Sweat Rate & Hydration Calculator
Weigh yourself before and after a workout, tell us what you drank, and we'll work out how much fluid you lose per hour — and how much to drink to replace it.
Hourly sweat rate
Hydration strategy sorted?
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How the calculation works
The gold-standard field method for measuring sweat rate — for runners, cyclists, and triathletes alike — is a simple weigh-in protocol: weigh yourself (ideally nude and toweled dry) right before and right after a session. Every kilogram lost is roughly 1,000 ml of fluid. Add back whatever you drank during the workout, subtract any urine output, and divide by the duration — that's your hourly sweat rate.
Why it matters: losing more than about 2% of your body weight to dehydration measurably hurts endurance performance, and thirst lags well behind actual losses. Your sweat rate changes with heat, humidity, and intensity, so retest in different conditions — a summer race plan needs a different bottle strategy than a winter trainer session.