Work out hydration for hot weather instantly with clear inputs, formula shown and shareable results.
Baseline intake is about 33 ml per kilogram of body weight, and sweat loss is the hourly rate over the exposure period scaled up by 3% for every degree above 25 C. Sweat carries roughly 900 mg of sodium per litre, so replacing volume alone during long heat exposure risks dilutional hyponatraemia.
Sweat loss
litres = sweat rate x hours x (1 + 0.03 x (temperature - 25))
Total intake
total = body weight kg x 0.033 + sweat loss
litres = sweat rate x hours x (1 + 0.03 x (temperature - 25)). Baseline intake is about 33 ml per kilogram of body weight, and sweat loss is the hourly rate over the exposure period scaled up by 3% for every degree above 25 C.
Sweat carries roughly 900 mg of sodium per litre, so replacing volume alone during long heat exposure risks dilutional hyponatraemia.
Enter body weight, hours of exposure or activity, sweat rate, air temperature. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.