General Fitness & Nutrition: Overvew of our Calorie Calculations

Many people have asked me how we calculate calories on this site, so I've decided to post an answer.

We use a combination of BMI, time taken, and MET score to estimate your calories burned.

BMI is your body mass index score and is based on your height/weight ratios. For more info: http://www.whathealth.com/bmi/formula.html

MET (Metabolic Equivalent) is the ratio of the work metabolic rate to the resting metabolic rate. One MET is defined as 1 kcal/kg/hour and is roughly equivalent to the energy cost of sitting quietly. A MET also is defined as oxygen uptake in ml/kg/min with one MET equal to the oxygen cost of sitting quietly, equivalent to 3.5 ml/kg/min.

Specific activities have a working MET -- ie, walking at a very slow pace (less than 2mph) for an hour will be the equivalent (ie., burn the same calories) to sitting still for two hours. The METS for this activity is 2.0

See: http://prevention.sph.sc.edu/tools/compendium.htm

So the final equation bringing this all together is:

Calories Burned = (MET value) X (BMI/24) X (duration of activity in hours)

To see this formula in action, visit our calorie calculator

-Kevin
Co-Founder, MapMyFitness

If we look at the MET compendium, we do break down activity by speed -- ie,

6.0 bicycling bicycling, 10-11.9 mph, leisure, slow, light effort
8.0 bicycling bicycling, 12-13.9 mph, leisure, moderate effort
10 bicycling bicycling, 14-15.9 mph, racing or leisure, fast, vigorous effor
12 bicycling bicycling, 16-19 mph, racing/not drafting or >19 mph drafting, very fast,

So the gaps you may be looking at breaks across these boundaries.  We're looking into ways to make this more incremental.

We're also looking at changing our BMI formula and was wondering if any users have experience with these calculations in determine a more accurate estimate:
http://blog.forret.com/2006/03/bmi-is-not-perfect/

Kevin,

 Sounds like you should hire aju to write the code and explanations of cal. use and various activities!!

<<Calories Burned = (MET value) X (BMI/24) X (duration of activity in hours) >>

 I think here you mean (MET) X (BMR/24) X (duration of activity in hours)

where BMR=basal metabolic rate (aka RMR-resting metabolic rate).... I'd need to know how you're calculating basal metabolic rate.  I'm assuming you're using the Harris-Benedict equation for BMR:

BMR = 66.5 + (13.75 * mass (kg)) + (5.003 * height (cm)) - (6.775 * age (y)) in men

BMR = 655.1 + (9.563 * mass (kg)) + (1.850 * height (cm)) - (4.676 * age (y)) in women

This formula has been around since 1919......which means it's old, but it works fairly well.  The problem with Harris-Benedict is that it doesn't take lean body mass into account.  Actual BMR is correlated more strongly to the amount of lean body mass, which is fairly stable, vs. total body mass, which we all know is not.  Harris Benedict tends to overpredict BMR in most populations and undershoot BMR in subjects with a lot of lean body mass (really bulky, "stocky" people).  Realistically, though, the variance is pretty small, so Harris-Benedict is probably as good as any for an estimate.

There are equations for figuring mets in Appendix D of the Guidelines for Exercise Testing and Prescription, 7th ed., American College of Sports Medicine.  The walking and running equations allow the inclusion of grade.  Unfortunately, these equations are more for steady-state activity rather than instantaneous activity.  What this means is that if you tried to use them to figure an instant met value for a 30 meter segment of hill they won't work as well as they would for a long stretch of hill at the same grade.  There are also some issues with the switchover point between walking and running (there is a sharp discontinuity in energy expenditure switching from a walking gait to a running gait).  However, using these equations might give a slightly more realistic determination of energy expenditure.  They also give cycling equations, but these equations are a bit tougher to implement on a website since they are dependent on power output.

The tough thing about setting up a web site to determine energy expenditure is the wide variance between individuals that simply can't be accounted for in some simple equations on a website.  And even then, energy expenditure is not the be-all and end-all either.  An hour's worth of intervals will burn about the same amount of calories as an hour's worth of a steady run, but current research shows that the intervals would have a much larger impact on increasing your VO2max (which basically measures your cardiovascular capacity).  I think this site has actually done a pretty good job at coming up with its figures, and I think the best use of the site is to not take the expenditures as gospel but use them to look at your trends over time and to set goals for the future.