Does your watch know how many calories you burned?
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Sort of. Your watch measures your steps and heart rate closely, and it estimates calories from them using your height, weight, gender and age. In a Stanford lab test, that estimate missed by more than 20% on every one of seven devices. It’s good for comparing your own days and poor for deciding what to eat.
What we’d do
Keep wearing it. Use the steps to pick your activity level and the daily totals to see which days you moved more. Don’t eat to its calories. Then find out how your watch leans for you: log your food and weigh in most mornings for four weeks, work out your real maintenance, and compare it with the watch’s average. After that, you know whether to treat its number as a little high or a little low. Keep your weight current in the watch’s settings, because the estimate uses it.
If you remember three things
- The watch measures movement and heartbeats. The calories are worked out from them.
- The estimate missed by 20% or more in lab tests. On walks most of all.
- Four weeks of logs and weigh-ins show you which way yours leans.
Why it seems to know
It’s on your wrist all day. It feels every step and counts every heartbeat, and it shows calories to the single digit, live, during your run. Something that close to your body and that precise seems like it must be measuring.
It’s more like the “feels like” temperature in a weather app. That number is worked out from real measurements, the air temperature, the wind and the humidity, run through a model of a typical person. It’s useful, and it isn’t a thermometer. Your watch’s calories are the “feels like” number of your day.
How the number is made
Only lab methods can measure the calories you burn, from the oxygen you breathe in and the carbon dioxide you breathe out, or from doubly labeled water. A watch has motion sensors and a heart rate sensor. Apple says its watch uses your height, weight, gender and age to calculate how many calories you burn, and that calibrating it improves the distance, pace and calorie numbers.
So the calorie number has two layers of guessing. Part of it comes from your size, age and gender, the same inputs calculator formulas use, and the best of those formulas misses by more than 10% for about one person in five. The active calories, in Apple’s words “the energy that the user has burned due to physical activity and exercise”, come from your movement and heart rate, run through a model.
How far off it gets
Strong The same answer from a large lab test and two pooled reviews.
Seven devices in a lab
60 adults, 2017. Shcherbina and colleagues, Stanford
No device got calories within 20%. Median errors ran from 27% to 93% depending on the device. Errors were highest when walking and higher for men, for people with a higher BMI and for people with darker skin.
The watches of 2017. The reviews below add newer models.
Apple Watch, 56 studies
Pooled review, 2025. Choe and Kang, Physiological Measurement
Heart rate stayed inside the 10% error mark in every group of studies, and steps did in most. Calories missed it in every group.
Studies of different models, settings and people.
Wrist and arm devices, 60 studies
Pooled review, 2020. O’Driscoll and colleagues, British Journal of Sports Medicine
Accuracy depended on the activity. Devices that added a heart rate sensor to the motion sensor did better for most activities.
The studies varied widely, so averages hide a lot of spread.
On a 2,500 calorie day, 20% is 500 calories. That’s a whole deficit, in either direction, and the watch shows it to the digit.
Where it’s right
The watch does know some things well. It knows how many steps you took and how fast your heart beat. It knows Saturday’s hike was a bigger day than Tuesday at your desk. It doesn’t forget to log a walk. And because it uses the same model on the same wrist, we’d trust its day-to-day differences more than any single total.
That’s the “sort of”. It knows your movement and roughly how much bigger one day was than another. It doesn’t know your calories, and nothing on your wrist does yet.
Test your own watch
- Wear it as usual for four weeks, and note its average daily total.
- Log everything you eat for the same four weeks, and weigh in every morning.
- Work out your maintenance from the log and the trend with the real TDEE calculator.
- Compare the two. If the watch averaged 2,700 and you maintain at 2,300, your watch runs about 400 high for you, and you can keep that in mind every day after.
7 sources
- Apple, Get the most accurate measurements using your Apple Watchsupport.apple.com
- Apple, HealthKit: activeEnergyBurneddeveloper.apple.com
- Shcherbina et al., Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort, Journal of Personalized Medicine, 2017pmc.ncbi.nlm.nih.gov
- Choe and Kang, Apple Watch accuracy in monitoring health metrics: a systematic review and meta-analysis, Physiological Measurement, 2025pubmed.ncbi.nlm.nih.gov
- O'Driscoll et al., How well do activity monitors estimate energy expenditure? A systematic review and meta-analysis of the validity of current technologies, British Journal of Sports Medicine, 2020pubmed.ncbi.nlm.nih.gov
- Mifflin et al., A new predictive equation for resting energy expenditure in healthy individuals, American Journal of Clinical Nutrition, 1990pubmed.ncbi.nlm.nih.gov
- Frankenfield, Bias and accuracy of resting metabolic rate equations in non-obese and obese adults, Clinical Nutrition, 2013pubmed.ncbi.nlm.nih.gov
