The fastest person in the world can run between 23 and 24 miles per hour for 100 meters. Usain Bolt from Jamaica set the 100meter sprint world record in 9.58 seconds at the 2009 Athletic Championships in Berlin. Germany. To calculate how fast you can run on average in miles per hour, you need to divide the distance you cover (in miles) by the time required to run that distance (in hours).
Things You'll Need
 Stopwatch
 Basic calculator

Measure the course that you have run or will run. Many runners wear a watch that measures the distance. There are 1,760 yards in a mile, and there are 3,600 seconds in an hour.

Say, for example, you run 50 yards in 6.9 seconds. Divide 6.9 seconds by 3,600 seconds (6.9/3600) and the result is .0019your time in hours. Divide 50 yards by 1,760 yards (50/1760) and the result is .0284your distance in miles.

Plug the distance and time into the following formula: Distance/Time=Miles Per Hour. The distance in miles is .0284 and the time in hours is .0019 so .0284/.0019=14.947 miles per hour. Thus, you are running at an average speed of 15 miles per hour at a 50yard distance.
References
 Photo Credit running image by Byron Moore from Fotolia.com global distances sign post image by Sirena Designs from Fotolia.com hour glass image by Warren Millar from Fotolia.com speed limits image by TA Craft Photography from Fotolia.com