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 you bladder and record your pre-exercise weight (nude or swim suit)
    ________lbs. (A) Pre-exercise weight
  2. Do your usual workout, and drink like you normally would.
  3. Record the approximate volume of fluid consumed during exercise.
    ________fluid ounces (E)
  4. Post-workout: Towel dry, empty your bladder and then record your weight (nude or swim suit):
    ________ lbs.(B) Post-exercise weight
  5. Subtract your post-exercise weight from your pre-exercise weight to get the number of pounds you lost during exercise:
    ________lbs.(A) –________lbs(B) = ________lbs.(C) Pounds lost during exercise.
  6. To find out how many fluid ounces of water you have lost, multiply (C) x 16:
    ________lbs(C) x 16 = ________ (D) fluid ounces of water lost during exercise
  7. To determine hourly fluid replacement needs, add number of fluid ounces you lost during exercise (D) to the number of fluid ounces you consumed during exercise (E) and divide by total number of hours spent training.
    (_______ (D) +_______ (E)) ÷_______hours =_______fluid ounces needed each hour*

Here is a downloadable Excel Calculator:

sweat rate calculator

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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