Hardness Conversion: Making Sense of Rockwell, Brinell, and Tensile Strength

A drawing calls out 45 HRC. Your supplier’s mill certificate lists Brinell. A spec sheet quotes tensile strength. Are they even talking about the same steel? Usually yes — hardness scales are convertible, within limits. This guide explains what each scale measures, how to move between them, and the surprisingly useful shortcut that links hardness to tensile strength.

What Hardness Actually Measures

Hardness is a material’s resistance to localized plastic deformation — how well it resists being dented. Every common test presses a defined indenter into the surface under a defined load, then reads either the depth or the size of the dent. Because the tests differ in indenter and load, each produces its own number on its own scale.

The Three Scales You Will Meet

Rockwell (HR)

Fast, direct-reading, and the shop favorite. It measures the depth of penetration under a minor then major load. Multiple scales exist for different materials; the two you will see most are HRC (diamond cone, 150 kgf — hard steels) and HRB (1/16″ ball, 100 kgf — softer steels, brass, aluminum). Always cite the scale letter: “45 HRC” and “45 HRB” are wildly different materials.

Brinell (HB / HBW)

Presses a 10 mm tungsten-carbide ball under a heavy load and measures the diameter of the dent. The large indenter averages over a big area, which makes Brinell excellent for coarse or non-uniform materials like castings and forgings — and it is why mill certs love it.

Vickers (HV)

A diamond pyramid under a light load, measuring the dent diagonal. One continuous scale spans soft foils to hardened tool steel, which makes Vickers the natural bridge when converting between systems and the standard for thin parts and coatings.

Approximate Conversion Table (Steel)

These values follow ASTM E140, the standard conversion tables. Use them for planning and cross-checking — not as a substitute for testing on the actual scale a spec requires.

The Handy Tensile-Strength Rule

For most carbon and low-alloy steels, tensile strength tracks Brinell hardness by a simple factor:

Tensile strength (psi) ≈ 500 × HB

So a steel at 200 HB is roughly 100,000 psi (100 ksi) tensile. This approximation is genuinely useful on the shop floor for a quick sanity check, but it holds only for steel and only in the normal hardness range — it breaks down for aluminum, and at very high hardness the relationship flattens out.

Why Conversions Are Only Approximate

Each test deforms the material differently, so no exact formula ties them together — the tables are empirical fits to test data. Conversions are most reliable for plain carbon and low-alloy steels, the materials the tables were built from. They drift for austenitic stainless, non-ferrous alloys, cold-worked material, and case-hardened parts, where a surface reading does not represent the core.

Rule of thumb: If a print specifies a scale, test on that scale. Convert only to sanity-check a certificate or to compare specs — never to certify compliance.

Choosing the Right Scale for the Job

The scales are not interchangeable in practice, even where they convert on paper. Pick by the part in front of you:

• Hardened steel, thin parts, tight tolerance — Rockwell C or Vickers. Small indents read cleanly on ground surfaces and thin sections without punching through.
• Castings, forgings, coarse or heterogeneous material — Brinell. The large ball averages over grain structure and porosity, giving a number that represents the bulk material rather than one hard spot.
• Soft metals — brass, aluminum, mild steel — Rockwell B or Vickers. HRC on soft material simply bottoms out and tells you nothing.
• Coatings, case-hardened layers, weld heat-affected zones — micro-Vickers, which reads a shallow layer without sampling the softer core underneath.

Matching the test to the part is not pedantry — a hardness number taken on the wrong scale or the wrong feature is worse than no number, because it looks authoritative while being meaningless.

A Note on Portable Testers

Bench testers are the reference, but portable rebound (Leeb) and ultrasonic hardness testers let you check parts too large for the lab — incoming forgings, weldments, installed shafting. They report in their own units and convert to HRC/HB on board; just remember those readings inherit all the conversion caveats above, so treat them as a screening tool.

Convert Any Value Instantly

Skip the interpolation: our Hardness Conversion Chart converts between HRC, HRB, Brinell, Vickers, and tensile strength in one click. When that hardness feeds a fastener grade, carry it into the bolt torque calculator, and find related references under Materials & Process.