The purpose of this study was twofold: (1) develop a refined 2-regression model for the Actical which predicts METs every 15 s, and (2) compare the refined and 2008 Crouter 2-regression models and the Klippel and Heil equations during free-living activity. To develop the refined 2-regression model, 48 participants (mean ± SD; age 35 ± 11.4 years) performed 10-min bouts of various activities ranging from sedentary to vigorous intensity. An Actical accelerometer was worn on the left hip, and a Cosmed K4b(2) was used to measure oxygen consumption. For the free-living measurements, 29 participants (age, 38 ± 11.7 years; BMI, 25.0 ± 4.6 kg m(-2)) were monitored for approximately 6 h during work (N = 23) or leisure time (N = 9) while wearing an Actical and Cosmed. Actical prediction equations were compared against the Cosmed for METs and time spent in sedentary behaviors, light physical activity (LPA), moderate PA (MPA), vigorous PA (VPA), and moderate and vigorous PA (MVPA). The refined 2-regression model developed used an exponential regression equation and a linear equation to predict METs every 15 s for walking/running and intermittent lifestyle activities, respectively. Based on the free-living measurement, the refined 2-regression model was the only method that was not significantly different from the Cosmed for estimating time spent in sedentary behaviors, LPA, and MVPA (P > 0.05). On average, compared to the Cosmed, the refined 2-regression model and the Klippel and Heil equations had similar mean errors for average METs.