Research: Commercially available smart watches have 15% to 25% error in measuring exercise calories

Many people use smart watches to monitor their exercise status and track calorie consumption.

After you finish exercising and see that your smart watch shows that you have burned several hundred calories, don’t rush to reward yourself with a big meal. A study by Florida International University in the United States found that commercially available smart watches may have significant errors in estimating exercise calorie consumption, with typical errors of about 15% to 25%. Moreover, the higher the body fat rate, the more inaccurate the measurement results are. Researchers caution that the calorie numbers on the watch are more suitable as a guide rather than precise values. The research results have been published in "PLOS One".

According to comprehensive foreign media reports, a research team from Florida International University found 58 adults for testing and recorded the subjects’ skin color, height, body fat rate and other information. The subjects used a recumbent cycle ergometer to alternately perform moderate and high-intensity exercise while wearing Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955. The actual energy consumption was measured with the COSMED K5 metabolic analyzer used in the laboratory to compare the accuracy of the four devices.

The results show that various smart watches have a certain degree of error in estimating heat consumption, with typical errors ranging from 15% to 25%. Among them, Garmin Forerunner 955 and Samsung Galaxy Watch 5 overestimated heat consumption most obviously, while Apple Watch Series 8 overestimated it to a relatively small extent.

Fitbit Sense 2's situation is more special. After excluding outliers, the average deviation did not reach statistical significance, but about 13% of the estimated results during the test were considered unreasonable. If these abnormal results are taken into account, Fitbit's performance will worsen significantly, so the researchers believe that the relevant results need to be interpreted with caution.

Another important finding of the study is that there is a clear correlation between error and body fat percentage. The higher the subject's body fat percentage, the greater the error in the device's estimated calorie consumption. As for the reason, the research team is currently inconclusive. It may involve the sensing technology that measures blood flow, or it may be related to the algorithms used by various manufacturers to convert physiological data into energy consumption.

Notably, the study found no significant differences in measurement error among subjects with different skin colors. However, this study only had a sample of 58 people, all of whom were Hispanic. The ethnic group covered was relatively single, and the test only used a single exercise mode. Therefore, whether the relevant results can be applied to a wider population, as well as other sports such as running, swimming, and weight training, still needs to be further verified in the future by expanding the sample and including more diverse ethnic groups and exercise types.

Jason Kostrna, the lead author of the study and a sports scientist, said bluntly that the calories burned displayed by the smart watch cannot be regarded as a number, because the accumulated calories per week may vary by hundreds of calories, which may even make the user think that they are in a "calorie deficit", but in fact they may have consumed more than they burned.

However, the research team emphasized that this does not mean that smart watches are useless. Kostrna suggests that instead of obsessing over how many calories you burn during a single exercise, you should use wearable devices to observe your exercise trends over the long term and regard the relevant data as a reference tool to help improve exercise habits.