Smartwatches are everywhere now

About 100 million U.S. adults own a smartwatch, and roughly 560 million people wear them worldwide. They've become a standard tool for tracking health and fitness.

But calorie counts are often wrong

Research shows that one of the most popular smartwatch metrics, calories burned, is frequently inaccurate. A new study examined how well four major brands actually measure energy expenditure.

How smartwatches estimate calories

Smartwatches use accelerometers to measure movement and light sensors to estimate heart rate. They combine this data with your age, sex, height and weight to calculate energy expenditure using an algorithm.

The study tested four popular brands

Researchers recruited 58 Hispanic adults in Miami with varying body fat percentages and skin tones. Each wore an Apple Watch Series 8, Garmin Forerunner 955, Samsung Galaxy Watch5 and Fitbit Sense 2 during controlled cycling sessions.

They compared watches to lab equipment

Participants wore the watches while cycling at moderate and vigorous intensity. Researchers measured actual energy expenditure using a metabolic analyzer, the gold standard in exercise science, to compare against each watch's estimate.

All four watches showed substantial error

The median error across all devices was 15 to 25%. Apple had the smallest systematic overestimate, while Garmin and Samsung overestimated more. Fitbit produced some implausible readings, including seven exceeding 450% of actual values.

Body fat percentage made errors worse

As body fat percentage increased, smartwatch measurement errors grew significantly for all four brands. For people with higher body fat, errors ranged from 50% to 100% or more, meaning a 100-calorie burn could display as 150 to 200 calories.

Skin tone didn't affect accuracy

The study found no significant effect of skin tone on caloric expenditure error under the conditions tested, suggesting the problem is not related to how light sensors read different skin colors.

The irony for weight loss seekers

People with higher body fat percentages are most likely to use smartwatches for weight loss support, yet they receive the least accurate calorie estimates. A 50-100 calorie daily error can add up to 1,200-2,400 calories per week.

“People with higher body fat percentages are likely among those most interested in using wearable devices to support weight loss.”

Smartwatches still have value

The same research team found that smartwatches measure heart rate with reasonable accuracy. They also provide useful data on workout duration, distance and pace depending on the activity.

Treat calorie counts as rough estimates

Experts recommend treating smartwatch calorie numbers as estimates, not exact measurements. Don't rely solely on them to decide how much to eat or judge workout success. Look at patterns over time alongside heart rate and body weight changes.

“Treat the calorie number as an estimate rather than an exact measurement.”

Technology will keep improving

Better sensors may help, but improving how energy expenditure is estimated across people with different body compositions is equally important. Researchers are now testing smartwatches during strength training, where accuracy may be even more challenging.