Smartwatches Overestimate Calorie Burn by Up to 25 Percent, Florida International University Study Finds

A user who completes a workout and sees 400 calories burned on their wrist display has likely only expended 300 to 340 calories in reality. That 60- to 100-calorie gap per session is the baseline hardware error identified in a September 2026 evaluation by researchers at Florida International University, who found that popular consumer smartwatches consistently overestimate energy expenditure by 15 to 25 percent.[1][6]

The findings, published this week and distributed via PR Newswire, evaluated the algorithms powering modern fitness trackers against clinical-grade metabolic testing. While the devices proved highly accurate at measuring heart rate—often within a 5 percent margin of chest straps—their ability to translate those beats per minute into total energy burned remains fundamentally flawed.[6]

"Your smartwatch is lying to you about how many calories you burned," the FIU research team stated plainly in their release, noting that the algorithms rely on generalized demographic data that cannot account for individual metabolic efficiency. The discrepancy is not a minor rounding error; over a week of daily exercise, a 25 percent overestimation can falsely add up to 700 phantom calories to a user's perceived output.[6]

The error margin does not apply equally across all body types. According to the data highlighted by BigGo Finance, the overestimation worsens significantly for individuals with higher body fat percentages. Because adipose tissue is less metabolically active than skeletal muscle, a tracker that calculates expenditure based primarily on heart rate and weight will assume a higher baseline calorie burn than the body is actually performing.[5]

Popular Science reports that this algorithmic blind spot creates a frustrating loop for users relying on the devices for weight management. When a tracker overstates the energy cost of a 45-minute brisk walk, a user eating back those reported calories will inadvertently place themselves in a caloric surplus, stalling their progress despite consistent physical effort.[2]

The hardware itself is not the limiting factor. Modern optical sensors capture blood volume changes with remarkable precision, but as ZME Science notes, the conversion from heart rate to kilocalories requires assumptions about a user's VO2 max, muscle mass, and mechanical efficiency. Without a direct measurement of oxygen consumption and carbon dioxide production—the gold standard used in laboratory settings—the watch is essentially making an educated guess.[1]

Sportswire Women emphasizes that the devices still hold substantial value for tracking relative progress. A workout that registers as 500 calories is reliably more intense than one that registers as 300 calories on the same device, even if neither absolute number is perfectly accurate. The utility lies in comparing your own sessions against one another over time.[4]

Gadget Review points out that manufacturers frequently update the software algorithms driving these calculations, meaning future firmware patches could narrow the 15 to 25 percent gap. Until those updates arrive, sports nutritionists advise users to treat smartwatch calorie counts as a gamified activity score rather than a ledger for dietary planning.[3]

The most practical adjustment for current users is to apply a mental discount to the wrist display. Shaving 20 percent off the reported total provides a much safer estimate for anyone tracking their energy balance, ensuring that the technology supports their fitness goals rather than quietly undermining them.