Calculate your hourly sweat rate the way exercise physiologists do — pre and post weigh-in, accounting for fluids consumed. Get evidence-based fluid replacement guidance from the National Athletic Trainers’ Association.
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Sweat rate
1900 mL/ hour
Total sweat lost during this session: 1900 mL
To replace 150 % of fluid lost (Casa et al., 2000): drink ~713 mL per hour over the next 4 hours
Add electrolytes (sodium 300–700 mg/L) if exercising over 60 min in heat.
Sweat-rate testing is the gold standard published by the National Athletic Trainers’ Association (NATA), American College of Sports Medicine (ACSM) and Casa et al. (J Athl Train, 2000). The formula assumes 1 mL of sweat ≈ 1 g of body mass. For accuracy, towel off and void completely before pre-weight, weigh nude or in dry clothing only, and re-weigh in the same clothing after towelling off. Rates over 1500 mL/h indicate a salty/heavy sweater — pre-loading with 0.5 L of sodium-containing fluid 2 h before exercise reduces dehydration risk.
Why personal sweat rate matters
Generic hydration advice (“drink 8 glasses a day”) breaks down for endurance athletes, especially in heat. Two cyclists doing the same ride in the same conditions can have sweat rates that differ by 2×. Some need 1.2 L/h; others need 0.5 L/h. Drinking to a generic prescription instead of your own rate either dehydrates you (under-drinking) or causes hyponatremia (over-drinking).
How to measure correctly
Towel off completely. Weigh nude (or in dry clothing of known weight) on a scale that reads to 0.1 kg. Void your bladder completely. Exercise. After exercise, towel off, return to the same scale, weigh in the same outfit. Track every mL of fluid consumed during the session and any urine voided.
Replace gradually
For sessions ending under 4 hours from your next workout, replace 150 % of fluid lost over the following 4 hours (this accounts for ongoing renal loss as your body re-equilibrates). For sessions ending more than 12 hours from your next workout, replace 100 % over the rest of the day at a normal pace.
This tool is for educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a licensed Canadian healthcare professional. Read our full disclaimer.