Site: http://www.runnersweb.com/running/bmr.html

Summary:

This formula is the Harris-Benedict equation for calculating basal metabolic rate (BMR). Enter your height and weight (in either metric - centimetres/kilograms or english - inches/pounds), your age in years, miles run per day (decimals allowed) and activity factor if applicable. The activity factor compensates for additional physical activity during the day beyond running, such as walking, gardening, mowing the lawn, vacuuming, etc. ADD 100 calories for each 15 minutes of the afore-mentioned activity.