Hydration Rate Calculator Formula

Understand the math behind the hydration rate calculator. Each variable explained with a worked example.

Use the Hydration Rate Calculator

Formulas Used

Sweat Rate

sweat_rate_oz_hr = round(sweat_loss_oz / (exercise_minutes / 60) * 10) / 10

Sweat Rate (mL/hr)

sweat_rate_ml_hr = round(sweat_loss_oz * 29.574 / (exercise_minutes / 60))

Dehydration Level

dehydration_pct = round(weight_loss_lbs / pre_weight_lbs * 10000) / 100

Target Fluid Intake

fluid_target_oz_hr = round(sweat_loss_oz / (exercise_minutes / 60) * 0.8 * 10) / 10

Variables

VariableDescriptionDefault
pre_weight_lbsPre-Exercise Weight(lbs)160
post_weight_lbsPost-Exercise Weight(lbs)158
fluid_consumed_ozFluid Consumed During Exercise(oz)16
exercise_minutesExercise Duration(min)60
weight_loss_lbsDerived value= pre_weight_lbs - post_weight_lbscalculated
sweat_loss_ozDerived value= weight_loss_lbs * 16 + fluid_consumed_ozcalculated

How It Works

How Sweat Rate Is Calculated

Formula

Sweat Loss (oz) = Weight Lost (lbs) x 16 + Fluid Consumed (oz)

Sweat Rate = Sweat Loss / Exercise Duration (hours)

Each pound of weight lost during exercise represents about 16 oz (473 mL) of sweat.

Hydration Guidelines

  • Replace 80% of sweat losses during exercise.
  • Dehydration above 2% of body weight impairs performance.
  • Dehydration above 4% can be dangerous.

    • Factors Affecting Sweat Rate

  • Temperature and humidity
  • Exercise intensity
  • Body size
  • Fitness level (fitter athletes sweat more efficiently)
  • Genetics

    • Worked Example

    Pre-exercise: 160 lbs. Post-exercise: 158 lbs. Drank 16 oz during 60 minutes.

    pre_weight_lbs = 160post_weight_lbs = 158fluid_consumed_oz = 16exercise_minutes = 60
    1. 01Weight lost = 160 - 158 = 2 lbs.
    2. 02Sweat loss = 2 x 16 + 16 = 48 oz.
    3. 03Sweat rate = 48 / 1 hr = 48 oz/hr (1420 mL/hr).
    4. 04Dehydration = 2 / 160 x 100 = 1.25%.
    5. 05Target fluid intake = 48 x 0.8 = 38.4 oz/hr.

    Frequently Asked Questions

    How do I perform a sweat test?

    Weigh yourself nude before exercise. Track all fluid consumed during the session. Weigh yourself nude again after, toweling off sweat first. Do not urinate between weigh-ins. The weight difference plus fluid consumed equals sweat loss.

    Is it possible to drink too much?

    Yes. Overhydration (hyponatremia) is dangerous. It dilutes blood sodium levels. Do not drink more than you sweat. If your post-exercise weight is higher than pre-exercise weight, you drank too much.

    Does sweat rate change with training?

    Yes. As you become fitter, your body sweats earlier and more efficiently to cool itself. Well-trained athletes typically have higher sweat rates. Heat acclimatization (training in heat for 10-14 days) also increases sweat rate.

    Ready to run the numbers?

    Open Hydration Rate Calculator