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Sweat Rate Calculator

Q: Why is fluid intake added and urine subtracted?

Weight loss understates sweating when you drank (the drink replaced some sweat) and overstates it when you urinated, so both are corrected to isolate sweat.

Q: Should I drink the full sweat rate during exercise?

Not necessarily: ACSM suggests limiting the loss to under 2 % of body mass, which for many people means drinking less than the full sweat rate; drinking more than you sweat is harmful.

Q: Why 150 % for recovery?

Urine production continues after exercise, so restoring fluid balance within a few hours needs about 1.5 L for every kilogram lost, ideally with sodium.

Calculates sweat loss and sweat rate from body weight before and after exercise, corrected for fluid drunk and urine passed, plus the percentage of body mass lost, a dehydration classification and the volume to drink afterwards (150 % of the deficit), following the ACSM position stand on exercise and fluid replacement.

When to use

An athlete weighed themselves before and after a session and wants their sweat rate, how dehydrated they became, or how much to drink during and after similar sessions.

Do not use when: You want general daily fluid needs (use water-intake), the weights were taken with wet clothing or after eating, or for clinical dehydration assessment; results are informational, not medical advice.

Formula

sweat_loss_liters = (pre_exercise_weight_kg − post_exercise_weight_kg) + fluid_intake_ml / 1000 − urine_output_ml / 1000; sweat_rate_l_per_hour = sweat_loss_liters / (duration_minutes / 60); body_mass_loss_percent = (pre − post) / pre × 100; fluid_to_replace_ml = 1.5 × (pre − post) × 1000

Body-mass change is the standard field measure of sweat loss (ACSM 2007); it ignores respiratory water loss and metabolic mass loss, which are small for sessions under about 3 hours. Drinking should not exceed the sweat rate, because over-drinking causes exercise-associated hyponatraemia. Informational, not medical advice.

Inputs

Parameter	Type	Unit	Required	Description
`pre_exercise_weight_kg`	number	kg	yes	Nude (or minimal dry clothing) body weight before exercise, after emptying the bladder. Range: > 0, ≤ 300
`post_exercise_weight_kg`	number	kg	yes	Body weight after exercise, towelled dry, same clothing and scale. Range: > 0, ≤ 300
`fluid_intake_ml`	number	mL	default 0	Volume of fluid consumed during the session. Range: ≥ 0, ≤ 10000
`urine_output_ml`	number	mL	default 0	Urine volume passed between the two weighings (0 if none). Range: ≥ 0, ≤ 5000
`duration_minutes`	number	min	yes	Exercise duration between the two weighings. Range: > 0, ≤ 1440

Outputs

Output	Type	Unit	Description
`sweat_loss_liters`	number	L	(pre − post) + fluid drunk − urine, in litres (1 kg of body mass ≈ 1 L).
`sweat_rate_l_per_hour`	number	L/h	Sweat loss divided by the duration in hours.
`sweat_rate_ml_per_hour`	number	mL/h	The same rate in millilitres per hour.
`body_mass_loss_percent`	number	%	(pre − post) / pre × 100; negative when weight was gained.
`dehydration_status`	string		< 2 % minimal; 2–3 % performance impaired; > 3 % significant (ACSM thresholds).
`fluid_to_replace_ml`	number	mL	150 % of the net body-mass deficit, drunk over the hours after exercise (ACSM).
`recommended_intake_ml_per_hour_next_session`	number	mL/h	Hourly intake that would match the sweat rate in the same conditions (an upper bound: ACSM advises keeping the loss below 2 % of body mass without drinking more than you sweat).

Example

70 → 69.2 kg, 500 mL drunk, 60 min: {"pre_exercise_weight_kg":70,"post_exercise_weight_kg":69.2,"fluid_intake_ml":500,"duration_minutes":60} → {"sweat_loss_liters":1.3,"sweat_rate_l_per_hour":1.3,"sweat_rate_ml_per_hour":1300,"body_mass_loss_percent":1.14,"dehydration_status":"Minimal (< 2 % body mass loss)","fluid_to_replace_ml":1200,"recommended_intake_ml_per_hour_next_session":1300}

62 → 60.5 kg, 750 mL drunk, 100 mL urine, 90 min: {"pre_exercise_weight_kg":62,"post_exercise_weight_kg":60.5,"fluid_intake_ml":750,"urine_output_ml":100,"duration_minutes":90} → {"sweat_loss_liters":2.15,"sweat_rate_l_per_hour":1.43,"body_mass_loss_percent":2.42,"dehydration_status":"Performance impaired (2–3 % body mass loss)","fluid_to_replace_ml":2250}

GET https://tttkmbb.com/api/v1/calculate/sweat-rate?pre_exercise_weight_kg=70&post_exercise_weight_kg=69.2&fluid_intake_ml=500&duration_minutes=60

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/sweat-rate (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/sweat-rate · Markdown: https://tttkmbb.com/fitness/sweat-rate.md · JSON definition: https://tttkmbb.com/fitness/sweat-rate.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="sweat-rate"
OpenAPI operationId: calculate_sweat_rate
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Sawka MN et al. (2007) American College of Sports Medicine position stand: Exercise and fluid replacement, Med Sci Sports Exerc 39:377-390 (peer_reviewed)

FAQ

Why is fluid intake added and urine subtracted?

Weight loss understates sweating when you drank (the drink replaced some sweat) and overstates it when you urinated, so both are corrected to isolate sweat.

Should I drink the full sweat rate during exercise?

Not necessarily: ACSM suggests limiting the loss to under 2 % of body mass, which for many people means drinking less than the full sweat rate; drinking more than you sweat is harmful.

Why 150 % for recovery?

Urine production continues after exercise, so restoring fluid balance within a few hours needs about 1.5 L for every kilogram lost, ideally with sodium.

Related calculators

Water Intake Calculator — Daily baseline fluid needs outside training.
Calories Burned Calculator — Energy cost of the same session.