Estimated Best Efforts are automatically calculated using your GPS-based running activity, and reflect your fastest times for benchmark distances such as 1 mile, 5km, 10km, and half marathon. Strava can find your best effort at any point in each running activity. We do not require that a best effort starts at a mile split.
Strava will look at the running activity and identify the fastest rolling-time for benchmark distances. They are considered estimates, because they are subject to normal discrepancies in GPS accuracy.
How are Estimated Best Efforts different from All-time Personal Records?
Estimated Best Efforts are a training level insight into GPS based activities
All-time Personal Records are a runner's fastest time ever run over a distance in an official race, or over a verified, known distance
Strava requires good quality GPS location information throughout the effort. If there are missing points, errant points or other false accelerations then that part of the activity will not be eligible to be recognized as a best effort.
Estimated Best Efforts are displayed on your profile page, as a link to the activity during which they were achieved:
...As well as on the activity during which they were achieved - provided they account for one of the "Top Results" for that activity.