Soil compaction is a serious issue that affects the growth of crop roots. Penetrometer resistance is often used as an indicator of the resistance of soil to root elongation. We developed a simple function to estimate penetrometer resistance from the soil compression characteristic. Five soils with contrasting textures and organic matter contents were used in this work. Air-dry soil samples were saturated on a tension table and then drained to different matric potentials not less than -30 kPa. The equilibrated soils were compacted in a uniaxial compression device to give precompression stresses in the range of 30 to 1000 kPa. The penetrometer resistance of the soils was then measured with a 2-mm-diameter 60 degrees cone penetrometer. Soil compression characteristics varied with soil texture, organic matter content, and initial soil water content. Penetrometer resistance values increased with decreasing void ratio and could be explained by the precompression stress and the slope of the compression characteristic, using a simple equation with parameters independent of soil type. Our equation explained 84% of the variance in penetrometer resistance.