Hardness Converter
By KAMP Inc. / UnitOwl · Last reviewed:
Material hardness measures resistance to permanent surface deformation — scratching, indentation, or penetration. Because hardness is not a fundamental physical property but a response to a specific test, multiple incompatible scales exist: Rockwell, Brinell, Vickers, Knoop, and Mohs each probe hardness differently. Converting between them requires empirical tables rather than exact formulas, and the appropriate scale depends on the material, its expected hardness range, and the required precision.
Popular Hardness Conversions
Rockwell, Brinell, and Vickers: Indentation Methods
Rockwell hardness (HR) measures the depth of indentation under a fixed load using either a diamond cone (HRC, for hard metals) or a steel ball (HRB, for softer metals). It is the most common scale in industry due to its speed and ease. Brinell hardness (HB or HBW) measures the area of an indentation made by a 10mm tungsten carbide ball under a specified load — appropriate for rough or inhomogeneous materials like castings. Vickers hardness (HV) uses a diamond pyramid indenter and is accurate across a very wide hardness range, making it popular for thin materials, coatings, and heat-affected zones in welds. For steel: a hardened tool steel might be HRC 60, equivalent to approximately HV 697 or HB 615.
| Material | Hardness (approximate) |
|---|---|
| Soft copper/aluminum | HRB 40–70 / HB 60–90 |
| Mild steel (annealed) | HRB 90–100 / HB 120–150 |
| Hard steel (quenched) | HRC 55–65 / HV 600–800 |
| Carbide cutting tools | HRC 70–80 / HV 1,400+ |
| Glass | HV ~600 / Mohs 5.5 |
| Diamond | HV ~10,000 / Mohs 10 |
The Mohs Scale: Scratch Hardness for Minerals
The Mohs scale, devised by Friedrich Mohs in 1812, ranks minerals by their scratch resistance: a material can scratch anything softer than itself. The ten reference minerals are: 1 Talc, 2 Gypsum, 3 Calcite, 4 Fluorite, 5 Apatite, 6 Orthoclase feldspar, 7 Quartz, 8 Topaz, 9 Corundum (ruby/sapphire), 10 Diamond. The scale is ordinal, not linear — diamond is about 1,600 times harder than corundum on an absolute scale (Vickers), not twice as hard. A fingernail scratches at about Mohs 2.5; a steel file at about Mohs 6.5; glass at about Mohs 5.5. Mohs hardness correlates roughly with wear resistance and is widely used in gemology, mineralogy, and abrasive material selection.
Why Hardness Conversions Are Approximate
Each hardness test measures a different aspect of material behavior: Rockwell measures elastic-plus-plastic recovery depth; Brinell measures permanent indentation area; Vickers measures projected indentation area with a sharp indenter; Mohs is a scratch test. Converting between them requires empirical data tables for specific material classes, not universal formulas. Conversion tables published by ASTM (Standard E140) are valid for homogeneous carbon and alloy steels but should not be applied to cast iron, non-ferrous alloys, or coatings without appropriate material-specific data. For safety-critical applications such as aerospace components, always use the hardness test specified by the engineering drawing.
Sources & References
- NIST — Units and Conversion Factors — Official unit conversion factors from the National Institute of Standards and Technology.
- BIPM — The International System of Units (SI) — International SI unit definitions from the International Bureau of Weights and Measures.