Measuring available forage mass is critical in making grazing management decisions. The rising plate meter (RPM) has been used to measure forage mass over extensive areas in an efficient and easy manner; however it must be calibrated to provide reliable estimates. Variation in RPM calibration equations within and across environments has not been reported in USA. Our objective was to document variation in weekly calibration coefficients of the RPM for estimating forage mass in six site-year environments in Ohio.  Data were analyzed to investigate differences due to environment, calendar week and environment x week interactions, and at one location for the effect of grass species on herbage mass. The statistical approach used a linear model with RPM-units as a continuous variable in addition to the other classification variables in the analysis. Significant (P <0.0001) effects were found for RPM-reading, week, environment, and environment x week. At one location there was an effect (P = 0.03) of species, but no week x species effect (P = 0.38). Most (83%) calibration equations of forage mass regressed on RPM-reading had an intercept similar to (P > 0.05) zero. Slope coefficients decreased rapidly during the first few weeks of the growing season then increased steadily during the remainder of the season, suggesting the existence of underlying processes responsible for the seasonal changes. We recommended that RPM calibration be performed frequently and by species to maximize accuracy of forage mass predictions.