Recognition of the importance of soil compaction is increasing, but instrument cost, measurement repeatability and data interpretation limit its measurement on agricultural and rangelands. The dynamic penetrometer described here follows ASAE standards but replaces the proving ring with a strike plate, a shaft extension and a sliding hammer. The penetrometer cone eis pushed into the soil by successive hammer blows. Penetration resistance is calculated as the work by the soil needed to stop cone movement divided by the penetration distance. The work by the soil is defined as the kinetic energy of the hammer when it impacts the strike plate. Construction cost is approximately US$100. The standard drop height and hammer mass ensure measurements are consistent between operators.