Estimate daily water intake, exercise fluids, and heat-related hydration needs with flexible metric or imperial inputs.
This tool provides estimates for informational purposes only. It is not a substitute for medical, nutrition, or fitness advice. Individual results vary based on your health status, training background, body composition, and personal circumstances. Always consult a qualified healthcare or fitness professional before making important decisions based on the output.
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A useful starting figure: 35 ml per kg body weight. For a 70 kg adult: 2.45 L/day (about 10 US cups or 8 UK cups). The old "8 × 8 oz glasses" rule (1.9 L) is a reasonable minimum but undershoots for larger, active, or hotter-climate people. The European Food Safety Authority recommends 2.5 L/day for men, 2.0 L/day for women — from all sources including food.
Exercise: add 500–700 ml per hour of moderate activity, 1 L per hour for intense or hot conditions. Heat: add 500 ml–1 L/day in summer above 30°C. Altitude (above 2,500 m): add 500 ml–1 L/day. Pregnancy: +300 ml/day. Breastfeeding: +700 ml/day.
Pale-yellow urine (like lemonade — not clear, not dark) is the simplest indicator. Clear urine often signals over-hydration; dark amber suggests dehydration. Frequent urination (every 2–4 hours), no dry mouth, and no headaches by mid-afternoon are good signs.
Fruits and vegetables are 80–95% water. A diet rich in produce can account for 600–900 ml of daily water needs. Cucumber, lettuce, watermelon, strawberries are 92%+ water. This is why "drink 2 L of water on top of food" produces over-hydration in many people — total fluid (food + drinks) is the target.
Hyponatraemia — sodium dilution from excessive water intake — has caused deaths in endurance athletes. Drink to thirst during long workouts. The kidneys can excrete about 800–1,000 ml of water per hour, so sustained intake above that level is dangerous. Sports drinks add sodium (300–600 mg/L) which prevents hyponatraemia in long efforts.
About 35 ml per kg body weight. 70 kg adult: ~2.5 L. Adjust up for heat, exercise, pregnancy, breastfeeding.
2 L = 8.5 US cups (240 ml each) or 8 UK cups (250 ml each).
No — at typical intake (1–4 cups), coffee's diuretic effect is small and net hydration is positive. Tea and coffee count toward daily fluid.
Yes. Over-hydration dilutes sodium (hyponatraemia) and can be fatal in endurance athletes. Drink to thirst.
Small sips, yes. Large volumes 1–2 hours before bed cause nighttime waking. Front-load fluid intake during the day.
The "eight glasses a day" rule is a handy slogan but not a precise target. Fluid needs scale with body size, activity, climate and diet. A common starting estimate is about 30–35 ml of water per kilogram of body weight per day, which the calculator then adjusts upward for exercise and hot conditions.
Roughly 20% of your daily fluid also comes from food — fruits, vegetables and soups are especially water-rich — so you do not need to drink your entire requirement. Caffeinated drinks count toward intake too; their mild diuretic effect is offset by the water they contain.
The simplest hydration check is urine colour: pale straw indicates good hydration, while dark yellow signals you need more fluid. Thirst, headache, fatigue and poor concentration are common early signs of mild dehydration. During prolonged exercise or heat, you also lose electrolytes — sodium and potassium — which water alone does not replace.
To stay consistent, keep a bottle within reach, drink a glass with each meal, and front-load fluids earlier in the day so you are not catching up at night. Increase intake around workouts: drinking before, during and after exercise maintains performance and helps recovery. People with kidney or heart conditions should follow their clinician's specific fluid guidance.