Hydration Calculator

Yes. Despite containing caffeine, which has a mild diuretic effect, the net fluid contribution of coffee and tea is positive. The European Food Safety Authority confirms that caffeinated beverages contribute to daily hydration. Only extremely high caffeine intake (above roughly 400 mg/day) produces a notable diuretic effect that would offset the fluid content.

The 8 × 8 rule (eight 8-ounce glasses = 64 oz = about 1.9 L) is a rough approximation for a sedentary person of average weight. The actual IOM recommendation is 3.7 L/day total water (from all sources including food) for men and 2.7 L/day for women. Needs vary significantly with body weight, activity, climate, and diet, which is why a personalized formula produces more relevant targets.

Yes. Hyponatremia (dangerously low blood sodium) can occur when large volumes of plain water are consumed rapidly without electrolytes - most often seen during endurance events when athletes drink far beyond thirst. Symptoms include nausea, headache, confusion, and in severe cases, seizures. Drinking to thirst during exercise and using electrolytes for efforts exceeding 90 minutes significantly reduces this risk.

Sweat rate varies widely: a typical adult loses 0.5-2.0 liters per hour during moderate to vigorous exercise depending on body size, fitness, temperature, and humidity. A practical guideline is to drink 400-600 mL about 2 hours before exercise, then 150-250 mL every 15-20 minutes during activity, and replace approximately 150% of estimated fluid lost (weigh yourself before and after) in the recovery window.