Water Intake Calculator
How much water does your body need every day?
About Water Intake Calculator
This calculator estimates your daily water requirement using a weight-based formula of 35 mL per kilogram of body weight, then adds an evidence-informed activity bonus to account for sweat and metabolic water losses during exercise. The result is expressed in millilitres, litres, and fluid ounces, with a per-hour sipping guide across 16 waking hours. EFSA (2010) adequate intake values and IOM (2005) total water figures are shown as reference benchmarks.
Daily Water (mL) = Body Weight (kg) x 35 mL/kg + Activity Bonus Activity Bonus: Sedentary +0 mL/day Lightly active +350 mL/day Moderately active +700 mL/day Very active +1,050 mL/day Extra active +1,400 mL/day Per-hour guide: Daily total / 16 waking hours
Source: EFSA Panel on Dietetic Products, Nutrition and Allergies (2010) · IOM Dietary Reference Intakes for Water (2005) · WHO Water, Sanitation and Health Technical Notes
Why Your Water Target Matters
Dehydration and chronic underhydration have measurable effects on physical performance, cognition, and long-term organ health. Even small daily deficits compound over time.
Cognitive and Physical Performance Decline
A fluid deficit of just 1–2% of body weight — achievable in under an hour of activity without drinking — measurably impairs concentration, working memory, and aerobic performance. WHO identifies this as a major occupational and athletic risk, particularly in warm climates.
Kidney Stones and Urinary Tract Infections
Chronically low water intake reduces urine volume and raises the concentration of stone-forming minerals. EFSA notes that adequate fluid intake is the single most modifiable dietary factor for preventing recurrent kidney stones and reducing UTI frequency.
Heat-Related Illness
In physically active individuals and older adults, inadequate hydration in hot conditions can progress rapidly from heat cramps and heat exhaustion to life-threatening heat stroke. WHO identifies heat-related illness as an increasing global health risk under climate change, with dehydration as the primary preventable driver.
How to Use This Calculator Most Effectively
This estimate covers typical conditions. Adjust your intake based on the practical signals below:
Use urine colour as your real-time guide
Pale straw yellow urine indicates good hydration. Dark yellow or amber means you need more fluid. Colourless urine after large volumes may indicate overdrinking. This visual check is more responsive than any fixed daily target.
Increase intake in heat, illness, and pregnancy
Hot or humid environments, fever, diarrhoea, and pregnancy significantly raise fluid needs beyond this estimate. EFSA recommends an additional 300 mL/day during pregnancy and approximately 700 mL/day during exclusive breastfeeding.
Count all fluids, not just plain water
Tea, coffee, milk, soups, and water-rich foods all contribute to your daily total. Caffeine at moderate intakes (up to 400 mg/day) does not cause net dehydration in habituated adults, per EFSA. Only high-alcohol beverages reliably increase net fluid loss.