Despite proprietary algorithms to account for differences, output from activity trackers worn on different wrists may not be comparable because individuals vary in their reliance on each hand during free-living activities. Participants (n = 48) wore Fitbit Flex or Flex2 monitors on each wrist for three days. T tests, equivalence tests, and correlations were used to compare steps, Calories, distance, active minutes, and sleep duration recorded by dominant and nondominant wrist-worn monitors and effect sizes and mean absolute and percent difference were calculated. The nondominant Flex2 monitor was not equivalent to the dominant wrist-worn monitor and recorded significantly more steps/day (absolute difference = 708), miles/day (0.3), and active minutes/day (7.9) than the dominant Flex2 monitor. For all variables, nondominant and dominant output was correlated (r>0.75). Nondominant and dominant Flex2 monitors are significantly different, but there were small differences for Flex monitors. Research should investigate effects on behavior and replicate findings using other monitors.