Apple Watch Series 12 treadmill readings varied across three reported runs: the watch was 20% above the treadmill display in a one-mile sample, then 6.7% and 4% below it in two longer runs. Those percentages compare individual readings, not a universal accuracy rate.
How close were the Series 12 readings?
Michael Sawh’s test for TechRadar compared the Series 12 with a Coros Pod 2 worn on the foot and the treadmill display. The watch’s reading was higher in the one-mile sample and lower in the interval and steady-run samples.
| Sample run | Apple Watch Series 12 | Coros Pod 2 | Treadmill display | Watch difference vs. display |
| One mile | 1.2 miles | 1.01 miles | 1 mile | +20% |
| Intervals | 4.2 miles | 4.4 miles | 4.5 miles | About −6.7% |
| Steady five-mile run | 4.8 miles | 5.02 miles | 5 miles | −4% |
Each percentage uses the treadmill display as the reference: (watch distance − display distance) ÷ display distance. That makes the short-run gap look larger proportionally than either longer-run gap. The display is a comparison point, though, not a verified measure of the true distance. Sawh noted that treadmill condition and belt wear varied across some sessions.
What the test compared
Sawh tested beta watchOS 27 on Apple Watch Ultra 3 and Apple Watch Series 11, then tested the public version on Apple Watch Ultra 4 and Apple Watch Series 12. The test series included one-mile runs, hour-long steady runs and intervals. For the reported sessions, he used Apple’s Workout app, kept the incline the same and calibrated each watch with an outdoor run of at least 20 minutes. The Coros Pod 2 was worn on the foot and paired with a Coros watch.
The test’s broad summaries put the differences at about 300 meters in the beta phase and roughly 200 meters with the public version. Those figures do not set a maximum: one listed Series 12 interval sample was 0.3 mile below the treadmill display. The individual readings also varied in direction, as the table shows.
What Apple says calibration can improve
Apple says calibrating an Apple Watch can improve distance, pace and calorie measurements, and help with accuracy when GPS is limited or unavailable. Its calibration guidance describes the purpose of calibration; the reported treadmill samples show how readings differed in specific sessions.
In Sawh’s test, calibration meant an outdoor run of at least 20 minutes for each watch. That is the setup he used, not a promise that every calibrated watch will match a treadmill display by a particular amount.
Why the test does not isolate watchOS 27’s effect
The beta and public-version phases used different watch models as well as different software states, and treadmill conditions sometimes varied. The results therefore do not show how much of the difference between phases came from watchOS 27 itself.
Apple described watchOS 27 as improving motion-tracking algorithms for estimating treadmill walking and running distance from the wrist. The samples show that Series 12 readings differed from the treadmill display in these sessions; they do not establish a single correction that applies to other workouts or treadmills.
