Can you trust the calories on your watch? See how far off it can be

Your watch says you burned 412 calories, and it feels exact. It isn't. Wrist devices measure heart rate well, but calories burned is a formula guess built around an average body.

In one Stanford study of seven popular devices, the best was off by 27 percent and the worst by 93. Slide the error band on a real workout to see how wide the range gets, and learn why eating back the watch's number is a trap.