Apple Watch Series 12 Heart Sensor Accuracy in Real-World Tests

Apple has overhauled the heart rate sensor in the Watch Series 12, claiming it can provide continuous data even when you are sitting still. To verify these claims, independent analyst Rob ter Horst from The Quantified Scientist conducted a rigorous comparison against the Polar H10, a chest strap widely considered the gold standard for cardiac monitoring.

The results suggest the new hardware is highly capable, particularly for steady-state activities like cycling. However, the testing also highlights that wrist-based sensors still face challenges during high-intensity, jerky movements. As reported by 9to5Mac, the findings indicate a strong performance but with specific trade-offs depending on the type of exercise.

Cycling and running show strong correlation

During indoor cycling, the Apple Watch Series 12 performed exceptionally well. The readings were nearly identical to the Polar H10 chest strap, achieving a near-perfect correlation score of 1.0. This suggests that for endurance athletes who maintain a steady pace, the watch is a reliable tool for tracking effort and recovery.

Running results were also positive, with a correlation of 0.98. While slightly lower than the cycling data, this level of accuracy is generally sufficient for most recreational and competitive runners. The minor deviations noted at higher heart rates are a known limitation of optical sensors, which can struggle to keep up with rapid blood flow changes.

Weightlifting remains a technical challenge

The most difficult test for any wrist-based sensor is weightlifting. The repetitive, high-impact motion and changing blood pressure can confuse optical photodiodes. In this scenario, the Series 12 posted the lowest correlation of the three tests, at 0.97.

Despite being the lowest score in the comparison, ter Horst described the result as quite good. The device outperformed 85% of other watches tested for this specific activity. This indicates that while not perfect, the new sensor is competitive in a category where accuracy has historically been the weakest link.

Step counting and algorithm maturity

Beyond heart rate, ter Horst tested the pedometer by manually counting approximately 5,000 steps. The watch was generally accurate, with only slight overcounting. This is a minor issue for most users, as step counts are rarely used for precise medical or training calculations.

One notable finding is that the Series 12 did not significantly outperform the previous generation in all scenarios. In fact, the older Series 11 was slightly more accurate during running in some tests. This suggests that Apple may still be refining the algorithms that interpret the new sensor data, meaning future software updates could improve performance without hardware changes.