Accumulated herbage mass (AHM) availability is a determining criterion for efficient pasture management and use. The Rising Plate Meter (RPM) is a tool for estimating this parameter; to do so, equations need to be developed for the pasture conditions in which it will be used. The objective of this study was to develop a calibration equation for the RPM to determine AHM in mixed pastures of Lolium perenne L. and Cenchrus clandestinus. The experiment was carried out at the San Pedro farm located in the municipality of Cota, Cundinamarca Department, Colombia. Sixteen experimental paddocks were used, composed of 30% L. perenne and 70% C. clandestinus; 25 samples per paddock were taken on a daily basis throughout the experimental period. A linear regression analysis was performed for compressed height (CH) and AHM expressed in kg DM ha-1. The resulting equation for the RPM calibration showed a high level of precision and indicated that AHM (kg ha-1) is highly dependent on CH. Given the level of accuracy of the regression equation developed in this study (R2 = 0.85), the RPM can be a useful tool for estimating AHM at a field level for mixed grasslands composed of L. perenne and C. clandestinus.