Garmin’s erroneous average power data
Apple’s Health system - including the Health and Fitness apps on the iPhone and the Workout app on the Apple Watch - calculates incorrect average values for a range of data types. It’s unfair to single out Apple for criticism, though, since other companies like Garmin share the same mistake. The first post in this series provided a brief primer on averages, and how they are calculated on a computer. The second post explored the magnitude of this error for cycling average power data using real world measurements from an Apple Watch. This post continues the series by examining how simultaneous data recorded by a Garmin Edge 130 plus device compares to data recorded on an Apple Watch.
Energy always equals Power × Time. Power is always Energy ÷ Time.
Average Power is simply Energy ÷ Total Time, where Energy is the sum of each individual Power sample multiplied by the Time interval of the sample.
Just like Apple’s health team might not grasp the basic physics of energy and power, it appears that Gamin’s engineers don’t as well. Like Apple, the reported average power in their app is simply the sum of all power readings divided by the number of readings. This leads to errors in average power of approximately 5% in the Garmin reported data. For example, Garmin reported an average power of 230 watts for a ride recorded on December 6, 2025 using a Specialized Tarmac SL7 S-Works equipped with a Shimano Di2 crankset and a 4iii Precision Pro dual-sided power meter. Apple reported an average power of 256 watts for the same ride. But, when using the correct equation for average power as the sum of each individual power measurement time time divided by the total ride time, the Garmin recorded data shows a true average of 218.3 watts, while the Apple recorded data show a true average of 221.0 watts. Through the Garmin and Apple data fail to match exactly, the 1.2% true average power difference is much less than the 5.5% error in the Garmin reported average power and the true average power. Apple fairs far worse with an error of 12.7% between the reported and true average power.
For the Garmin data, the simple average power value is 228.4 watts, which is close to the reported value of 230 watts. Similarly, the simple average power value is 249.3 watts, which is somewhat close to the reported value of 256 watts. This suggests that Apple and Garmin are reporting simple average values - the sum of all power readings divided by the total number of readings.
Garmin claims an average power of 230 watts, but the properly calculated the average power is only 218.3 watts - a 5.5% error.
For a ride recorded on December 7, 2025, Garmin reported an average power of 259 watts. Apple reported an average power of 269 watts for the same ride. The Garmin recorded data shows a true average of 248.1 watts, while the Apple recorded data show a true average of 244.4 watts. Through the Garmin and Apple data fail to match exactly, the 1.5% true average power difference is much less than the 3.5% error in the Garmin reported average power and the true average power. Apple again fairs far worse with an error of 9.2% between the reported and true average power. Finally, for a ride recorded on December 10, 2025, Garmin reported an average power of 269 watts, with Apple reporting an average power of 277 watts. The Garmin recorded data showed a true average power of 253.9 watts, while the Apple data showed a true average power of 252.8 watts, for a smaller true average power difference of only 0.4%.
Note that the average power reported by Apple and Garmin shows a significant difference from the true values, but when properly calculated as a time averaged value, the data shows a far smaller difference in values. The average power reading from the December 7, 2025 Garmin ride data is 257.1 watts, which is close to the reported value of 259 watts. The corresponding average power from the Apple ride data is 266.6 watts, which approximates the 269 watts reported by Apple. The average power reading from the December 10, 2025 Garmin ride data is 268.2 watts, which is close to the reported value of 269 watts. The corresponding average power from the Apple ride data is 275.2 watts, which approximates the 277 watts reported by Apple.
Taken together, the evidence that the reported average power values from Garmin and Apple differ substantially, the fact that the time-weighted average values match reasonably closely, and the fact that the calculated average power reading closely matches the reported values, all suggest that Apple and Garmin are reporting the average power reading instead of the average power.
| Apple Average Power | Garmin Average Power | ||
|---|---|---|---|
| Reported Value | True Value | Reported Value | True Value |
| 256 | 221.0 | 230 | 218.3 |
| 269 | 244.4 | 259 | 248.1 |
| 277 | 252.8 | 269 | 253.9 |