With this calculator, you can precisely predict and estimate how much you need to rehydrate in any given temperature and humidity.

Sweat loss calculator instructions:

Step 1: Enter your pre-run weight in box 1 (preferably without shoes and clothes on)

Step 2: Weigh yourself after your run and enter  in box 2 (again, without clothes or shoes)

Step 3: Enter how much, if any, fluid you loss by going to the bathroom on your run (estimate to the nearest ounce) in box 3

Step 4: In box 4, enter how much fluid you drank to the nearest ounce

Step 5: Enter the duration of your run in minutes

Step 6: Enter the distance of your run in miles

The calculator will do the rest for you, showing you exactly how much fluid you lost during your run.

Sweat loss calculator

Make a Spreadsheet

Finally, record the temperature and humidity levels during your run (average the starting and finishing temperatures if they changed significantly while you were running) and enter them into the first and second column of the spreadsheet and fill in the rest of the data based on the numbers the calculator provided.

Now, you have a very accurate assessment of exactly how much fluid you lose in a given heat and humidity. By filling out the chart multiple times, you can precisely predict how much fluids you’ll need to consume during workouts and races and adequately rehydrate yourself for optimum performance.