Sweat Rate & Hydration Calculator (Exercise)
Calculate your sweat rate from a before/after exercise weigh-in, and find your fluid replacement needs for training and racing.
Sweat Rate & Hydration Guide
Why Sweat Rate Matters
Knowing your individual sweat rate lets you hydrate precisely for exercise — neither under-drinking (dehydration, reduced performance) nor over-drinking (rare but dangerous hyponatraemia). Sweat rates vary enormously between individuals — from 0.5 to 2.5+ litres per hour. Genetics, fitness, acclimatisation, body size, and conditions all affect it. Two athletes side by side can have completely different fluid needs. The impact of dehydration: losing 2% of body weight as sweat begins to impair per
The Weigh-In Method
The most accurate way to measure sweat rate at home: weigh yourself naked (or in minimal dry clothing) immediately before exercise. Exercise for a known duration at a representative intensity. Note any fluid you drink during. Towel off and weigh again immediately after (naked/minimal dry clothing). Calculate: sweat loss (litres) = weight before - weight after + fluid consumed (in kg/litres). Sweat rate = sweat loss / hours of exercise. Example: 75kg before, 73.8kg after, 90 min, drank 500ml. Wei
Replacing Fluids
During exercise: aim to replace fluid to limit losses to under 2% body weight. For most, drinking 400-800ml/hour suffices, but heavy sweaters in heat may need 1-1.5L/hour. Don't try to fully match a very high sweat rate during exercise — the gut can only absorb ~1-1.2L/hour, and forcing more causes discomfort. Some controlled dehydration during long efforts is normal and acceptable. After exercise: replace 125-150% of fluid lost (you continue to lose fluid via urine and ongoing sweat). Lost 1.5L
Practical Hydration Strategy
Before exercise: arrive well-hydrated (pale yellow urine). Drink 400-600ml in the 2-3 hours before. Don't overhydrate immediately before (causes mid-exercise bathroom stops). During exercise: drink to your calculated rate, in regular small amounts (150-250ml every 15-20 min) rather than large gulps. Use sports drinks/electrolytes for sessions over 60-90 min or in heat. After exercise: rehydrate with 125-150% of losses, including sodium. Monitor urine colour (pale yellow = well hydrated). Conditi
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