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How Much Water Should You Drink Based on Weight and Activity?
Your daily water target starts with a baseline tied to body weight, then adjusts upward for exercise, hot climates, and life stages like pregnancy or breastfeeding that increase fluid needs.
Example: A 70kg person with 30 minutes of exercise in a temperate climate → (70 × 33ml) + 350ml ≈ 2.66 liters/day.
Daily Water Intake Calculator by Weight (Baseline, No Activity)
| Body Weight | 30 ml/kg | 33 ml/kg | 35 ml/kg |
|---|---|---|---|
| 50 kg (110 lb) | 1.5 L | 1.65 L | 1.75 L |
| 60 kg (132 lb) | 1.8 L | 1.98 L | 2.1 L |
| 70 kg (154 lb) | 2.1 L | 2.31 L | 2.45 L |
| 80 kg (176 lb) | 2.4 L | 2.64 L | 2.8 L |
| 90 kg (198 lb) | 2.7 L | 2.97 L | 3.15 L |
| 100 kg (220 lb) | 3.0 L | 3.3 L | 3.5 L |
This calculator uses 33 ml/kg as its baseline multiplier — the midpoint of the commonly cited 30-35 ml/kg range — before activity, climate, and life-stage adjustments.
Water Intake for Athletes and Active People
Exercise increases fluid loss through sweat. A common guideline adds roughly 350-700 ml (12-24 oz) per 30 minutes of moderate-to-intense activity, scaling up further in hot conditions or for heavy sweaters. Athletes should also watch for signs of both under- and over-hydration around long training sessions.
Water Needs During Pregnancy and Breastfeeding
Pregnancy
Increased blood volume and amniotic fluid raise water needs by roughly 300 ml (about 10 oz) per day above baseline.
Breastfeeding
Milk production significantly increases fluid needs — often by 700-1,000 ml (24-34 oz) per day above baseline.
Is the "8 Glasses a Day" Rule Actually True?
The 8x8 rule (eight 8-ounce glasses, about 1.9 liters) is a simple, memorable guideline — not a precise scientific target. It roughly matches the baseline for a smaller, sedentary adult, but underestimates needs for larger, more active people, and doesn't account for climate, exercise, or life stage. Weight- and activity-based targets, like the one this calculator provides, are more individually accurate.
Signs of Dehydration and Overhydration
- Dehydration: dark yellow urine, dry mouth, fatigue
- Dehydration: headache, dizziness, reduced urine output
- Severe dehydration: rapid heartbeat, confusion
- Overhydration: nausea, headache, confusion
- Overhydration: most common during endurance events with excess plain water intake
- Overhydration: can dilute blood sodium (hyponatremia) in rare, severe cases
Related Calculators
Scientific References
- Institute of Medicine (US). "Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate." National Academies Press, 2005.
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. "Scientific Opinion on Dietary Reference Values for water." EFSA Journal, 2010.
- Popkin BM, D'Anci KE, Rosenberg IH. "Water, hydration, and health." Nutrition Reviews, 2010.
- Sawka MN, et al. "American College of Sports Medicine position stand: exercise and fluid replacement." Medicine & Science in Sports & Exercise, 2007.