Our bodies compute our caloric needs like the ultimate algebraic equation.  Subtract calories consumed from calories burned.  Factor in basic metabolism, body functions and exercise.  Adjust for donuts and divide by skinny jeans.  Wait!  It's got to be easier than that.  Just how many calories do we really burn on an average day?  How about when we exercise?  How many does the average man or woman need and how many do we need to cut to lose weight? 

The Mayo Clinic explains how calories affect our weight.  "Weight is dependent on the balance of total calories consumed versus total calories burned. Take in more calories than your body needs, and you gain weight. Take in less and you lose weight. Metabolism is the engine that burns these calories." 

The number of calories we burn each day is your total energy expenditure and is broken down into three factors: Basal Metabolic Rate, food processing and physical activity.

1. Our Basal Metabolic Rate (BMR) is the amount of energy it takes to make our bodies work, including:  vital organs, circulation, respiration, hormone manufacturing, etc.  It's what we burn just being alive. Our BMR accounts for two-thirds to three-quarters of the calories used each day. Our BMR is relatively stable on a day-to-day basis and uses about the same number of calories.

2. Food processing. About 10 percent of our daily calories are burned dealing with the food we eat through digestion, absorption, transportation and storage. This rate is also relatively stable on a day-to-day basis.

3. Physical Activity is the part we have some control over and accounts for the remainder of calories used. The number of calories burned depends on the frequency, duration and intensity of your activities.  Sitting on the couch burns almost none.  To lose one pound, you have to burn 3500 calories more than your body needs, either by increasing physical activity or by reducing calorie intake.  The most successful weight loss plans include both.

Just how many calories do we need?  It varies from person to person depending on body type, how much exercise they get, and health status.  The U.S. Department of Agriculture provides a chart that helps calculate your specific caloric needs.

The chart below is provided by the National Institutes of Health (NIH) and estimates the number of calories burned in an hour by an average healthy person weighing 150 pounds.

Activity Calories Burned per Hour*
Bicycling, 6 mph 240
Bicycling, 12 mph 410
Cross-country skiing 700
Jogging, 5½ mph 740
Jogging, 7 mph 920
Jumping rope 750
Running in place 650
Running, 10 mph 1280
Swimming, 25 yds./min. 275
Swimming, 50 yds./min. 500
Tennis, singles 400
Walking, 2 mph 240
Walking, 3 mph 320
Walking, 4½ mph 440


Once you've figured out your BMR, factored in your activity calories, you can do your own algebra about how many donuts you can afford before your jeans won't fit.