Sweat Rate Calculator
Sweat rate (L/hr) = (pre-weight − post-weight + fluid intake) / duration.
Result
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How to use this calculator
- Weigh yourself nude before exercise.
- Log every mL of fluid consumed during.
- Weigh again after exercise (towel off, nude).
- Calculate; aim to replace 60-80% during, finish hydrating after.
About this calculator
Sweat rate measures fluid loss per hour: the foundation of hydration strategy. Typical recreational athletes lose 500-800 mL/hr; competitive athletes 1.0-1.5 L/hr; elite endurance athletes in heat 2.0-2.5 L/hr. Replace 60-80% during exercise (more isn't absorbed). Losing >2% of body weight as sweat impairs performance. To measure: weigh nude before and after a 60-90 min effort, log everything drunk and any urine.
Frequently asked
Why nude weight?+
Sweat-soaked clothes hold ~200-500g of water — would under-count sweat loss. Towel off, weigh nude.
How does heat affect this?+
Sweat rate doubles or triples in 90°F+ vs. 60°F. Re-test for hot conditions; don't assume your fall sweat rate applies in summer.
Should I weigh sodium too?+
If you're a heavy sweater (>1.5 L/hr) or doing 3+ hour events, yes — get a sweat sodium test. Most are 600-800 mg/L; salty sweaters 1200+.
Why "replace 60-80%"?+
Above ~1 L/hr of intake, gut absorption can't keep up — extra fluid sloshes and causes nausea. Match output but don't exceed gut capacity.
How does altitude affect sweat?+
Increases by 10-25% at 2,000m+ due to higher respiratory loss and altered thermoregulation. Plan accordingly.
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