Calculate your personalized fluid needs with our free Water Intake Calculator. Find optimal hydration goals based on body weight and exercise minutes using IOM and WHO formulas with 100% offline privacy.
Select Imperial (lbs / fl oz) for US measurements or Metric (kg / liters) for international units.
Set your body weight and average daily exercise minutes using the sliders to personalize your target.
View your daily fl oz or liters goal and tap each glass icon as you drink to track progress in real time.
Your body weight, exercise habits, and hydration goals are processed entirely inside your browser using client-side JavaScript. Nothing ever travels over a network to any server — making BunchTool 100% private.
Hydration targets recalculate in real time as you move the weight and exercise sliders — no page reloads or server delays whatsoever.
Uses IOM (Institute of Medicine) weight-based formulas for Imperial mode and WHO/EFSA guidelines for Metric mode, plus a clinically-supported exercise sweat-loss bonus.
Why Body Weight Determines Your Water Needs: The human body is approximately 60% water by mass. The kidneys must excrete a minimum of 500 ml (17 fl oz) of urine daily to eliminate metabolic waste. Larger body mass generates more metabolic heat and waste, requiring proportionally more water intake to maintain fluid homeostasis.
The mathematical hydration formulas are:
\(\text{Imperial Target (fl oz)} = \frac{\text{Weight (lbs)}}{2} + (\text{Exercise Mins} \times 0.4)\)
\(\text{Metric Target (ml)} = (\text{Weight (kg)} \times 32.5) + (\text{Exercise Mins} \times 11.833)\)
Exercise Sweat Compensation: Physical activity causes sweat loss of approximately 0.5–1.5 liters per hour depending on intensity and climate. For every 30 minutes of moderate exercise, an additional 12 fl oz (355 ml) of water is needed to replenish electrolyte-carrying fluids and prevent dehydration-related performance decline.