Developing a uniform and reliable way to measure hardness that can replace the many hardness scales available has been a discussion in the metrology field for a long time. However, little progress has been made in finding a solution. The Rockwell hardness scale, the Vickers hardness scale, the Brinell hardness scale, the Mohs hardness scale, the Knoop hardness scale, and the Shore hardness scale each come with pros and cons, and creating a uniform method that combines them may be impossible. Therefore, conversion between the different hardness scale remains common practice. The central problem with conversion is there is no definitive way to clearly convert one hardness scale value to another without a small amount of variation. This variation can have a lasting impact on acceptance or rejection of parts with specified requirements. Conversion tables exist and are currently the best option. Importantly, knowing and understanding the differences between hardness scales and how hardness is tested will be crucial to making the best conversion decisions possible and ensuring as much accuracy as possible.