Sports Hydration Calculator
Estimate sweat rate from pre/post body mass, fluid intake, urine output and exercise duration, with visible body-mass change.
Estimate exercise sweat rate
Use a before-and-after weigh-in plus measured fluid intake to estimate sweat loss during a training session.
Sports hydration and sweat-rate formula
A practical field estimate is: sweat loss ≈ pre-exercise body mass − post-exercise body mass + fluid consumed − urine produced. When body mass is measured in kilograms, a 1 kg change is treated approximately as 1 litre of water. Divide sweat loss by exercise hours to estimate litres per hour.
Why body mass is measured before and after exercise
The change helps account for fluid that left the body during the session. Adding recorded fluid intake prevents drinking during exercise from hiding part of the loss.
Do not turn one test into a universal drinking prescription
The American College of Sports Medicine maintains evidence-based guidance on exercise and fluid replacement, but practical hydration should be individualized. Sweat rate can change substantially across weather, pace, clothing and acclimatization. ACSM position stands.
Important safety note
This calculator estimates sweat rate; it does not diagnose dehydration or prescribe a medical fluid or electrolyte plan. Excessive fluid intake can also be unsafe. People with medical conditions or unusual symptoms should follow professional guidance.
Turn sweat-rate data into a practical observation
Repeat the sweat-rate measurement in conditions similar to the sport because heat, humidity, clothing, acclimatization and intensity can change losses substantially. Body-mass change should be measured on the same scale and with similar clothing, while drinks consumed and urine produced during the session should be recorded. The result helps describe observed fluid loss; it is not a universal prescription to replace every millilitre during exercise.