Based on some theory, some experience, and some modeling, here is a simple formula to scale running results and allow a fair comparison among people of diverse ages, sizes, and shapes:
age + body mass index - 50 + 10 (if you are female)
The result is a percentage adjustment. Take that much off your time in a race, and you have got your scaled result. For example, with a body mass index of 24 and an age of 51, the handicap would be 24+51-50 = 25%. So when running a 10 minute mile, subtract 25% from the finishing time, or equivalently add 25% to the speed. It is like doing a 7:30 mile when being half this age, which makes sense and feels about right. The logic, in brief:
Age hurts performance by approximately 1% per year, as is widely recognized.
BMI (body mass index) is a standard measure of obesity or lack thereof. To compute Body Mass Index take your weight in kilograms and divide by the square of your height in meters (or use one of the widely-available tables or online calculators). Since each pound of excess weight traditionally is thought to slow a runner down by approximately 2 seconds per mile, converting the units shows that a 1 point change in Body Mass Index means approximately 1% penalty at a typical 7:30 minutes/mile pace.
-50 is simply a linear shift in the zero point of the scale, a reasonable choice so that most people in their mid-twenties end up with near-zero handicap factors.
+10 for females corrects for the normal difference in body fat percentage and strength between the sexes.
One certainly could design a much more complex equation, and likewise could quibble over various of the terms in the formula - but age + body mass index -50 +10 (if female) is probably about as accurate as the uncertainties in the input factors allow, and it has the singular advantage of being trivial enough to compute, just barely, during the final stages of exhaustion during a long run.
Run The Planet thanks ZhurnalWiki (www.zhurnal.net) for the permission to reprint the article "Handicap Jogging" by Mark Zimmermann. Text © by ZhurnalWiki. Illustration © 2006 by Run The Planet.