The calorie number is the least accurate thing your fitness tracker shows you. In a Stanford study of seven wrist devices, the typical calorie error ran from 27% to 93%, depending on the device. Heart rate was a different story. Six of those same seven devices had a median heart rate error under 5% while people pedaled a stationary bike.
So I’d trust the heart rate more than the calories, and I’d read the calories as a trend, not a fact.
I get this question across the counter more often than you’d think. Somebody holds up a wrist and tells me the watch says a walk burned 600 calories, and they want to know if that’s real. Honestly, the display makes it look exact. It shows one whole number, never a range, so the honest answer tends to surprise people.
One thing before we start. I’m a pharmacist, not your doctor. I can tell you what the research says about these devices and how to get a better reading out of yours. What your numbers mean for your health, and whether anything about your exercise or your eating should change, is a conversation for your doctor.
Here’s what I’ll go through:
- what the Stanford team measured, and how far off the calorie numbers were
- what the bigger reviews have found since
- how your watch comes up with a calorie number in the first place
- what throws the reading off (cold hands are on the list)
- why a chest strap reads heart rate better than your wrist does
- how I’d use the calorie number, and how I wouldn’t
- when a reading means call someone
What the Stanford Study Measured
In 2017, a team at Stanford put wrist devices on 60 volunteers, 29 men and 31 women, with a wide spread of ages, body sizes, skin tones and fitness levels. Seven devices were tested:
- Apple Watch
- Basis Peak
- Fitbit Surge
- Microsoft Band
- MIO Alpha 2
- PulseOn
- Samsung Gear S2
Each volunteer sat still, then walked and ran, then rode a stationary bike. While they did, the researchers measured the real numbers the hard way. Heart rate came from continuous telemetry, the kind of heart monitoring used in hospitals. Calories came from indirect calorimetry, which works out the energy you burn from the oxygen you breathe in and the carbon dioxide you breathe out. That’s the lab’s gold standard, and it’s the number your watch is trying to guess.
Heart rate held up. On the bike, six of the seven devices had a median error under 5%. Across every activity, the Apple Watch had the lowest heart rate error, 2.0%, and the Samsung Gear S2 had the highest, 6.8%.
Calories didn’t hold up. Not one device got its calorie error below 20%. The median error across the activities ran from 27.4% for the Fitbit Surge to 92.6% for the PulseOn.
Here’s what that looks like at home. Say your watch tells you a walk burned 400 calories. If it’s off by 27%, the low end of that study’s range, the real burn could be anywhere from about 290 to about 510 calories. That’s the error band, and the display never shows it to you.
The team also found more error for men, for people with a higher body mass index, for darker skin tones and during walking. I’ll come back to skin tone, because a later study didn’t agree.
The Bigger Reviews Since Then Found the Same Thing
One study is one study. So I went looking for the reviews that pool many of them together, and it turns out they say the same thing.
A team at McMaster University in Canada went through 65 studies of wrist trackers and published its review in 2022. For heart rate, the Apple Watch’s average error was under 10% in the two studies that measured it. For calories, the average error was over 30% for every brand. Their conclusion was plain: none of the devices tested proved accurate for energy expenditure, which is the research term for calories burned.
A 2020 review from Memorial University in Newfoundland looked at 158 publications covering nine brands, through May 2019. For calories, no brand was accurate. Heart rate depended on the brand, with Apple Watch and Garmin the most accurate and Fitbit tending to read low. In lab tests, step counts were the strong point for Fitbit, Apple Watch and Samsung.
Those reviewers made another point I repeat to people at my counter. These devices get redesigned all the time, so a study of an older model doesn’t prove much about the one on your wrist today. That cuts both ways. Yours may well be better than the ones tested. It may not be.
How Your Watch Comes Up With the Calorie Number
Nothing you wear on your wrist can measure calories directly. It estimates them.
Apple explains its approach on its support site. The watch uses your personal information, such as your height and weight, plus your gender and age, to calculate how many calories you burn. Then it adds what the sensors pick up while you move. The Activity app relies on arm motion and the accelerometer, and the Workout app can also use the heart rate sensor and GPS.
So the calorie number stacks two guesses on top of each other. One is a formula built on averages for people of your size and age. The other is your heart rate and movement, read through the skin of your wrist. If either one is off, the total is off.
That’s why it pays to keep your profile current. If you’ve lost 20 pounds since you set the watch up and never changed your weight in the app, the formula is still working from the old you. Apple also recommends calibrating the watch, which it says helps it learn your stride and fitness level. Apple says that improves the calorie reading along with distance and pace.
READ: What is The Simplest Fitness Tracker for Seniors?
What Throws the Reading Off
Most of what goes wrong with the calorie number starts with the heart rate reading underneath it. The sensor on the back of the watch shines light into your skin and reads how much comes back as blood pulses through. The technical name is photoplethysmography, or PPG. Anything that gets between that light and your blood flow shows up in the reading.
Here’s what Apple’s own support page lists, with my notes from behind the counter:
- A loose fit. Apple says the watch should sit snugly on top of the wrist, with the sensor close to the skin. It suggests tightening the band for a workout and loosening it afterward.
- Cold. When you exercise in the cold, blood flow in the skin of your wrist can drop so low that the sensor can’t get a reading. I see the same thing with fingertip pulse oximeters. Cold hands make for a poor reading every time.
- Some tattoos. Apple says the ink, pattern and saturation of some tattoos can block the sensor’s light.
- Irregular movement. Rhythmic exercise like running or cycling gives better results than stop-and-start sports like tennis or boxing.
What you’re doing matters for calories, too. In the Stanford study, the calorie error was 31.8% for walking and 31.0% for running. It was worst while people simply sat, at 52.4%.
A Duke University team that went looking for the sources of error found that heart rate error during activity was about 30% higher, on average, than at rest. On skin tone, the two studies disagree. Stanford found more error with darker skin, while the Duke team found no statistically significant difference in accuracy across skin tones. In my experience, when two good studies disagree, the fair thing is to call the question open.
A Chest Strap Reads Heart Rate Better
If heart rate is the number you really care about, there’s a better tool than a wrist sensor. A chest strap picks up the heart’s electrical signal through electrodes against your skin, the same basic idea as an ECG.
Cleveland Clinic researchers put that to the test in 2017. Fifty healthy adults, 54% of them women, exercised on a treadmill, a stationary bike and an elliptical while wearing a Polar H7 chest strap, a forearm monitor and two wrist monitors. Every reading was checked against an ECG.
The chest strap agreed with the ECG almost perfectly, at 0.996 on a scale where 1.0 is a perfect match. The Apple Watch came next at 0.92, and the Fitbit Blaze came last at 0.67. On the elliptical with the arm levers in use, none of the devices read heart rate accurately.
The researchers’ advice was short: when an accurate heart rate really matters, use a chest monitor with electrodes. Apple says its watch can connect to a Bluetooth chest strap when the wrist sensor can’t get a steady reading.
Keep in mind that a better heart rate doesn’t fix the calorie formula. It removes one source of error, not all of them.
READ: Heart Rate Monitor: Which should I choose?
How I’d Use the Calorie Number
I don’t tell people to ignore the number. I tell them to read it the right way.
Compare it with itself. The same watch, on the same wrist, doing the same kind of walk, should make roughly the same mistake each time. So if your weekly total has crept up from about 1,500 active calories in September to about 1,800 now, you really are moving more, even though neither total is exact.
Don’t compare it across devices. Two different watches can disagree by a lot, and the Stanford range shows you why.
Don’t plan your meals by it. If a watch overstates a workout by a third and you eat that number back, the extra food can quietly cancel the workout. If you’re trying to lose or gain weight, your doctor or a registered dietitian can give you a target built on more than a wrist sensor.
Heart rate is the stronger number, as long as you wear the watch well. With a snug fit on a warm wrist during something rhythmic, the better devices in these studies read within a few percent. At a 2.0% error, a reading of 120 beats a minute could be 2 or 3 beats either way. At 6.8%, the worst device’s error in the Stanford study, the same reading could be off by about 8 beats.
If you check your blood pressure after a workout, the same patience applies there.
READ: Blood Pressure After Exercise: When to Take the Reading (and What’s Normal)
When a Reading Means Call Someone
A fitness tracker is not a diagnosis, and a calorie number is the last thing on it I’d make a health decision with.
Here’s my plain rule. If you feel unwell, how you feel wins over what the display says. Chest pain, fainting, sudden shortness of breath, or a racing heartbeat at rest that won’t settle: call 911 or your doctor, whatever the watch reads. A normal-looking number on your wrist is no reason to wait.
If the watch shows you something that worries you, like a heart rate that seems far too high or too low for what you’re doing, don’t sit at home with it. Call your doctor’s office and tell them what you saw, and bring the watch to the appointment. They’d much rather hear about it than not.
The short version I give at the counter goes like this. Heart rate: fairly good, if the watch fits. Steps: decent. Calories: a rough estimate with a wide error band, best used to watch your own trend over weeks.
If there’s a device you’d like me to look into next, leave it in the comments. I read them all.
