Water Intake Calculator
Get a personalised daily hydration target adjusted for your body weight, activity, climate, and other factors. Goes beyond the generic "8 glasses a day" advice.
Your Details
If provided, hydration is calculated from lean body mass — more accurate for high or low body fat. Don't know yours? Use our body fat calculator.
Minutes of exercise per day (enter 0 if sedentary).
Your Result
Enter your details to get a personalised hydration target.
📋 What Your Result Means
For educational purposes only — not medical advice.
Your daily water target is personalized to your body weight, activity level, and environmental conditions. For active individuals, this will typically exceed the generic "8 glasses a day" advice, which has no strong scientific basis (Valtin 2002, DOI: 10.1152/ajpregu.00365.2002).
Body composition matters. Hydration needs are driven primarily by lean body mass, not total weight. Muscle tissue is roughly 75% water, while fat tissue is only about 10%. If you enter your body fat percentage (optional field above), the calculator switches to a lean-mass-based formula that gives more accurate results — especially if your body fat is significantly above or below average. Don't know your body fat? Our body fat calculator can estimate it from simple tape measurements.
All fluids count. Coffee, tea, milk, and water-rich foods (fruits, vegetables, soups) all contribute to your daily total. Contrary to popular belief, moderate caffeine intake (up to 400 mg/day) does not cause net dehydration in habitual consumers (Killer et al. 2014, DOI: 10.1371/journal.pone.0084154).
Urine color is your best real-time indicator. Aim for pale straw color. Dark yellow means you are behind; completely clear may indicate overhydration. During exercise, weigh yourself before and after — each kg lost represents roughly 1 liter of fluid to replace.
Electrolytes matter as much as volume. During prolonged exercise (over 60 minutes) or heavy sweating, plain water alone is insufficient. Sodium, potassium, and magnesium losses must be replaced to maintain performance and prevent hyponatremia (Sawka et al. 2007, DOI: 10.1249/mss.0b013e31802ca597).
Method 1 — Total body weight (default):
Base = weight × 35 mL/kg (male) · 31 mL/kg (female)
Source: IOM (2004) / EFSA (2010)
Method 2 — Lean body mass (when body fat % is entered):
Lean mass = weight × (1 − body fat / 100)
Base = lean mass × 40 mL/kg
Source: Sawka et al. (2005)
Why two methods? Adipose tissue is approximately 10% water, compared to 75% for lean tissue. The total-weight method works well for individuals near average body composition, but overestimates for those with high body fat and underestimates for very lean individuals. The lean mass method accounts for this directly.
Activity: +500 mL per 60 min moderate exercise
Climate: ×0.9 cold · ×1.0 temperate · ×1.2 hot/humid
Pregnancy: +300 mL · Breastfeeding: +700 mL (IOM 2004)
Reference Benchmarks
EFSA adequate intake: 2.5 L/day (men), 2.0 L/day (women) — for average-weight, sedentary adults in temperate climates. IOM total water intake (including food): 3.7 L (men), 2.7 L (women). These benchmarks do not account for individual variation in body composition, exercise, or environment, which is why this calculator adjusts per-person.
Limitations
This calculator provides a general estimate. Actual hydration needs vary with body composition, kidney function, medication, altitude, and individual sweat rates. The reduced scaling above reference weight is an approximation — individuals with high lean mass at high body weight (e.g. strength athletes) may need more than estimated. People with kidney disease, heart failure, or on diuretics should consult their physician for individualized guidance.
✓ Strong EvidenceWant to learn more? Read our in-depth article: How Much Water Should You Drink Per Day? →
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