Water Intake Calculator
Calculate your personalised daily water intake based on weight, activity level, and climate. Evidence-based hydration guidelines from EFSA and Institute of Medicine. Free.
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Disclaimer: Results are for informational purposes only and do not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional for personal guidance.
Beyond the "8 Glasses" Rule
The popular "8 glasses a day" guideline has no scientific basis in peer-reviewed literature (Valtin, American Journal of Physiology, 2002). Actual needs depend on body size, activity, climate, diet water content, and individual kidney function. The EFSA and Institute of Medicine recommendations provide population averages, but individuals vary enormously — a 100 kg male athlete training twice daily in summer needs far more than a 55 kg sedentary woman in a cool climate.
Signs of Adequate Hydration
Pale yellow urine (straw colour) indicates good hydration. Dark yellow or amber urine suggests dehydration. Clear/colourless urine may indicate overhydration (rare but possible). Thirst is generally a reliable guide in healthy adults — drinking when thirsty and stopping when no longer thirsty is sufficient for most sedentary people in temperate climates. Athletes and outdoor workers in heat need more deliberate hydration strategies.
FAQs
How much water per day?
EFSA recommends 2.0 L/day (women) and 2.5 L/day (men) total, including food water. From beverages: ~1.6 L and 2.0 L respectively. Varies widely with activity and climate.
Does exercise increase water needs?
Sweat rate 0.5–2.0 L/hr. ACSM: 400–600 ml 2–3 hrs before exercise. Replace losses during and after. Sessions over 60–90 min may need electrolytes.