A hardness conversion calculator is a lookup-and-interpolation tool that turns one steel hardness value you already know into the equivalent value on every other major scale, plus an approximate ultimate tensile strength in MPa and ksi. The tool accepts a single number in any one of four input scales: Rockwell C (HRC), Rockwell B (HRB), Vickers (HV) or Brinell (HB). As soon as you type, it returns the matching Rockwell, Vickers, Brinell and approximate tensile figures from the same standard steel table, which is built from ASTM E140 and SAE J417 non-austenitic conversion data. There is no exact equation, because each scale measures a different combination of material behavior, and the relationship is necessarily empirical. A steel-grade-specific table with linear interpolation between anchor rows is the standard way engineers reconcile readings taken on different machines, on the shop floor on a phone, in an engineering office, or pasted into an inspection log. That cross-reading is what a hardness conversion calculator does in practice: not a closed-form calculation, but a published table you can run in a browser.
What a Hardness Conversion Calculator Actually Does
The point of a hardness conversion calculator is reconciliation. A shop might report a part as "HRC 40", a vendor's mill certificate might quote the same batch as "HB 371", and a quality engineer might want to know what either of those numbers means in HV or as an ultimate tensile strength in MPa. Rather than thumbing through a paper table or a PDF chart, you open the tool, choose the scale you know, and let it cross-read the other three plus the tensile estimate. Because the calculation runs in the browser and updates as you type, the same workflow works on the shop floor on a phone, in an engineering office, or embedded in a spreadsheet-backed inspection record.
The output is approximate by design. The four scales do not measure identical things, so no closed-form formula can map one to another with exactness. The calculator stores discrete anchor points from a single standard table and linearly interpolates between them: an input that falls exactly on an anchor returns the published value, an input between anchors is interpolated proportionally, and an input past the end of a scale shows an out-of-range message instead of being extrapolated into a fabricated number. There is no submit button, nothing is uploaded, and the answer updates as you type, which is why the tool feels as quick as a calculator while still being anchored to a real standard.
The Four Hardness Scales You'll See in the Tool
The conversion table is built around four indenter-based scales. Each uses its own geometry, load and applicable hardness range, which is why the four scales cannot be reduced to one universal formula.
| Scale | Indenter | Load | Typical range | Common use |
|---|---|---|---|---|
| HRC (Rockwell C) | Diamond cone | 150 kgf | ≈ HRC 20 to 68 | Hardened steels |
| HRB (Rockwell B) | 1/16 in steel ball | 100 kgf | Up to ≈ HRB 100 | Sofer steels |
| HV (Vickers) | Diamond pyramid | Variable | Defined across the whole scale | Reference scale, used to back the table |
| HB / HBW (Brinell) | 10 mm ball (steel or tungsten-carbide) | 3000 kgf | ≈ HB 100 to 650 (HBW above) | Forgings, castings, coarse-grain steels |
Vickers is the most general of the four because the diamond pyramid geometry produces a defined hardness number across virtually the whole range. That generality is why engineers tend to treat HV as the backbone of the conversion table and report everything else in HV when accuracy matters. Rockwell C is undefined at the soft end and saturates near the top of its scale, so a value below about HRC 20 will read as out of range for that scale. Rockwell B saturates near HRB 100, so values harder than that fall outside HRB and the calculator marks the scale as not defined for that level. Brinell is quoted from about HB 100 to 650 on the standard steel table, and a tungsten-carbide ball (HBW) is needed above roughly HB 650 because the steel ball itself would deform past that point.
Why Hardness Conversion Is Always an Estimate
Each test presses a different indenter into the surface and measures a different combination of plastic deformation, elastic recovery and geometry of the impression. Diamond-cone Rockwell C, ball-indenter Rockwell B, pyramidal Vickers and spherical Brinell do not measure the same property, only correlated ones. There is no exact closed-form equation that converts one scale to another; the relationships come from empirically measuring the same specimens on several machines and tabulating the results.
The published charts used across engineering references trace back to a small set of large interlaboratory programs on non-austenitic carbon and low-alloy steels. The standard reproductions of that work live in ASTM E140 (Standard Hardness Conversion Tables for Metals) and SAE J417. The calculator stores anchor rows from those standards and interpolates linearly between them, which is why a value at an anchor returns the published number and a value between anchors is returned proportionally. The anchors are well documented: HRC 60 lines up with about HV 697 and HB 654, HRC 40 with HV 392 and HB 371, and HRC 20 with HV 238, HB 222 and HRB 97. For a deeper look at how the same steel table is presented as a chart, the Hardness Conversion Chart for Steel: HRC, HRB, HV, HB guide lays the same anchors out side by side.
Because the table is empirical, the calculator deliberately refuses to extrapolate. Inputs past the ends of a scale come back with an out-of-range notice; scales that do not apply at that hardness return "not defined". That boundary handling matters in inspection: a converted number for a scale that does not apply to the material is a manufactured figure, and the tool's job is to flag the limit instead of producing a number that looks plausible.
How to Use the Hardness Conversion Calculator
The tool is designed for a single-input, single-output workflow. Pick the scale you already have on the part or test report, type the number, and read the other three scales plus the tensile estimate as the result updates in place. There is no submit button; the result table refreshes as you type.
- Open the Hardness Conversion Calculator in your browser. Nothing is uploaded to a server.
- Pick the input scale that matches what you already measured: HRC for hardened steels, HRB for softer steels, HV for a Vickers reading, or HB for a Brinell reading.
- Type the measured hardness number into the value box. The accepted range for that scale is shown directly beneath the input so you stay inside it.
- Read the equivalent values on the other three scales (Rockwell, Vickers, Brinell) plus the approximate ultimate tensile strength in MPa and ksi as the result updates.
- If the input is past the end of the chosen scale, treat the result as out of range and re-check the original reading or use a different one. If a target scale returns "not defined", the scale does not cover that hardness and you should not treat the blank as zero.
For a worked example, take HRC 40. That input lands exactly on the HRC 40 anchor row of the table, so the calculator reads the anchor row directly from the published table without interpolation: HV 392 and HB 371 come straight from the anchor row, and HRB is left as "not defined" because Rockwell B does not cover that hardness range. The tensile estimate is drawn from the same anchor row and reported in both MPa and ksi alongside the other scale equivalents. That anchor step is the cleanest way to see the calculation in a verification sense, since no interpolation is involved.
Which Materials the Conversion Table Is Calibrated For
The anchors in the table are taken from non-austenitic carbon and low-alloy steel programs. That calibration is what makes the conversions trustworthy for ordinary structural steel, alloy steel bar, plate and most hardened-steel components. The numbers get less reliable when the material moves off that baseline.
Austenitic stainless steel, for example, work-hardens very differently from carbon steel, so a converted value can drift by a wide margin. Tool steel, hardened surfaces and case-hardened layers have gradients that defeat a single-number reading. Cast iron has a graphite-flake structure that changes the way the indenter sits, and non-ferrous metals such as aluminum, copper and brass follow completely different hardness relationships. For any of those, the conversion is a rough guide rather than a measurement, and the safe move is to test on the actual scale rather than trust a cross-read. The tensile-strength row is a similar case: it tracks hardness closely enough for carbon and low-alloy steel to be useful, but it is only an estimate and is not a substitute for a real tensile test on a critical part.
Reading the Out-of-Range and 'Not Defined' Responses
Two boundary conditions show up in real inspection work. The first is out-of-range, where the input itself is outside the table for that input scale, say an HRC 12 on a part that should be reporting HRC 40. The tool shows an out-of-range notice rather than extrapolating, because a fabricated number outside the table is worse than no number. The second is undefined scale, where the input is valid for the input but the scale to which you are trying to convert does not apply at that hardness. Rockwell B on a hardened HRC 60 part is the standard example: HRB is calibrated only up to about 100, so the tool marks HRB as not defined rather than inventing one.
Treating those messages as part of the result, rather than as errors, is what keeps the conversion trustworthy. A blank scale in the output means the scale does not cover that hardness; an out-of-range notice means the input is not supported. Either way, the safe next step is to go measure on a scale that does apply to the material and hardness of the part rather than to trust a number the table cannot honestly produce. For related steel calculations, a separate Brinell Hardness Calculator can be used to work HB out from an indentation when the load, ball and indent are known.