The 2017 Australian and Canadian 24-hour movement guidelines recommend infants receive 30 minutes of tummy time daily. Currently, there are no validated objective measurement tools or devices to assess tummy time. The purpose of this study was to: 1) test the practicality of using devices on infants as an objective measure of tummy time, and 2) test the accuracy of developed algorithms and cut-points for predicting prone posture. Thirty-two healthy infants aged 4 to 25 weeks completed a protocol of 12 positions. Infants were placed in each position for 3 minutes while wearing a MonBaby (chest), GENEActiv (right hip) and two ActiGraphs (right hip and ankle). Direct observation was the criterion measure. The accuracy of the algorithms or cut-points to predict prone on floor, non-prone and prone supported positions were analyzed. Parents also completed a practicality questionnaire. Algorithms and cut-points to classify posture using devices from MonBaby, GENEActiv and ActiGraph (hip and ankle) were 79%, 95%, 90% and 88% accurate at defining tummy time and 100%, 98%, 100% and 96% accurate at defining non-prone positions, respectively. GENEActiv had the smallest mean difference and limits of agreement (-8.4s, limits of agreement [LoA]: -78.2 to 61.3s) for the prone on floor positions and ActiGraph Hip had the smallest mean difference and LoA for the non-prone positions (-0.2s, LoA: -1.2 to 0.9s). The majority of parents agreed all devices were practical and feasible to use with MonBaby being the preferred device. The evaluated algorithms and cut-points for GENEActiv and ActiGraph (hip) are of acceptable accuracy to objectively measure tummy time (time spent prone on floor). Accurate measurement of infant positioning practices will be important in the observation of 24-hour movement guidelines in the early years.