Apple Watch Series 12 and Garmin Fenix 9 Pro Show Heart Rate and Calorie Gaps in First Test

The Apple Watch Series 12 and Apple Watch Ultra 4 are reportedly the most accurate heart rate-tracking smartwatches on the market and the best Apple Watches yet, largely because of a new Health Sensing System. That system uses a redesigned LED sensor array and is said to be more accurate than previous generations, measuring active and resting heart rates up to 60 times more often. The TechRadar reviewer said earlier tests with the Apple Watch Ultra 3 had come remarkably close to an electrical heart rate monitor.

After receiving an Apple Watch Series 12 review unit, the reviewer went to the gym and tracked the same workout on both the Apple Watch Series 12 and the Garmin Fenix 9 Pro, wearing one on each wrist. The Garmin Fenix 9 Pro was described as a five-star watch, an expensive do-it-all device with advanced health tracking credentials. It uses Garmin's Elevate V5 heart rate sensor, which in previous tests with a Garmin Venu 4 fell within around 5 bpm of an Apple Watch Ultra 3 for average heart rate.

During the 40-minute strength session, the Apple Watch recorded an average heart rate of 125 bpm, while the Garmin Fenix 9 Pro recorded 132 bpm. The seven-point difference was slightly outside the reviewer's 5 bpm threshold for insignificant differences, but only just. The reviewer said the theory is that as each watch is worn more, their algorithms will tighten up and produce closer results, and that comparing both with an electrical Polar H10 heart rate monitor will be very interesting.

The difference in calorie expenditure was described as more worrying. Garmin recorded 340 kcal, while Apple recorded 242 kcal as total expenditure for the session. The 88 kcal difference is equivalent to one smallish apple or a couple of spoonfuls of average Ben & Jerry's ice cream, according to the article.

Calorie expenditure on a smartwatch is not an exact science. A research paper published this year in the journal PLOs One said that proprietary differences in device algorithms and sensor design introduce another layer of variability, with comparative studies reporting an error of more than 20% in energy expenditure across widely used models. The article said deviations between watches should be expected based on algorithm differences and should not matter so long as measurements on a device are consistent. However, an 88 kcal difference is over that 20% margin for error. TechRadar said the Apple Watch Series 12 will be tested further, over longer workouts and against different wearables, to determine whether its accuracy claims hold water.

Editor's Summary

TechRadar's first comparison of the Apple Watch Series 12 and Garmin Fenix 9 Pro found a seven-bpm average heart rate gap and an 88-kcal calorie expenditure difference during a 40-minute strength workout. The test did not include an electrical heart rate monitor, and the reviewer plans more testing against other wearables and a Polar H10. The article also notes that smartwatch calorie estimates can vary by more than 20% because of proprietary algorithms and sensor design.