Sweat Rate Calculator

Sweat carries sodium (around 1 gram per liter on average). When you replace heavy fluid losses with plain water across a session over 2 hours, blood sodium drops. That causes exercise-associated hyponatremia, which feels like nausea and confusion and can be dangerous. Adding 300-700 mg sodium per liter prevents the dilution.

For a single sweat-rate baseline, weigh before and after one representative session. To dial in long-term hydration, repeat the measurement across different conditions (hot vs cool, indoor vs outdoor, easy vs intense) and track the spread. Sweat rate changes with heat acclimation, fitness level, and humidity.

Under 0.5 L/hr is low, 0.5-1.0 L/hr is moderate, 1.0-1.5 L/hr is high, and above 1.5 L/hr is very high. Endurance athletes in hot conditions routinely hit 1.5-2.5 L/hr. Heavily acclimated marathoners can exceed 3 L/hr in race conditions.