Your smartwatch buzzes. Workout done: 500 calories burned. You figure you’ve earned the muffin.
Not so fast. Researchers at Florida International University recently tested four of the most popular smartwatches against lab equipment, and three of them handed out credit for calories nobody burned.
That matters if you spent a few hundred dollars on a gadget partly to help manage your weight. The calorie number is the one you’re most likely to act on — and it’s the one least worth trusting.
Here’s why, and what to watch instead.
1. It gives you credit for calories you didn’t burn
The FIU team had 58 adults pedal a recumbent exercise bike while wearing an Apple Watch Series 8, a Fitbit Sense 2, a Samsung Galaxy Watch 5 and a Garmin Forerunner 955.
Each watch’s calorie count was checked against a portable metabolic analyzer, a mask-and-sensor rig that measures the oxygen you use. It’s the kind of equipment researchers use as their yardstick.
The watches missed. According to FIU’s summary of the research, typical errors ran about 15% to 25%, and three of the four brands leaned high.
The researchers’ verdict in the journal PLOS One: Consumer smartwatches “substantially misestimate” how many calories you burn exercising.
2. The heavier you are, the further off it gets
Here’s the cruel twist. The error grew with body fat for every brand tested.
In other words, the people most likely to be counting calories to lose weight are the ones getting the worst numbers.
That’s not your fault. Your watch can’t actually measure calories — the lab needed a mask on your face to do that.
It’s making an educated guess from sensors on your wrist, and the guess gets worse the further you are from whatever body type the formula was built around.
3. Your brand matters more than you’d think
Over one short bike session of about 10 minutes, here’s how far each watch overshot on average, according to the study:
- Garmin: about 69 calories too many
- Samsung: about 57 too many
- Apple: about 22 too many
Fitbit came out closest on average, but that’s misleading. It sometimes showed a single calorie for an entire workout, or nothing at all, and the researchers had to toss many of its readings before they could score it.
Now do the math on a real workout. If a Garmin overcounted at that same rate for 45 minutes, you’d be credited with roughly 300 phantom calories. McDonald’s lists a Big Mac at 580 calories. Eat that back, and you’ve had half a Big Mac you never earned.
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4. No brand has ever gotten this right
Don’t assume a newer model fixes the problem. This isn’t a one-study fluke.
A 2020 review in JMIR mHealth and uHealth pooled 158 published studies covering nine brands of wearables. Its conclusion on calorie burn was blunt: No brand is accurate.
Some devices tended to run high and others ran low. Either way, the number on your wrist was rarely the number your body actually burned.
5. You can’t check the company’s math
Every watchmaker uses its own secret formula to turn heartbeats and arm swings into calories. Jason Kostrna, the FIU study’s lead author and an associate professor of kinesiology, notes that outsiders have no idea what data those formulas were built and tested on.
That’s a real gap. The FIU volunteers were all between 18 and 50. If you’re older, this study can’t tell you how far off your watch runs.
And it isn’t only calories. A California lawsuit claims the Oura Ring overstates how well it tracks sleep stages. You can read more in “Oura Ring Doesn’t Accurately Measure Sleep Quality, Lawsuit Says.”
What your watch is actually good for
I’m not telling you to toss the thing in a drawer. It’s just better at some jobs than others.
Heart rate is the standout. The FIU team says heart rate and time spent in each heart-rate zone are where these devices earn their keep.
On that score, the 2020 review found Apple Watch and Garmin the most accurate, with Fitbit tending to read low.
The other thing your watch does well is show direction. Even a number that’s consistently wrong can tell you whether you’re moving more this month than last. Kostrna’s advice is simple: “Track your own trends over time.”
The scoreboard that doesn’t lie
If you’re trying to lose weight, the bathroom scale is the referee. Weigh yourself at the same time each week and watch the trend, not any single day.
Treat the watch’s calorie count as a rough guess, round it down and don’t plan your meals around it. If your weight holds steady while your watch says you’re in a big deficit, you’ve got your answer about which one to believe.
And whatever you do, don’t let a fuzzy number talk you out of exercising. Moving your body pays off no matter what the tally says — even if you’re getting a late start, as Stanford Medicine researchers have found.
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