Hydration Calculator
Estimate your fluid and sodium needs based on body weight, duration, and race-day conditions.
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How it works
This calculator estimates your sweat rate based on body weight, exercise intensity, temperature, and humidity. It recommends replacing about 80% of sweat losses, the ACSM guideline for maintaining performance without over-hydrating.
Sodium recommendations are based on 500mg per liter of fluid consumed, the mid-range of ACSM guidelines. If you're a heavy sweater or see salt stains on your gear, consider going toward the higher end (600-700mg/L).
For runs over 2 hours, consider using a drink mix to get both hydration and carbs in one step. This reduces the number of gels you need and makes your fueling plan simpler.