Sweat Rate Calculator

by Barry Stokes, USA Triathlon Level II Triathlon Coach

Hydration is extremely important. The body is approximately 60% water. An athlete who is not fully hydrated is forcing his or her body to work at less than full capacity, especially during physical activity. Dehydration can occur at any time, but it is especially important and beneficial when exercising. Studies have shown that just a small amount of dehydration can have negative effects on athletic performance and may predispose them to heat illnesses. Dehydration occurs when fluid loss (via sweat, urine and through respiration) is greater than fluid intake (via drinking and food).

Sweat rate is KEY to understanding your body and how much fluid you require. Once you know your fluid loss, over DIFFERENT temperatures, then you know how much you need to drink per hour to stay hydrated. Different temperature on race day, and different humidity levels will affect your numbers so you need to baseline test multiple times and record the number so you have an average. We did this on the 'run', since it would be the easiest way to make the calculations and we basically run three times a week, so you'll get three baseline readings quickly. Try to do it on one hour or two hour runs or rides.

It's a basic formula that's very simple, and you can just create an Excel spreadsheet and plug in the numbers to get an average. You want to test yourself over hot, humid, cool, cloudy and sunny conditions so you begin to see a pattern or average.

The goal is to see exactly how much dehydration you incur during your workout and in turn, determine your hourly fluid replacement (how much you need to drink). When you are done with this homework, you will have a hydration target that you can use in both training and racing.

Here's how to calculate your sweat rate:

Sweat Rate Assessment

  1. Empty your bladder and step on the scale. (Do this nude or in a swimsuit for accuracy.)

  2. Record your starting weight.

    Pre-exercise weight lbs A

  3. Record the starting environmental conditions.

    Temperature
    Humidity
    Dew Point

  4. Do your usual workout. Hydrate like you normally would and track your fluid consumption.

    Important: Do not use the bathroom during the workout or prior to your post-workout weigh-in. The results will not be accurate.

  5. Record the approximate volume of fluid consumed and total workout duration.

    Fluid consumed fluid ounces E
    Workout duration hours F

  6. Record the environmental conditions at the end of the workout.

    Temperature
    Humidity
    Dew Point

  7. Towel dry and record your post-exercise weight.

    Post-exercise weight lbs B
    Note: If you have long and/or thick hair, dry your hair before weighing for the most accurate results.

  8. Determine the amount of weight lost during exercise.

    Weight Lost = A - B = C

  9. Determine how many fluid ounces of sweat you lost.

    C x 16 = D

  10. Determine hourly fluid replacement needs.

    ( D + E ) ÷ F = G
    G = Fluid ounces needed per hour to replace sweat losses under similar conditions.

Conditions that will increase sweat rate include heat, humidity, and elevated heart rate (high intensity training). Athletes should measure sweat rate across several workouts to determine their fluid replacement needs in various environmental conditions.

On race day, based on the forecast, the athlete than can go back to their log and know exactly what fluid they need to plan on ingesting to prevent performance declines associated with dehydration.

Deaths have occurred when the air temperature was less than 75 degrees F (24 degrees C) but the relative humidity was above 95%. Humidity levels over 75% will contribute to an increased risk of heat injury.

*Note: A factor of 1.2-1.6 can be multiplied to hourly replacement needs when heat & humidity are extreme (>75%).

Key points to remember:

  1. Sweat rates generally increase after 10 to 14 days of heat exposure. For the most accurate results, calculate your sweat rate after becoming heat acclimatized.
  2. Higher sweat rates are commonly observed in men and individuals with a high level of fitness.
  3. When beginning an exercise program in hot conditions, your body may lose more sodium through perspiration. Consider a modest increase in dietary sodium intake until you become heat adapted, which typically occurs after 10 to 14 days.

Here is a downloadable sweat rate worksheet courtesy of Infinit Nutrition.

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