Short of these scientific methods of measurement, only estimates can be made. Estimates, however, can be very helpful in gaining an understanding of the relative value of different activities.
Our calculator uses METs, body weight and duration of activity to calculate the number of calories burned. A fourth very significant parameter is the intensity level at which the activity is performed.
We base our activity listings on intensity level whenever the data is available. For example, running is very specific and is listed at various speeds; downhill skiing is listed as light, moderate, and vigorous. Activities such as attending class and bailing hay, however, are much more general with no indication of intensity.
Intensity would obviously play a greater role in determining the number calories burned bailing hay than attending class. Considering such factors will help you understand the value of our estimates.
We are frequently asked about the meaning of terms such as light, moderate, and vigorous. While we do not know the specifics of how all of our data was collected, one common method is to simply take a group of people and ask each of them to perform the activity at light, moderate, and vigorous levels of intensity and take measurements.
Using this method the measurements are based on what the participants "feel" is the correct level of intensity. While this method may not sound very scientific, if the intensity level were measured with instruments then using the data would require the same instruments. Thus using "feel" is a more practical method of making estimates.
There is the potential for inaccuracy in calculating the calories burned by very heavy individuals. While an individual's weight is used to calculate the number of calories burned, the additional energy required for a very heavy individual to bear their own weight performing weight bearing activities is not factored into the equation. Thus the actual number of calories burned may be higher than estimated.
Conversely, when performing non-weight bearing activities, the heavy weight used to calculate the number of calories burned may play too great a role in the equation. Thus the actual number of calories burned may be lower than estimated.
Just the opposite would be true for a very light individual. Performing weight bearing activities, the actual number of calories burned may be lower than estimated; performing non-weight bearing activities, the actual number of calories burned may be higher than estimated.
There is also the potential for inaccuracy in calculating the calories burned by very strong and very weak individuals. The formulas used for calculations do not take into account your body composition (percent muscle vs. fat) and are therefore less accurate if you have a non-typical amount of muscle. This is because muscle burns calories, while fat does not.
A person with an above average amount of muscle will burn more calories than calculated, and a person with a below average amount of muscle will burn fewer calories than calculated.
Other factors that are not accounted for in the equation include:
The Compendium of Physical Activities, our primary source of data, provides the most complete and accurate collection of data available. However, even these figures can only be used as general guidelines. Not only because of all the variables described above, but because the data was compiled from a variety of sources and in some cases where different sources provided varying data averages were taken. Also, the data for some activities was estimated from the data of activities that were believed to require similar levels of energy expenditure or METs.
In addition, we believe that the METs assigned to a few of the activities are either too high or too low. We originally assumed that some activities had been assigned very low METs because they were based on a period of time that included periods of rest. For example, in Gymnastics a notable amount of time is spent resting and waiting ones turn to perform; in Surfing much time is spent waiting for a good wave. This could explain why these activities have been assigned the same MET as Bakery Work which is much less strenuous but is performed on a continuous basis.
Perhaps similar logic applies to Sexual Activity - Vigorous, which has the same MET as Sewing and Knitting. We get a lot of e-mail about this.
However, the latest update of the Compendium instructs users to "recall only the time spent in movement." This implies that the METs were calculated based on time spent in movement only, contradicting our assumption. On the other hand, the updated Compendium lists certain activities with the note "only active periods." This implies that activities listed without this note could include periods of rest, and this supports our assumption. This issue remains unclear.
The solution? If you believe that the burn rate for an activity is incorrect, we suggest that you find an activity with a rate that appears more realistic and use it for your calculations. For example, Motocross has also been assigned the same MET value as Bakery Work. For this highly intensive activity which does NOT have periods of rest, you might instead use Mountain Biking to make your calculations.
The Compendium states that the most accurate way to calculate calories burned is to measure the calories burned at rest (Resting Metabolic Rate or RMR) and multiply that number by the MET value. Acknowledging that this is not always practical, the Compendium further suggests that estimates can be made using a constant and the individual's weight. This is the method we use.
The Compendium is, by definition, a collection of information from a variety of sources. It is not clear exactly how all of these sources prepared their information, specifically whether they used RMR or a constant. The opinion has been expressed that in most cases a constant was likely used, making it likely that our method of calculation is more accurate than it would be if we were to use RMR.
As we stated at the top of the page, "only estimates can be made." Our goal is to make them the best possible. Your comments and suggestions are always welcome.