Convert a hardness reading across every common scale — Rockwell (C, B, A, D, superficial N and T), Brinell, Vickers and Knoop for metals; Shore A and D for plastics and elastomers — and estimate tensile strength. Based on standard ASTM E140 steel correlations.
One Reading, Every Scale
Hardness gets measured on a dozen different scales depending on the material, the part size and the test rig on hand. A heat-treat shop reads Rockwell C, a foundry reads Brinell, a lab reads Vickers or Knoop, an inspector checking a thin case reads superficial Rockwell. This tool takes a reading on any one metal scale and gives you the equivalent on all the others, plus an estimate of tensile strength — using the standard steel correlations of ASTM E140.
Which Scale for What
Rockwell C (HRC) is the workshop standard for hardened and tool steels, knives and heat-treated parts, valid above about 20 HRC. Rockwell B (HRB) covers softer steels, brass, copper and aluminium alloys. Rockwell A (HRA) and D (HRD) use lighter loads for thinner or harder pieces. Brinell (HBW) suits structural steel, castings and large forgings with its big ball indenter. Vickers (HV) spans the entire range, from soft metals to the hardest steels, which makes it the natural pivot for conversion. Knoop (HK) is a microhardness test for thin coatings, ceramics and small areas; at low loads it reads close to Vickers.
Superficial Rockwell: N and T
When a part is too thin or its hardened case too shallow for a standard test, the superficial Rockwell scales apply a lighter load so the indenter stays near the surface. The N scales (15N, 30N, 45N) are the superficial equivalent of the C and A range for harder material; the T scales (15T, 30T, 45T) shadow the B range for softer material. The number is the load in kilograms-force.
Hardness and Tensile Strength
For carbon and low-alloy steels, tensile strength tracks hardness closely: ultimate tensile strength in MPa is roughly 3.38 times the Brinell number, or about half a ksi per Brinell point. That is why a quick hardness test often stands in for a tensile test on the shop floor. The rule does not transfer to non-ferrous metals or work-hardened surfaces.
| HV | HB | HRC | HRA | Tensile (ksi) |
|---|---|---|---|---|
| 300 | 285 | 29.8 | 65.3 | 140 |
| 400 | 381 | 41 | 70.9 | 187 |
| 500 | 485 | 50 | 75.9 | 238 |
| 600 | 587 | 56 | 79.0 | 288 |
| 700 | 670 | 60 | 81.2 | 329 |
Shore Hardness for Plastics and Rubber
Plastics and elastomers use the Shore durometer scales instead. Shore A measures soft rubber, silicone and flexible elastomers; Shore D measures hard rubber and rigid plastics like nylon, Delrin and polycarbonate. The two overlap only at the hard end of A. Switch the tool to the Plastics and Elastomers family for an approximate Shore A to Shore D conversion. Note that durometer hardness has no physical relationship to metal scales — there is no valid conversion between Shore and Rockwell, Brinell or Vickers.
Frequently Asked Questions
How do you convert Rockwell C to Brinell or Vickers?
Use a standard correlation table such as ASTM E140. For example, 40 HRC is about 371 HB and 390 HV. This calculator interpolates the full table for you across every scale at once.
What is superficial Rockwell (15N, 30T)?
A lighter-load Rockwell test for thin parts and shallow cases. N scales cover hard material, T scales cover soft material, and the number is the test load in kgf.
Is Knoop the same as Vickers?
They are close, especially at low test loads, so this tool shows Knoop as approximately equal to Vickers. They can diverge at the extremes and for anisotropic materials.
Can you convert Shore hardness to Rockwell?
No. Shore durometer measures polymers and has no physical correspondence to metal hardness scales. Any chart claiming a Shore-to-Rockwell conversion is not metallurgically valid.
What about Rockwell G?
HRG is rarely specified and not reliably tabulated, so it is omitted here to avoid presenting false precision.
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