Sweat rate is the volume of fluid lost through sweating per hour of exercise, measured in litres/hour. Average endurance athletes sweat 0.5–2.5 L/hr; heavy sweaters can exceed 3 L/hr in heat.

Which formula is used?

The ACSM (American College of Sports Medicine) standard formula: Sweat Rate (L/hr) = (Pre-weight − Post-weight + Fluid consumed − Urine produced) / Duration in hours. Each 1 kg body weight loss equals approximately 1 litre of sweat.

How do I weigh myself accurately?

Weigh yourself nude immediately before exercise, then nude again immediately after — before eating or drinking anything post-exercise. Use the same scale both times.

Do I need to track urine output?

For most workouts under 90 minutes, urine output is negligible and you can leave it at 0. For ultra-endurance events, tracking urine output improves accuracy.

Why does sodium loss matter?

Sweat contains sodium (average ~920 mg/L). High sweat rates deplete sodium faster, risking hyponatremia in long events. If you lose more than 800 mg/hr, consider electrolyte drinks or salt tabs.

Is this tool free and private?

Yes. AI Fit Hub tools are free, browser-based, and no-signup. Your inputs stay in your browser.

Methodology: Sweat Rate Calculator

Scope

Measures personal sweat rate from pre/post-exercise weigh-ins and estimates fluid and sodium losses.

Formula

sweat_rate_l_per_h = (pre_kg - post_kg + fluid_l - urine_l) / hours. sodium_loss_mg = sweat_l * sodium_concentration.

Coefficients

Parameter	Value	Note
Typical sodium in sweat	`500–1500 mg/L`
High-sweat athletes	`>2 L/h`

Data sources

Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390. — PMID 17277604. Source of fluid-replacement and sweat-rate testing protocol.
Baker LB. Physiology of sweat gland function: the roles of sweating and sweat composition in human health. Temperature (Austin). 2019;6(3):211-259. — PMID 31608304. Reference for the 500-1500 mg/L sweat-sodium range.
McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers' Association position statement: fluid replacement for the physically active. J Athl Train. 2017;52(9):877-895. — PMID 28985128. NATA guidance on individualized hydration.

Assumptions

Weigh-ins are nude and dry; fluid intake during session is logged accurately.

Approximation range

Sweat rate is highly individual — repeat the test 2–3 times to establish your personal baseline.

Limitations

Sodium concentration without a sweat-test kit is a population estimate; true value varies 3x between athletes.
Does not model acclimatization, which can change sweat rate 20% within 10 days.

Reproducibility

Pre 80 kg, post 78.5, fluid 0.5 L, no urine, 1 h ride: sweat = (80-78.5+0.5)/1 = 2.0 L/h.

Change log

2026-04-24: methodology page first published.

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Worked example

Computed by the same engine bundle served at /engines/sweat-rate-calculator.js. Re-runnable: the values below are the literal output of compute(engineInput).

Input

tool: sweat_rate_calculator
pre_weight_kg: 70
post_weight_kg: 69.2
duration_minutes: 60
fluid_consumed_ml: 500
urine_produced_ml: 0

Output

sweatRateLPerHr: 1.3
totalFluidLostL: 1.3
weightLostKg: 0.8
weightLostPercent: 1.1
hydrationStatus: mild_dehydration
hydrationLabel: Mild Dehydration
fluidReplacePerHourMl: 1050
sodiumLossMgPerHr: 1196
recommendation: You lost 1.1% body weight. Mild dehydration can slightly reduce performance. Increase fluid intake by ~1050 mL/hr next session.

FAQ

What is sweat rate?: Sweat rate is the volume of fluid lost through sweating per hour of exercise, measured in litres/hour. Average endurance athletes sweat 0.5–2.5 L/hr; heavy sweaters can exceed 3 L/hr in heat.
Which formula is used?: The ACSM (American College of Sports Medicine) standard formula: Sweat Rate (L/hr) = (Pre-weight − Post-weight + Fluid consumed − Urine produced) / Duration in hours. Each 1 kg body weight loss equals approximately 1 litre of sweat.
How do I weigh myself accurately?: Weigh yourself nude immediately before exercise, then nude again immediately after — before eating or drinking anything post-exercise. Use the same scale both times.
Do I need to track urine output?: For most workouts under 90 minutes, urine output is negligible and you can leave it at 0. For ultra-endurance events, tracking urine output improves accuracy.
Why does sodium loss matter?: Sweat contains sodium (average ~920 mg/L). High sweat rates deplete sodium faster, risking hyponatremia in long events. If you lose more than 800 mg/hr, consider electrolyte drinks or salt tabs.
Is this tool free and private?: Yes. AI Fit Hub tools are free, browser-based, and no-signup. Your inputs stay in your browser.