Estimate sweat loss during exercise.
Sweat loss cannot be measured directly, but it can be reconstructed: the weight you lost during a session is fluid, and any fluid you drank replaced some of what you sweated out, so sweat equals weight lost plus fluid consumed. One kilogram of body mass corresponds to one litre of fluid. Rehydration targets 150 percent of the deficit because some of what you drink is passed as urine rather than retained.
Sweat rate from mass change
Sweat L = (pre kg - post kg) + fluid ml / 1000; rate = sweat L / hours; rehydration ml = mass lost kg x 1500
These figures are population-average estimates for general planning only and are not medical or dietetic advice. Individual energy and nutrient needs vary widely with body composition, activity, medication and health status. Consult a qualified health professional or registered dietitian before making significant changes to what you eat, and especially if you are pregnant or breastfeeding, are under 18, or have a medical condition or a history of disordered eating.
Under 2 percent is generally tolerated. Above 2 percent, endurance performance and thermoregulation start to decline measurably, and above about 4-5 percent the risk of heat illness rises sharply.
Because drinking dilutes plasma sodium, which triggers urine production. Replacing only 100 percent typically leaves you still slightly dehydrated a few hours later. Including sodium with the fluid improves retention considerably.
Ideally, and towel dry first. Sweat retained in clothing counts as weight you still carry, so weighing in soaked kit understates the loss — often by several hundred grams after a hard session.