Article — Hardness Conversion Calculator
Hardness Conversion: How the Five Scales Compare
Hardness conversion translates a reading on one scale (Brinell, Rockwell B, Rockwell C, Vickers or Shore D) into approximate values on the others. ASTM E140 publishes the anchor tables for steels. Converted values are accurate to about 5 to 15 percent depending on alloy family, and conversions outside the published ranges should be treated as advisory only.
Hardness is not a fundamental physical property. It is whatever the test measures. A Rockwell number reflects depth of penetration under a defined load, a Brinell number reflects the diameter of an indent left by a ball, and a Vickers number reflects the diagonal of a pyramidal indent. Different geometries produce different numbers from the same metal, so converting between them depends on empirical correlations rather than first-principles math.
What is hardness conversion?
The conversion takes a reading on one scale and looks up (or interpolates) the equivalent on another. ASTM E140 lists conversion tables for steels (Table 1), nickel alloys (Table 2), cartridge brass (Table 3), and austenitic stainless steels (Table 4). Each table was built from thousands of side-by-side measurements where one sample was tested on every scale and a best fit was published.
This calculator uses Table 1 anchors and linearly interpolates between adjacent rows. The result is fast and reasonable for routine work, but it will not match published tables to the last decimal because the underlying data scatter by a few percent even within the same alloy family.
The ASTM E140 main table is built from tests on low-alloy steels in the quenched and tempered condition. A martensitic stainless or a cold-worked stainless at the same Brinell can give a Vickers reading 30 to 50 points different from the table.
Rockwell C hardness conversion
HRC dominates production hardness testing in tool and bearing steels. The test pushes a 120 degree diamond cone with 150 kgf, and the depth of penetration relative to a 10 kgf preload gives the reading. Valid HRC range is 20 to 70. Below 20 the diamond bottoms out on the soft material; above 70 the diamond starts to deform.
The polynomial HV = 1.4882 HRC squared - 16.5 HRC + 104 fits the steel anchor data inside that range with under one percent error. For Brinell the linear approximation HBW = 1743 - 11.3 HRC works between HRC 20 and 60, where the 10 mm tungsten carbide ball still produces a clean indent.
- HRC 30 ≈ HV 300 ≈ HBW 285 (mild tool steel anneal)
- HRC 45 ≈ HV 446 ≈ HBW 419 (typical machinable hard)
- HRC 58 ≈ HV 654 ≈ HBW 533 (knife edge, bearing race)
- HRC 62 ≈ HV 740 ≈ HBW 580 (HSS cutting tool)
- HRC 65 ≈ HV 832 ≈ HBW limit (extreme tool grades)
Brinell hardness conversion
Brinell hardness uses a 10 mm tungsten carbide ball pressed at 3000 kgf for 10 to 15 seconds. The indent diameter is measured with an optical eyepiece and converted to HBW through the standard formula. Because the test averages over a large area, Brinell is preferred for forgings, castings and any material with a coarse microstructure.
The valid range runs from HBW 85 to about HBW 650. Above 650 the carbide ball starts to wear unevenly and the test loses accuracy. Brinell to Vickers conversion uses HV approximately 1.055 HBW - 51, which fits steel anchors well across the range. For aluminum alloys the slope is different because aluminum work-hardens less under the ball.
ASTM E10 retired the steel Brinell ball in 2001. Any pre-2001 chart marked HBS is incompatible with current HBW readings above 200, because the steel ball deformed under load and under-reported hardness. Reject any HBS reading from a modern instrument.
Vickers hardness conversion
Vickers is the most universal hardness scale. A 136 degree diamond pyramid produces a clean square indent under any load from 10 grams (microhardness) up to 100 kgf (macrohardness). Readings span HV 5 to HV 2000 plus, covering polymers through tungsten carbide on a single scale. The result is independent of load above HV 25 except for very thin coatings or curved surfaces.
Because Vickers spans the widest range, the calculator routes every conversion through HV as an intermediate. The input value is mapped onto HV via the source-scale anchor table, then HV is mapped back to each target scale. This avoids compounding errors that would appear if you converted HRC directly to HBW without an intermediate.
HV ≈ 1.4882 HRC² - 16.5 HRC + 104 HRC 50 → HV 513 (approximate; for precise conversions consult ASTM E140)HBW ≈ 1743 - 11.3 HRC HRC 50 → HBW 478HV ≈ 1.055 HBW - 51 HBW 200 → HV 160HRC ≈ (Shore D - 10) / 2.2 Shore D 70 → HRC 27Shore D hardness conversion
Shore D was developed for hard rubber and rigid polymer testing. A spring-loaded steel cone is pressed against the sample and the depth of penetration drives a dial reading from 0 to 100. The test is fast, portable, and non-destructive, so it dominates field testing of plastics and elastomer parts.
Cross-scale conversion to Rockwell or Brinell is approximate at best. The empirical fit HRC approximately (Shore D - 10) / 2.2 works for Shore D 40 and above on hard polymers, but accuracy drops sharply outside that window. For softer elastomers use the Shore A scale and avoid Rockwell conversions entirely.
Hardness conversion and ASTM E140
ASTM E140 is the primary international reference for hardness conversion of metals. ISO 18265 covers the same ground with very similar tables. Both standards explicitly warn that converted values must not be used to qualify material against a specification that calls for a specific scale. If a drawing calls for HRC 58 to 62, the part must be tested in HRC; a Vickers reading converted to HRC will not satisfy QA.
The standards also publish uncertainty estimates. For steels in the working range, ASTM E140 gives conversion uncertainty of plus or minus 1 HRC, plus or minus 25 HV, or plus or minus 15 HBW at the 95 percent confidence level. These ranges widen at the ends of each scale.
Hardness conversion pitfalls
When in doubt, test directly. A converted hardness number carries the scatter of the original test plus the conversion uncertainty. For critical applications, run the test in the scale your specification names.
The most common errors involve thin samples, cold-worked surfaces, and the wrong alloy table. A Rockwell C reading on a sample under 1 mm thick is meaningless because the indent penetrates the support anvil. A surface that has been cold worked by grinding shows higher hardness than the bulk material; remove at least 0.1 mm before measuring. And using the steel anchor table for aluminum or copper alloys can overstate hardness by 20 percent or more.