From the BarryS Coaching Training Library


Sweat Rate Calculator

by Barry Stokes

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:

  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 temperature and humidity:
    (for future records and comparison)
    Temperature: ___________ ?
    Humidity: ___________ %
    Dew Point: ___________ ?
  4. Do your usual workout. Hydrate like you normally would, and track your consumption.
    Important Note: Hold it! If you go to the bathroom during your workout or prior to your post-workout weigh in, the results will not be accurate.
  5. Record the approximate volume of fluid consumed during your workout, as well as the total duration.
    How much you drank = ___________ fluid ounces (E)
    Duration of workout: ___________ hours (F)
  6. Record the temperature and humidity at the end of the workout. (for your records)
    Temperature: ___________ ?
    Humidity: ___________ %
    Dew Point: ___________ ?
  7. Towel dry and then record your weight. (nude or swimsuit)
    Post-exercise weight = ___________ lbs.(B)
    Note: If you have long and/or thick hair, blow dry your hair before weigh-in for the most accurate results.
  8. Determine the amount of weight you lost during exercise by subtracting your post-exercise weight from your pre-exercise weight.
    Weight Lost = _____lbs.(A) - ______lbs(B) = ________lbs.(C)
  9. Determine how many fluid ounces of sweat you lost by multiplying the number of pounds you lost by 16.
    _____lbs lost (C) x 16 = ________ fluid ounces of water you lost (D)
  10. To determine hourly fluid replacement needs, add number of fluid ounces you lost (D) to the number of fluid ounces you consumed during the workout (E) and divide by the total duration of the workout (F).
    (_____fluid ounces lost (D) + ___fluid ounces consumed (E)) ___hours (F)
    =_____fluid ounces needed to replace what is lost via sweat each hour

Here is a downloadable worksheet courtesy of Infinit Nutrition:

sweat rate worksheet

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

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