Rockwell Hardness Conversion Calculator
Enter a hardness value on any supported scale and this calculator instantly converts it to all other common scales: Rockwell C (HRC), Rockwell A (HRA), Rockwell B (HRB), Brinell (HBW), Vickers (HV), and Knoop (HK). It also estimates the approximate tensile strength in MPa and ksi for non-austenitic steel. Conversions follow the ASTM E140 reference table with linear interpolation between tabulated values.
What is Rockwell hardness and how is it tested?
Rockwell hardness is measured by pressing an indenter into the surface of a material under a minor preload, then a major load, and measuring how deep the indenter penetrates after the major load is removed. The depth of permanent indentation determines the hardness number: the shallower the indent, the harder the material. Scale C (HRC) uses a diamond cone (Brale) indenter with a 150 kgf major load and is the standard for hard steels, typically heat-treated tool steels, case-hardened parts, and high-strength alloys. Scale B (HRB) uses a 1/16 inch tungsten-carbide ball with 100 kgf and is suited to soft steels, brass, bronze, and other relatively soft materials. Scale A (HRA) uses the same diamond cone as HRC but with only 60 kgf, making it suitable for cemented carbides and thin case-hardened layers.
Brinell (HBW), Vickers (HV), and Knoop (HK) scales
Brinell hardness (HBW) is determined by pressing a tungsten-carbide ball under 3000 kgf into a flat surface and measuring the diameter of the residual impression. Because the impression is visible to the naked eye, Brinell is favoured for castings, forgings, and rough surfaces. Vickers (HV) uses a square-based diamond pyramid and a wide range of test loads, producing a very small indent ideal for thin specimens, coatings, and microstructure studies. Vickers is also the closest thing to a universal scale since it is continuous from very soft to very hard without a scale change. Knoop (HK) uses an elongated rhomboid pyramid under light loads for measuring hardness of very thin layers, ceramics, and brittle materials. It is reported alongside HV in ASTM E140 as an approximate equivalent.
How to convert between hardness scales
There is no single mathematical formula that relates all hardness scales across all materials. Conversions rely on empirical reference tables compiled from measurements on a large number of test specimens. ASTM E140 (and its international counterpart ISO 18265) is the most widely recognised standard, providing tables for several material groups: non-austenitic steel, nickel alloys, cartridge brass, annealed austenitic stainless steel, copper alloys, and white cast iron. This calculator uses the non-austenitic steel table (the most common engineering reference). If your material is a nickel alloy, stainless steel, or non-ferrous alloy, the conversions are only approximate guides. The standard explicitly states that values outside the established table range should not be extrapolated.
Tensile strength from hardness
Brinell hardness has a well-documented empirical relationship to ultimate tensile strength (UTS) for non-austenitic steel: UTS (MPa) is approximately 3.45 times the Brinell number. This approximation, referenced in ASTM E140 and SAE J417, is accurate to within about 10 percent for carbon and alloy steels in the range HBW 100-400. It does not apply reliably to stainless steels, cast irons, aluminium alloys, or non-ferrous materials. The ksi value shown is the MPa result divided by 6.895.
Hardness conversion quick reference (non-austenitic steel)
| HRC | HRA | HBW | HV | HK | Tensile (MPa) | Category |
|---|---|---|---|---|---|---|
| 68 | 85.6 | - | 940 | 920 | - | Very Hard |
| 65 | 83.9 | - | 832 | 846 | - | Very Hard |
| 60 | 81.2 | - | 697 | 732 | - | Very Hard |
| 55 | 78.5 | - | 595 | 630 | - | Hard |
| 50 | 75.9 | - | 513 | 542 | 1772 | Hard |
| 45 | 73.1 | - | 446 | 466 | 1551 | Hard |
| 40 | 70.4 | - | 392 | 402 | 1379 | Medium Hard |
| 35 | 67.9 | - | 345 | 351 | 1207 | Medium Hard |
| 30 | 65.3 | - | 302 | 311 | 1034 | Medium Hard |
| 25 | 62.8 | - | 266 | 278 | 862 | Soft-Medium |
| 20 | 60.5 | - | 238 | 251 | 738 | Soft-Medium |
Approximate values from ASTM E140. All conversions are for non-austenitic steel unless otherwise stated. Values are linear interpolations and may differ slightly from the full table.
Frequently asked questions
What is the difference between HRC and HRB?
HRC (Rockwell C) uses a diamond cone indenter and a 150 kgf major load. It is used for hard materials such as heat-treated steels, typically above HRB 100 or about HRC 20. HRB (Rockwell B) uses a 1/16 inch steel or tungsten-carbide ball and a 100 kgf load. It is designed for soft to medium materials: soft steels, brass, aluminium, and similar. When a material reads above HRB 100, you should switch to the HRC scale; when it reads below HRC 20, HRB is the appropriate scale.
Why can I not get an HRC reading from an HRB input here?
The two scales intentionally overlap only in a very narrow range. HRB 100 corresponds roughly to HRC 20. Below HRB 100 the material is too soft for meaningful HRC readings; the Rockwell standard does not define HRC equivalents for most of the HRB range. This calculator reports the scales that are physically valid for the input you provide.
Are hardness conversions exact?
No. Hardness conversions are approximations, and ASTM E140 explicitly states that it is not possible to give confidence limits for the errors. Different heat treatments, surface conditions, specimen thickness, and alloy compositions all shift conversion values. Treat converted numbers as engineering guides rather than precise equivalents. For critical applications, measure hardness directly on the required scale.
Which hardness scale is best for thin coatings or surface layers?
Vickers (HV) and Knoop (HK) both allow very light test loads, which means a smaller indent and a shallower measurement depth. For surface coatings, case-hardened layers, and thin specimens, micro-Vickers (loads of 0.01-1 kgf) or Knoop is preferred. Brinell and Rockwell indentations are too deep for thin layers and will include the substrate material in the result.
What material does this calculator assume?
The reference tables used here are the ASTM E140 values for non-austenitic steel, which is the standard choice for carbon and alloy steels. For nickel alloys, austenitic stainless steels, copper alloys, or aluminium, the actual conversion values will differ. Those materials have separate annexes in ASTM E140; if you are working with them, consult the relevant annex or a material-specific chart.
Can I convert from Vickers HV to Rockwell HRC using this calculator?
Yes. Select "Vickers (HV)" as the input scale, enter your HV value, and the calculator will look up the corresponding HRC from the ASTM E140 table (valid when HV is above about 238, which is the lower limit of the HRC range). If your HV value is below 238, the material is in the HRB range and HRC is not reported.