How to Get the Most Accurate VO2 Max Estimate from Your Running WatchMost coaches and exercise physiologists tell you to take the VO2 max estimate generated by your running watch with a grain of salt. However, a separate study conducted in the same year found that the Apple Watch’s VO2 max estimate can be off by about 13 percent. Only outdoor runs impact your VO2 max estimate, as the watch relies upon GPS technology. Related Story The 6 Best Advanced GPS WatchesHow to Do Your Own VO2 Max Test with a Watch TimerIf your watch doesn’t have a VO2 max function, you can conduct your own test using a basic stopwatch and either a treadmill or a track. Related Story What to Know About VO2 Max as You Age“[Watch estimators] typically work better for runners who are kind of in the middle range of VO2 max.