Rockwell Hardness Conversion Calculator

Convert a measured indentation depth into the standardized Rockwell hardness number using the ASTM E18 formula HR = N − h ⁄ 0.002 mm, for any of the nine standard Rockwell scales (A, B, C, D, E, F, G, H, K).

Quick Facts

Core formula
HR = N − h ⁄ 0.002 mm
h is the permanent depth increase left by the major load, measured in millimeters.
Scale factor N
100 (diamond) or 130 (ball)
Diamond-cone scales (A, C, D) use N = 100; ball-indenter scales (B, E, F, G, H, K) use N = 130.
Minor load
10 kgf (98.07 N)
Applied first on every regular Rockwell scale to seat the indenter before the major load.
Reliable range
≈ 20 to 100 HR
Readings outside this band are considered unreliable; pick a scale suited to the material's hardness.

Your Results

Calculated
Rockwell Hardness
-
HR = N − h ⁄ 0.002 mm
Total Test Force
-
Scale's standard total force (10 kgf minor load included)
Indenter
-
Defines the scale's geometry
Reading Validity
-
Rockwell numbers are reliable roughly 20-100

Ready

Choose a Rockwell scale and enter the measured indentation depth, then press Calculate.

About the Rockwell Hardness Conversion

The Rockwell hardness test measures a material's resistance to indentation by pressing a diamond cone or hardened ball into the surface under a known force and measuring how deep the indenter sinks in permanently. Unlike Brinell or Vickers testing, the Rockwell method reads hardness directly from that depth — no optical measurement of an indent's diameter is needed. This calculator converts a measured indentation depth into the standardized Rockwell hardness number for any of the nine standard (non-superficial) Rockwell scales defined in ASTM E18.

The depth-to-hardness formula

Every regular Rockwell scale follows the same relationship: HR = N − h ⁄ S, where h is the permanent increase in indentation depth (in mm) caused by the major load, S = 0.002 mm is the scale's depth increment, and N is a scale constant — 100 for scales that use a diamond cone indenter (A, C, D) and 130 for scales that use a ball indenter (B, E, F, G, H, K). A shallower indentation (smaller h) produces a larger HR value, so higher numbers always mean a harder material on a given scale.

Test forces and indenters

  • Diamond-cone scales (A, C, D) use a 120° diamond cone with a 0.2 mm spherical tip (the "Brale" indenter) and a scale constant N = 100.
  • Ball-indenter scales (B, E, F, G, H, K) use a hardened steel or tungsten-carbide ball — 1/16 in (1.588 mm) for B, F, G or 1/8 in (3.175 mm) for E, H, K — with a scale constant N = 130.
  • Every regular scale applies a 10 kgf (98.07 N) minor load first to seat the indenter, then a major load; the scale's standard total test force - 60, 100, or 150 kgf depending on the scale, which already includes that minor load - is what's shown in the results.
  • The depth increment S = 0.002 mm per Rockwell unit applies to all nine regular scales; superficial scales (15N, 30N, 45N, 15T, 30T, 45T, and similar) use S = 0.001 mm and are not covered by this calculator.

Choosing the right scale and reading validity

Rockwell numbers are only meaningful within roughly 20 to 100 on a given scale — below 20 the indenter penetrates too deep for the geometry to behave consistently, and a scale with a lighter load or larger indenter (for example HRB instead of HRC) usually gives a more reliable reading. Because each scale is defined by its own combination of indenter and load, a hardness number from one scale cannot be converted to another scale by simple arithmetic — comparisons between scales, or between Rockwell and other hardness methods such as Brinell or Vickers, require the empirical conversion tables published in ASTM E140, which are calibrated to specific material types and should not be treated as universal.

Frequently Asked Questions

What does "HR = N − h ⁄ 0.002 mm" actually mean?
It converts the permanent indentation depth h (in mm) left by the major load into a hardness index. N is a fixed scale constant — 100 for diamond-cone scales or 130 for ball scales — and dividing by 0.002 mm expresses the depth in Rockwell units. A shallower indentation gives a higher HR value, so bigger numbers mean a harder material on that scale.
Why do some Rockwell scales use N = 100 and others N = 130?
The constant sets where the scale's zero point falls. Diamond-cone scales (A, C, D) are calibrated so a very hard material with almost no penetration reads close to 100 using N = 100. Ball-indenter scales (B, E, F, G, H, K) test softer materials that indent more deeply, so N = 130 keeps typical readings within the same 0-100 window.
Can I convert an HRC reading directly into HRB or Brinell hardness?
Not with a simple formula. Each Rockwell scale uses a different indenter and load, so the relationship between scales depends on how a specific material deforms — it is not a fixed geometric ratio. Standardized approximate conversions between scales, and to Brinell or Vickers hardness, are published as empirical tables in ASTM E140 for particular material classes, mainly steels, and should not be applied outside that scope.
Why is a Rockwell hardness number reported without physical units?
The result is an index built from a depth measurement divided by 0.002 mm and subtracted from a fixed constant N, so the millimeter units cancel out. That's why a Rockwell value is always written with its scale letter attached, such as 62 HRC, rather than with a physical unit — the number is meaningless without knowing which scale produced it.

Related calculators