Formula, in endurance

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.

6 variables. Published Mar 12, 2026. Updated May 10, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

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

Sodium-loss estimation: most adults lose 500–1500 mg sodium per liter of sweat; salty sweaters (visible salt rings) trend higher.

Heat acclimation: 2 weeks of heat exposure raises sweat rate by ~10% and drops sweat-sodium concentration by ~50% — same volume but with less salt loss.

Body weight % rule: don't lose more than 2% body weight during a session. At 78 kg that's 1.56 kg / 1.5 L.

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Sources

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