Sweat Rate Calculator

About this calculator

Sweat rate is calculated from the change in body weight before and after exercise, adjusted for what you drank and lost as urine: sweat rate (L/hr) = [(pre-exercise weight − post-exercise weight) + fluid intake − urine loss] ÷ exercise duration. Body weight is used as a proxy for fluid loss because one liter of sweat weighs almost exactly one kilogram.

Athletic trainers, endurance coaches, and sports dietitians use this number to build individualized hydration plans — a marathoner who loses 1.5 L/hr needs a very different fluid strategy than one who loses 0.5 L/hr, even at the same pace. It's also used in military and occupational heat-stress settings to set safe work-rest cycles in hot environments.