Formula, in endurance
Sweat Rate Formula
Sweat rate is the body mass you lose during exercise, plus what you drink, minus any urine, divided by the duration; 1 kg lost is about 1 L of sweat. ACSM advises keeping losses under 2% of body weight and drinking about 1.5 L per kg still lost afterwards. The calculator plans about 80% of your measured rate for the next session.
6 variables. Published Mar 12, 2026. Updated Oct 19, 2026. By AI Fit Hub.
sweat_rate (L/h) = (mass_pre_kg − mass_post_kg + fluid_consumed_L − urine_L) / duration_hours
body_mass_loss_pct = (mass_pre_kg − mass_post_kg) / mass_pre_kg × 100
next_session_drinking (mL/h) ≈ 0.8 × sweat_rate × 1000, rounded to 50 mL
sodium_lost (mg/h) ≈ sweat_rate × 920 mg/L (a typical concentration) Variables
sweat_rate
Sweat rate
Litres per hour. Endurance athletes averaged 1.28 ± 0.57 L/h in a 1,303-athlete sweat-testing dataset (Barnes 2019), so individual rates vary widely.
mass_pre
Weight before
Kilograms, nude or in minimal dry clothing, after emptying the bladder.
mass_post
Weight after
Kilograms, towelled dry, in the same clothing state.
fluid_consumed
Fluid drunk
Litres drunk during the session, from bottle volumes or weighed bottles.
urine
Urine passed
Litres passed during the session, if any; it is mass lost that is not sweat.
duration
Duration
Hours, as a decimal: 90 minutes = 1.5.
Step by step
- 1
Weigh in just before exercise, after using the toilet, in minimal dry clothing.
Before: 78.4 kg.
- 2
Track what you drink during the session.
600 mL = 0.6 L.
- 3
Weigh in straight after, towelled dry.
After: 77.5 kg.
- 4
Apply the formula; 1 kg of mass lost ≈ 1 L of sweat.
(78.4 − 77.5 + 0.6 − 0) / 1.0 h = 1.5 L/h.
- 5
Check the loss against ACSM's 2% guidance and plan the next session below the sweat rate.
0.9 / 78.4 = 1.1% lost; 80% of 1.5 L/h is 1,200 mL an hour.
Worked example
A 1-hour outdoor run at 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 body-mass loss = 0.9 / 78.4 = 1.1% next session ≈ 0.8 × 1,500 = 1,200 mL/h sodium lost ≈ 1.5 × 920 = 1,380 mg/h
Sweat rate 1.5 L/h and a 1.1% body-mass loss, inside ACSM's 2% guidance, which the calculator rates minimal dehydration. For the next session in similar conditions it suggests about 1,200 mL an hour, and it estimates sodium losses at about 1,380 mg an hour.
Common variations
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Sources
- Sawka et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. , Medicine & Science in Sports & Exercise — the 2% guidance and post-exercise replacement
- Baker (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. , Sports Medicine — sweat-testing methods and variability
- Barnes et al. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. , Journal of Sports Sciences — 1,303-athlete norms
- Casa et al. (2000). National Athletic Trainers' Association position statement: fluid replacement for athletes. , Journal of Athletic Training — the body-mass-change bands the calculator uses
- Hew-Butler et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. , Clinical Journal of Sport Medicine — overdrinking and hyponatremia