The Apple Watch Series 12 and Apple Watch Ultra 4 are said to be the most accurate smartwatches for tracking heart rate, thanks to their new Health Sensing System. This system includes a redesigned LED sensor array, which measures heart rates more frequently—up to 60 times more than earlier models—both during activity and when resting. The improvements are expected to provide more reliable data for users tracking their health and fitness. In a recent test, a 40-minute strength training session showed that the Apple Watch Series 12 recorded an average heart rate of 125 beats per minute (bpm). In contrast, the Garmin Fenix 9 Pro recorded a higher average of 132 bpm, a difference of seven beats. The discrepancy in calorie expenditure was even more notable. The Garmin Fenix 9 Pro estimated the session burned 340 kilocalories (kcal), while the Apple Watch Series 12 calculated 242 kcal. This 88 kcal difference is roughly equivalent to the energy in one small apple or a couple of spoonfuls of typical Ben & Jerry's ice cream. Tracking calorie expenditure using smartwatches is not an exact process. A study published this year in the journal PLOS One noted that differences in the algorithms and sensor designs of various devices can lead to significant variations in measurements. The study found that energy expenditure estimates can vary by more than 20% across popular models, highlighting the challenges in achieving precise readings. While smartwatches like the Apple Watch Series 12 and Garmin Fenix 9 Pro offer valuable insights into physical activity and health, users should be aware that these measurements are estimates. The accuracy of these devices can be influenced by a range of factors, including sensor design, software algorithms, and individual physiological differences. As technology continues to advance, improvements in accuracy and consistency are likely to follow.