The variation within production agriculture fields changes from year to year. Fast, real time, and inexpensive methods in precision agriculture are being developed to account for in-field variation in crop management systems. Research has found that Normalized Difference Vegetation Index (NDVI) times height is a good predictor of forage yields and N uptake. This study was initiated to establish a relationship between plant height measured by hand and calibrated sonar. A total of 50 in-field observations were taken at several growth stages for corn (Zea Mays L.), cotton (Gossypium spp.), sorghum (Sorghum bicolor), and winter wheat (Triticum aestivum L.) in Stillwater, OK from 2006 to 2007. Height measurements were determined by hand readings taken with a ruler and sonar readings taken with a MassaSonic sonar. Hand and sonar height measurements were regressed to determine the coefficient of determination (r²). In 2006, data collected from 23 to 73 days after planting (DAP) showed that 95% of the variation in actual cotton height could be explained by the sonar readings. The r² obtained from hand and sonar heights were 0.81 and 0.42 for winter wheat (128 to 151 DAP) and for sorghum (22 to 72 DAP), respectively. Establishing the relationship between hand and sonar measurements is essential to include plant height in “on the go” predictions of forage biomass and N fertilizer rate requirements.