.5–2 L) daily. The 35 ml/kg rule adjusts this for body size and is widely used by dietitians.
- Does coffee or tea count towards water intake?
- Yes — caffeinated drinks count towards hydration despite mild diuretic effects. Fruit and vegetables also contribute significant water.