Rehydration Calculator

The rehydration calculator compares pre- and post-exercise body weights, treating only a drop in weight as a fluid deficit. Each pound lost becomes 16 fluid ounces. Fluid consumed during the session is added to that weight-derived amount to estimate total sweat loss, because replacing fluid while exercising can hide part of the loss on the scale. Exercise duration then converts total sweat loss into an hourly sweat rate. Post-exercise replacement follows a separate path. The engine multiplies the weight-derived deficit by 150 percent, without adding fluid already consumed, to produce the rehydration target -- a ratio drawn from the American College of Sports Medicine's position stand on exercise and fluid replacement (Sawka et al., 2007), which calls for rehydrating beyond a 1:1 replacement since some of what's drunk afterward is processed and excreted rather than retained. It estimates drinking time by dividing that target by 24 ounces per hour and enforcing a minimum of one hour.

Dehydration percentage is weight lost divided by pre-exercise weight. If post-exercise weight equals or exceeds pre-exercise weight, weight loss is floored at zero; rehydration target and dehydration percentage become zero, although fluid consumed can still appear in total sweat loss and sweat rate. Accurate scale measurements therefore matter more than duration for the deficit outputs. The model does not account for urine produced during exercise, food weight, wet clothing, electrolyte losses, or fluid consumed after the weigh-in. All quantities are in pounds and fluid ounces, not kilograms and liters. Use repeated comparable sessions to estimate a practical personal sweat rate rather than overinterpreting one noisy measurement.