Three scales, three different measurements
All three push something hard into a surface and see what happens. What they measure afterwards is not the same quantity.
- Rockwell measures depth. A diamond cone or a steel ball is pressed in under a minor load, then a major load, then back to the minor load, and the machine reads the permanent depth left behind. For the C scale one point is exactly 0.002 mm — so HRC 60 and HRC 61 differ by two micrometres of penetration. It is fast, it needs no microscope, and it is why Rockwell dominates the shop floor.
- Vickers measures force over area. A diamond pyramid with a 136° included angle leaves a square impression; you measure the diagonals under a microscope and divide the force by the surface area. The 1.8544 constant is not empirical — it is 2 sin(68°), straight out of the pyramid geometry.
- Brinell also measures force over area, but with a ball, so the impression is a spherical cap and the arithmetic is uglier. Its virtue is size: a 10 mm ball leaves a large impression that averages over a coarse microstructure, which is exactly what cast iron and coarse-grained castings need.
One scale is continuous from soft aluminium to the hardest steel — Vickers. Rockwell is a family of scales, each with its own indenter and load, because a depth measurement has a limited useful window; Rockwell B runs out around HV 240 and Rockwell C becomes unreliable below about HRC 20, which is why they overlap in the middle and neither covers the whole range.
Why there is no conversion formula
This is the part most converters skip, and it is not a technicality.
Rockwell C is linear in depth. Every 0.002 mm is one point, anywhere on the scale. Vickers is linear in reciprocal area — halve the diagonal and the number quadruples. Those are different functional forms, so no constant and no simple expression can carry one into the other.
What exists instead is measurement. Somebody took specimens, tested each one on several scales, and tabulated what corresponded to what. That is ASTM E140, and every hardness conversion anywhere — including this page — is reading off that kind of table or interpolating between its rows.
Which has three consequences worth stating plainly:
- The values are approximate, and the standard says so in its own text rather than in a footnote. Scatter of several HRC points between a converted value and a direct measurement on the same part is normal.
- They apply to the materials they were fitted to, and to nothing else.
- A converted value is not a test result. If a drawing calls for 58–62 HRC, the part is tested on the C scale. Testing Vickers and converting is a different measurement with extra uncertainty stacked on it, and an inspector is entitled to reject it.
None of that makes conversion useless — comparing a supplier’s Vickers certificate against a drawing in Rockwell is an entirely reasonable thing to want. It makes it a estimate, which is a different thing from an arithmetic conversion, and worth knowing you are doing.
The material decides, and mostly it says no
The table everybody uses is scoped to non-austenitic steels: carbon and alloy steels, tool steels, and most wrought and cast irons in the hardened and tempered condition. That covers a great deal of engineering, and it is why the table feels universal.
It is not. The common cases where it does not apply:
Materials the steel table does not cover, and why| Material | The problem |
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| Austenitic stainless | It work-hardens under the indenter, so the surface being measured is harder than the material was. A separate table exists and differs materially. |
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| Cold-worked material | The surface and the bulk have different properties, and each scale samples a different depth. |
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| Copper, brass, aluminium | Each alloy family has its own table. The steel one is wrong for them by large margins, not marginal ones. |
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| Grey and ductile cast iron | Graphite flakes mean a small indenter may land in graphite or in matrix. Brinell with a big ball is used because it averages; converting away from it reintroduces the problem it solved. |
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So the calculator asks the material first, and for anything on that list it refuses rather than quietly using the steel table. That is a deliberate choice about what a tool owes its user: the wrong number here would be indistinguishable from a right one, and a plausible wrong answer is worse than no answer.
A Brinell number needs its test conditions
Brinell has a second requirement the other scales do not: two results are only comparable if the force-to-diameter ratio was the same.
The reason is geometric similarity. A ball pressed into a surface produces a geometrically similar impression only if F/D² is held constant, so the standard defines a set of ratios and expects results to state which was used.
The standard force-to-diameter ratios| F/D² | A typical setup | For |
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| 30 | 3000 kgf on a 10 mm ball | Steel and cast iron |
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| 15 | 1500 kgf on 10 mm | Harder light alloys |
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| 10 | 1000 kgf on 10 mm | Copper and copper alloys |
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| 5 | 500 kgf on 10 mm | Aluminium alloys |
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| 2.5 | 250 kgf on 10 mm | Softer alloys |
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| 1 | 100 kgf on 10 mm | Lead, tin |
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The similarity law is checkable, and it holds: 3000 kgf on a 10 mm ball, 750 kgf on 5 mm and 187.5 kgf on 2.5 mm all give F/D² = 30 and produce the same Brinell number on the same material. That is why a portable tester with a small ball can be trusted against a bench machine.
Change the ratio and you have changed the test. 1000 kgf on a 10 mm ball is a perfectly valid Brinell test — it is the copper-alloy ratio — but its result is not interchangeable with a steel result at 30. And the Brinell scale has a ceiling regardless: above about 500 HBW the ball itself deforms, so the impression stops describing the specimen alone. That is where the Brinell column in the conversion table simply ends, rather than continuing with numbers nobody should trust.
Reading a hardness spec
A hardness callout is a scale, a range and often a test condition. Each part is doing work.
58–62 HRC — a hardened tool steel, tested on the C scale.
200 HBW 10/3000 — Brinell, tungsten carbide ball, 10 mm at 3000 kgf, so F/D² = 30.
350 HV 30 — Vickers at a 30 kgf test force. The load is part of the designation because at very light loads the number drifts upward.
Three things worth checking before treating any of it as settled:
- Is it a surface or a bulk requirement? A case-hardened gear is 60 HRC on the surface and perhaps 30 in the core. Both are correct. Which one the drawing means determines which test is valid, since a Brinell impression may punch straight through a thin case.
- Is the section thick enough? The standards require roughly ten times the indentation depth beneath the test point. Testing a thin part on a heavy scale measures the anvil as much as the part.
- Was it tested or converted? A certificate quoting HRC on a thin sheet that could only realistically have been tested on a superficial or Vickers scale has probably been converted, and carries that extra uncertainty.
The tensile estimate this page offers deserves the same caution. For steel, tensile strength in MPa is roughly 3.45 times the Brinell number, which is a genuinely useful sanity check and holds reasonably to about 400 HBW. Above that the relationship flattens as steels become less ductile, so the calculator stops offering it rather than extrapolating a rule past where it works.
Sources and methodology
The three test methods are each defined by their own standard, and those definitions are exact — the formulas on this page come straight from them. The conversion tables are a separate document, E140, and it is the one that carries the warning: the values are approximate, they are scoped to stated material families, and the standard says so itself rather than leaving anyone to discover it.