Sweat Rate Formula
Sweat rate is body weight lost during exercise plus fluid consumed, divided by duration. Replacing 125–150% of weight lost per hour during the same activity is a typical guideline.
Formula
Copy the exact expression or work through it step by step below.
sweat_rate (L/h) = (mass_pre_kg − mass_post_kg + fluid_consumed_L − urine_L) / duration_hours Variables
sweat_rate
Sweat rate
Liters per hour. Typical: 0.5–1.5 L/h moderate exercise, 1.5–2.5 L/h heat or intense work, up to 3.0 L/h elite athletes in heat.
mass_pre
Pre-exercise body mass
Kilograms, measured nude or in dry minimal clothing, post-void.
mass_post
Post-exercise body mass
Kilograms. Same conditions: towel-dry, void if possible, minimal clothing.
fluid_consumed
Fluid drunk during exercise
Liters. Weigh bottles at session start and end, or measure cups.
urine
Urine produced
Liters during the session, if any. Usually 0 for sessions under 90 minutes.
duration
Session duration
Hours (decimal). 90 minutes = 1.5.
Step By Step
- 1
Weigh in just before exercise, towel-dry, minimal clothing.
Pre: 78.4 kg.
- 2
Track fluid consumed during the session.
Consumed 600 ml = 0.6 L.
- 3
Weigh in immediately after, towel-dry again.
Post: 77.5 kg.
- 4
Apply the formula. Mass lost in kg ≈ liters of fluid lost.
(78.4 − 77.5 + 0.6 − 0) / 1.0 hour = 1.5 L/hour sweat rate. Replace 1.6–1.8 L/hour during the same conditions next time.
Worked Example
1-hour outdoor run, 25°C
Pre-mass (kg)
78.4
Post-mass (kg)
77.5
Fluid in (L)
0.6
Duration (h)
1.0
sweat_rate = (78.4 − 77.5 + 0.6 − 0) / 1.0 = 1.5 L/h
Sweat rate 1.5 L/h. To replace 125% next time: target 1.9 L/hour intake. Add 500–700 mg sodium per liter for sessions over 90 minutes.
Common Variations
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Sources & References
- ACSM Position Stand: Exercise and Fluid Replacement — Medicine & Science in Sports & Exercise
- Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability — Sports Medicine