Measurement of forage mass is critical to the management of forage allowance in grazing systems. The rising plate meter (RPM) was developed to monitor forage mass quickly and easily; however, it must be calibrated to provide reliable calculations of forage mass and the calibration coefficients may vary across seasons. Our objective was to investigate seasonal patterns in calibrations of the RPM for estimating forage mass in six cool‐season grass swards in Ohio. The RPM reading was highly significant (P < 0.01) in explaining variation in forage mass across sites and weeks; however, the RPM × week, RPM × site, and RPM × week × site interactions (P < 0.01) for forage mass indicated the relationship between RPM reading and forage mass varied among sites and over time. At one site, the RPM to forage mass relationship was investigated for three cool‐season grass species, but the RPM × species and RPM × week × species interactions were not significant (P > 0.05). Although there was variation among and within sites for the slope coefficients of forage mass regressed on RPM reading, a broadly similar seasonal pattern was found at most sites. The slope coefficients were usually high in early spring, decreased rapidly during the first weeks of the growing season, and then increased from early summer to the autumn. We concluded the RPM should be calibrated at least monthly over the growing season to define the seasonal pattern and changes in the slope coefficients for forage mass regressed on RPM readings.