Hardness Conversion Calculator

Hardness numbers look like units, but they are really results from different indentation tests. A Brinell value comes from pressing a ball into a surface and measuring the indentation. A Vickers value comes from a diamond pyramid and diagonal measurement. Rockwell B and Rockwell C use penetration depth under specified preliminary and major loads, with different indenters and scale ranges. Because the tests do not measure the same geometry, a hardness conversion is not like converting inches to millimeters. It is an empirical, table-based estimate.

This calculator estimates hardness among Brinell HB, Vickers HV, Rockwell B HRB, and Rockwell C HRC. It is deliberately labeled approximate because the conversion depends on material, heat treatment, microstructure, work hardening, surface preparation, and the hardness range. ASTM E140 and related standards provide comparison tables for particular metal families; this page uses a compact steel-oriented interpolation that mirrors the calculator’s compute logic. Use it to understand a supplier note, compare a rough range, or translate a familiar shop value. Do not use it as acceptance evidence for a drawing, purchase specification, or certificate.

If your task is a true unit conversion rather than a material property comparison, the pressure converter, force calculator, and density calculator are more direct tools. For tool sizing around hardened fasteners, see the Metric to SAE Calculator.

How this calculator maps scales

The form asks for a hardness value, a source scale, and a target scale. The default example is 200 HB converted to HRB. Internally, the calculator first converts the input to a reference Vickers value. For HB and HV, it treats the number as the same reference value. For HRC and HRB, it uses a short comparison table and linear interpolation. It then converts the reference HV value into the requested output scale, again using interpolation for Rockwell scales.

That means two important things. First, values outside the stored Rockwell tables are clamped to the end of the table rather than extrapolated forever. Second, the answer is only as broad as the reference table behind it. The calculator is useful for a quick steel estimate; a laboratory report, inspection plan, or heat-treat specification should cite the actual test scale and standard.

Formula

For this simplified calculator, Brinell and Vickers are treated as equivalent reference values:

$HV \approx HB$

For Rockwell scales, the calculator interpolates between neighboring comparison points:

$y = y_1 + \frac{x - x_1}{x_2 - x_1} \times \left(y_2 - y_1\right)$

When converting from Rockwell to HV, x is the Rockwell value and y is the Vickers bridge value. When converting from HV to Rockwell, the same relationship is inverted: x is the Rockwell value being solved for and y is the HV reference. The output is formatted to one decimal place.

Reference tables used by the compute logic

These are the compact table points coded in the calculator, not a full ASTM E140 table. They are included so the interpolation can be audited.

Example: converting a hardness value

The default input is 200 HB converted to Rockwell B HRB. The calculator first maps Brinell to Vickers:

$200\ HB \approx 200\ HV$

To convert 200 HV to HRB, the inverse interpolation uses the HRB table segment from 90 HRB to 100 HRB:

$90\ HRB \rightarrow 176\ HV$

$100\ HRB \rightarrow 222\ HV$

Now solve within that segment:

$HRB = 90 + \frac{200 - 176}{222 - 176} \times \left(100 - 90\right)$

$HRB = 90 + \frac{24}{46} \times 10 = 95.2173913$

The calculator formats this as 95.2 HRB. The details show 200.0 HV as the reference Vickers value, 200.0 HB as the approximate Brinell value, 95.2 HRB, and an approximate Rockwell C value. For HRC, 200 HV is below the first coded HRC reference point of 238 HV at 20 HRC, so the calculator clamps the HRC estimate to 20.0 HRC. That boundary behavior is another reason to treat cross-scale output as a comparison, not a certification.

Where hardness conversions are used

Hardness conversion is common when a supplier, heat treater, machine shop, or quality lab reports a scale that differs from the one used in a design note. Steels are often discussed in HRC after heat treatment, softer metals may be reported in HRB, and older or broader material tables may list Brinell. Vickers is a convenient reference for microhardness and international comparison. A conversion lets teams talk about the same approximate range while they decide whether a more specific test is needed.

Conversions also help during quoting. If a drawing says a wear plate should be around 40 HRC, a supplier catalog that lists HV can be screened before a technical review. In that range, the calculator table maps 40 HRC to 375 HV. That is useful context, but the purchase order should still state the required scale, method, load, and acceptance range. Surface condition, decarburization, case depth, and part thickness can all affect the measured value.

Common mistakes

• Treating hardness conversion as exact dimensional conversion.
• Applying a steel comparison to aluminum, brass, cast iron, or coatings without a material-specific table.
• Reporting a converted value as if it were directly tested on that scale.
• Ignoring scale range. HRB is for softer ranges; HRC is for harder steels.
• Forgetting that this calculator clamps outside its compact Rockwell tables.
• Over-rounding a borderline value and then comparing it with a tight drawing limit.

Sources

• WJE, ASTM E140 testing standard summary — accessible summary identifying ASTM E140 hardness conversion tables for metals.
• NIST, Hardness Standardization and Measurements — NIST hardness measurement and standardization work.
• NIST, Hardness Standard Reference Materials — traceable hardness SRMs and calibration context.
• NIST, Recommended Practice Guide: Rockwell Hardness Measurement of Metallic Materials — Rockwell hardness measurement practice and sources of uncertainty.