Are Fitbit, Garmin and Oura calories accurate?

Not for calories, whatever the brand. A review of 158 studies across nine brands found that no brand measured calories accurately, while steps and heart rate were often good. So use these devices for steps and heart rate, treat their calorie total as a rough guess, and use your scale for the real answer.

It’s natural to think a pricier device or a newer model fixes this. Every wearable faces the same problem, though: it can’t measure the energy you burn, so it estimates it from movement and heart rate. Better sensors sharpen the inputs. The estimate is still an estimate.

The simple version

Think of a car’s range estimate. Every brand shows one, it’s handy for comparing days, and none of them knows exactly how far you’ll get.

The studies

• Fitbit, Garmin and others. A 2020 review of 158 studies found Fitbit, Apple Watch and Samsung counted steps accurately in the lab, and Apple Watch and Garmin were the most accurate for heart rate. For energy burned, no brand was accurate.
• Oura. In a 2023 study, 32 people wore the Oura Ring for 14 days next to research monitors. Its numbers rose and fell with theirs, but it ran high or low for individual people, and in the lab it fell further behind as exercise got harder.
• Twelve devices at once. In a 2019 study, 19 adults wore 12 devices for 15 days while their real burn was measured with doubly labeled water, a lab method. 10 of the 12 devices significantly underestimated the calories spent on activity.

What to do

• Use the device for daily steps, and pick your activity level from them on the calories from your steps calculator.
• Don’t eat back its exercise calories. See should you eat back your exercise calories?
• Find your real burn from four weeks of meals and weigh-ins on the real TDEE calculator.