The hardness conversion calculator above takes one indentation hardness reading and gives you the equivalent on every other common scale, plus the tensile strength that usually accompanies it in steel. It follows the relationships published in the standard steel conversion tables, interpolating between the tabulated rows rather than rounding you to the nearest one, so a reading of 45.5 HRC does not silently become 45.
Arb Digital builds free tools that do one job properly and say plainly where that job ends. The most important thing on this page is not the number the calculator returns. It is the explanation below of why that number carries an uncertainty that no calculator can remove, and why converting a measured value is something metallurgists actively discourage in most situations.
What This Hardness Conversion Calculator Does
It converts between six indentation and rebound hardness scales: Rockwell C, Rockwell B, Brinell with a 3000 kgf load and a 10 mm carbide ball, Vickers, Knoop at a 500 gram-force load, and the Shore scleroscope. It also reports an approximate ultimate tensile strength in megapascals, thousands of pounds per square inch, or kilograms-force per square millimetre.
The underlying data is the correlation set for non-austenitic steels: carbon steels, alloy steels, tool steels, and wrought irons in the annealed, normalised, quenched and tempered conditions. That is the population the conversion tables were built from, and it is the only population they are properly valid for. Austenitic stainless steel, nickel alloys, cartridge brass and aluminium each have their own separate tables, because the same Vickers number in a different alloy family sits against a different Rockwell number.
The calculator interpolates linearly between the tabulated points. Where a scale runs out — Rockwell B has no meaning above roughly 100 and Rockwell C is unreliable below about 20 — the corresponding output is shown as a dash rather than an extrapolated guess. A dash is the honest answer, and a tool that manufactures a number there is worse than useless.
How to Use It
- Pick the scale the reading was actually taken on. Not the scale the drawing calls out. If the part was tested on Rockwell C and the print specifies Brinell, the measured scale is Rockwell C.
- Enter the value as reported, including the decimal. Rockwell scales are normally quoted to the nearest half point and the tool interpolates properly between table rows.
- Choose the scale you want as the headline. Everything else is reported in the grid underneath, so you do not have to convert twice.
- Treat the tensile strength as an estimate for steel only. It is a correlation with a real scatter band, not a substitute for a tensile test.
- Check that the result falls inside the sensible range of the target scale. A Rockwell C conversion below 20 or a Rockwell B conversion above 100 is a signal that you are using the wrong scale for that material, not that the material is unusual.
How Hardness Conversion Actually Works: There Is No Formula
This is the part most conversion pages skip. There is no equation that turns Vickers into Rockwell. The conversion is a table, and the table is experimental data. Somebody took a large batch of steel specimens across the full hardness range, tested each one on several scales, and tabulated which readings tended to occur together. ASTM E140, Standard Hardness Conversion Tables for Metals, is the document that publishes those relationships and defines which table applies to which alloy family.
The reason no formula exists is that the scales do not measure the same physical quantity. Vickers and Knoop are true pressure measurements: an indenter of known geometry is pressed in with a known force, the diagonal of the resulting impression is measured optically, and the hardness is the load divided by the contact area. Brinell works the same way with a ball rather than a pyramid. Rockwell measures something else entirely — it reads the depth difference between a minor preload and a major load, and then converts that depth into an arbitrary number on a scale that runs backwards, so deeper means softer. Shore scleroscope measures none of these; it drops a diamond-tipped hammer and reads the rebound height, which depends on elastic recovery as well as plastic resistance.
Because a depth measurement and an area measurement respond differently to the same material, the relationship between them is a curve fitted to data rather than an identity. Work the default. A reading of 45 HRC interpolates to roughly 446 HV, 421 HB and 466 HK, with an estimated tensile strength near 1,480 MPa, about 215 ksi. Those numbers travel together in the standard steel table. They are not equal to each other in any physical sense.
Why the Standard Warns Against Converting Measured Data
ASTM E140 carries a caution that is worth reading twice: conversions should be applied mainly to values such as specification limits established by agreement, and the conversion of actual test data should be avoided wherever possible. That is a strong statement from a standards body about its own tables, and it exists because the scatter is real.
The tables were built from averages. Any individual heat of steel, with its own grain size, carbide distribution and residual stress state, sits somewhere in a band around that average. In the middle of the Rockwell C range a conversion is typically good to within one or two HRC points for ordinary carbon and alloy steel, which is usually fine. Near the ends of the range it is worse, and for materials outside the table's alloy family it can be wildly wrong.
The practical consequence is a workflow rule rather than a maths rule. If a drawing specifies 58–62 HRC, test on Rockwell C. Do not test on Vickers and convert, because you have then introduced a conversion uncertainty on top of the measurement uncertainty, and a part sitting at the edge of the tolerance band can be passed or failed by the conversion rather than by its actual condition. Convert when you need to understand a number, not when you need to accept or reject a part.
Choosing the Right Scale in the First Place
Scale selection is driven by how hard the material is, how thick it is and how big a mark you can tolerate. Rockwell B uses a 1.588 mm ball and a 100 kgf load and covers soft to medium materials: mild steel, annealed alloy steel, brass, aluminium alloys. Rockwell C uses a diamond cone at 150 kgf and covers hardened steel from about 20 to 70. Between them they span nearly everything, which is why they dominate production testing.
Brinell uses a 10 mm ball and leaves a large impression, which makes it excellent for castings, forgings and anything coarse-grained, because the indenter averages over many grains. That same large impression makes it useless for thin sections, small parts or finished surfaces. Vickers uses one indenter geometry across the entire hardness range from soft lead to the hardest carbide, which is what makes it the reference method in laboratories, but it requires optical measurement of the diagonal and is slower.
Knoop uses an elongated pyramid that penetrates shallowly for a given diagonal length, so it suits thin coatings, case-hardened layers and brittle materials. The Shore scleroscope is portable and non-marking, which is why it survives for large in-service components, but its rebound principle makes it sensitive to specimen mass and support stiffness in a way the indentation methods are not. NIST publishes Hardness Standard Reference Materials — certified test blocks — precisely because every one of these machines drifts and has to be verified against a traceable artefact.
Hardness and Tensile Strength: Where the Correlation Breaks
The rough rule that ultimate tensile strength in MPa equals about 3.3 times the Brinell number is genuinely useful for steel, and it is what the tensile column here is built on, refined by the tabulated values. It works because both properties are dominated by the same thing: resistance to plastic flow. An indenter and a tensile specimen are both asking the material how hard it is to move dislocations.
It breaks in three situations. In heavily work-hardened material the surface is harder than the bulk, so the reading overstates the tensile strength of the section. In other alloy families the correlation constant shifts substantially, which is exactly why separate tables exist. And above roughly 60 HRC the correlation is meaningless, because a steel that hard fractures before reaching a proper ultimate tensile stress. Hardness keeps rising; usable tensile strength does not.
None of this touches yield strength, which is the number most designers actually want. Hardness correlates with ultimate strength far better than with yield, because the indentation involves large plastic strain rather than the onset of yielding. If you need a yield figure, the stress and strain calculator and the Young's modulus calculator deal with the elastic side of the same material, and neither can be derived from a hardness test.
Surface Effects That Corrupt a Reading
A hardness test is a surface measurement, and several ordinary things make the surface unrepresentative. Decarburisation from heat treatment leaves a soft skin on carbon steel that can read ten HRC points low if it is not ground off. Case hardening does the opposite: a nitrided or carburised part has a hard shell over a soft core, so the reading depends entirely on how deep the indenter goes, which means the scale and load choice change the answer.
Thickness matters more than people expect. The specimen must be at least ten times the indentation depth, and for Rockwell C on thin stock that is frequently violated; when it is, the anvil beneath is being measured too and the reading comes out high. Curvature biases results as well, since a convex surface reads low. Surface finish matters far more for Vickers and Knoop, where a diagonal is measured optically, than for Brinell.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Converting a measured value to accept or reject a part — test on the scale the drawing specifies. Converting adds an uncertainty band on top of the measurement and can flip a borderline result.
- Using the steel table for another alloy family — austenitic stainless, nickel alloys, brass and aluminium each have their own conversion tables and the same HV maps to a different HRC in each.
- Quoting Brinell without the load and ball — HB depends on the test conditions, and a 3000 kgf carbide-ball figure is not interchangeable with a 500 kgf steel-ball figure on the same part.
- Reading through a decarburised or plated surface — grind or machine away the affected layer first, otherwise you are measuring the skin rather than the material.
- Ignoring specimen thickness — if the part is thinner than roughly ten times the indent depth, the anvil is contributing and the reading comes out too hard.
Related Free Tools From Arb Digital
Hardness sits alongside the properties you get from a tension test, and the stress and strain calculator handles stress, strain and the elastic region directly, while the Young's modulus calculator covers stiffness, which hardness tells you nothing about. If you are sizing the section rather than checking the material, the section modulus calculator and the beam deflection calculator are the next steps. For fasteners, the bolt torque calculator ties material grade to clamp load, and the material weight calculator gives the mass of the stock you are specifying. The full free online tools hub lists everything Arb Digital publishes.
Frequently Asked Questions
No. The relationship is empirical and published as a table of experimentally paired readings, not as an equation. Any formula you see is a curve fitted to that table, and it inherits all of the table's scatter and all of its alloy-family restrictions.
In the middle of the range for ordinary carbon and alloy steel, typically within one or two Rockwell C points. Near the ends of a scale and for materials outside the table's alloy family it is considerably worse, which is why standards discourage converting measured test data.
Not from this table. It covers non-austenitic steels. Austenitic stainless, nickel alloys, cartridge brass and aluminium each have their own separate conversion tables because the same Vickers reading corresponds to a different Rockwell reading in each family.
They use different indenters and loads. Rockwell B uses a ball at 100 kgf and loses resolution once the material is hard enough to resist it, while Rockwell C uses a diamond cone at 150 kgf and is unreliable below about 20 because the indentation becomes too shallow to read precisely.
For steel, yes, approximately. Ultimate tensile strength in MPa is roughly 3.3 times the Brinell number, because both properties are dominated by resistance to plastic flow. It is an estimate with a real scatter band, it does not apply above about 60 HRC, and it says nothing about yield strength.
Hardness testers drift, indenters wear and anvils deflect. This is why certified reference blocks exist: a machine is verified against a traceable artefact of known hardness, and an unverified machine can easily be several points out.
It matters most for Vickers and Knoop, where the impression diagonal is measured optically and a rough surface makes the edges hard to resolve. Brinell is far more tolerant because the impression is large. Decarburised, plated or work-hardened surfaces distort every method.
This tool is provided for educational and reference use. Hardness conversions are approximate correlations and must not be used to accept or reject parts against a specification; test on the scale the specification calls out.
