The BMR formula uses the variables of height, weight, age and gender to calculate the Basal Metabolic Rate (BMR). This is more accurate than calculating calorie needs based on body weight alone. The only factor it omits is lean body mass and thus the ratio of muscle-to-fat a body has. Remember, leaner bodies need more calories than less leaner ones. Therefore, this equation will be very accurate in all but the very muscular (will underestimate calorie needs) and the very fat (will over-estimate calorie needs).

Height:  feet
and  inch(es)
Weight:  pounds
Age:  years
  
  
  
Your BMR: 
   
Harris Benedict Formula
Activity Factor Daily Caloric Needs
sedentary (little or no exercise)
lightly active (light exercise/sports 1-3 days/week)
moderatetely active (moderate exercise/sports 3-5 days/week)
very active (hard exercise/sports 6-7 days a week)
extra active (very hard exercise/sports & physical job or 2x training)

The Harris Benedict Equation is a formula that uses your BMR and then applies an activity factor to determine your total daily energy expenditure (calories). The only factor omitted by the Harris Benedict Equation is lean body mass. Remember, leaner bodies need more calories than less leaner ones. Therefore, this equation will be very accurate in all but the very muscular (will under-estimate calorie needs) and the very fat (will over-estimate calorie needs).