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Who uses it: materials-science and quality-assurance laboratory staff, and anyone specifying or checking metals and polymers. After reading you should be able to say which hardness test measures what, how Rockwell scales are built, and what keeps a tester’s results trustworthy.
What is a hardness tester?
A hardness tester is an instrument that presses a defined indenter into a material under a defined force and turns the response into a hardness number. Indentation tests such as Rockwell, Brinell and Vickers are the common examples. The number has no independent physical definition: NIST notes that there is no independent way to measure Rockwell hardness, so the value depends on the test-method definition (NIST SP 960-5, section 2.2). That is why the standard, the scale and the verification routine matter as much as the instrument itself.
How a Rockwell tester works
NIST describes the Rockwell test as based on the difference in indenter depth at two specific times in the test cycle. The indenter contacts the material, a preliminary force (minor load) is applied and held, and the depth is recorded. An additional force brings the load to the total force (major load); the force is then removed back to the preliminary force. The Rockwell hardness number comes from the depth difference with the preliminary force maintained, and it is reported without units (NIST SP 960-5, sections 2.2 and 2.4).
Because the depth is read by the machine during the cycle, a Rockwell tester does not need an optical measurement of the indentation afterwards. In Brinell and Vickers testing, by contrast, the size of the indentation is measured following the indentation process (NIST SP 960-5, section 2.2).
Types of hardness testers
| Type | What is measured | Source note |
|---|---|---|
| Rockwell | Difference in indenter depth at two points in the cycle, between preliminary and total force | NIST SP 960-5, 2.2 |
| Brinell | Size of the indentation, measured after the indentation process | NIST SP 960-5, 2.2 |
| Vickers | Size of the indentation, measured after the indentation process | NIST SP 960-5, 2.2 |
| Knoop and Vickers microindentation | Microhardness; NIST supplies reference blocks for checking microhardness testers | NIST hardness standardization page |
Shore A and D durometers, used for rubbers and plastics, are a separate family; see our guide to the durometer and the Shore A and D scales.
Rockwell scale forces
NIST states that the ASTM standard defines thirty Rockwell scales, each a combination of an indenter type and a force level, in two categories (NIST SP 960-5, section 2.3):
| Category | Preliminary force | Total forces | Intended for |
|---|---|---|---|
| Regular | 98.07 N (10 kgf) | 588.4 N (60 kgf), 980.7 N (100 kgf), 1471 N (150 kgf) | Materials of sufficient thickness (see thickness limits below) |
| Superficial | 29.42 N (3 kgf) | 147.1 N (15 kgf), 294.2 N (30 kgf), 441.3 N (45 kgf) | Thinner materials |
Standards that govern the test
NIST notes that in North America most Rockwell testing follows ASTM standards, that ISO Rockwell standards are increasingly used elsewhere, and that OIML publishes recommendations (NIST SP 960-5, section 2.5). Quote the standard and scale used alongside any result, because the same number under a different scale means something different.
How to choose a Rockwell scale
NIST lists the factors that drive scale selection (SP 960-5, section 3.1):
- the type of test material;
- its thickness;
- the area or width available for testing;
- its homogeneity;
- the limits of each scale.
Thickness is the common trap. NIST warns that thin material can flow at the anvil interface and give erroneous results, that stacking layers cannot make up for insufficient thickness, and that testing material that is too thin can damage a steel anvil.
Verification and handling
NIST recommends verifying the tester at minimum each day tests are made, for every Rockwell scale in use, and also whenever the indenter, anvil or test force changes. Periodic indirect verification uses reference test blocks (NIST SP 960-5, section 6.2). NIST produces hardness test blocks as Standard Reference Materials for Vickers, Knoop and Rockwell scales; its Rockwell blocks are transfer standards for the HRC and HRN scales (NIST hardness standardization page). For the wider idea of keeping instruments traceable, see what calibration is and standard reference materials.
Common mistakes
- Comparing numbers from different scales or different test types as if they were interchangeable.
- Testing material too thin for the chosen scale, or stacking layers to compensate.
- Skipping the daily verification, or not repeating it after changing the indenter, anvil or force.
- Reporting a hardness value without naming the scale and standard.
FAQ
What is a hardness tester?
It is an instrument that applies a defined indenter and force to a material and converts the result into a hardness number. Rockwell testers read the depth difference during the cycle; Brinell and Vickers testers measure the indentation size afterwards (NIST SP 960-5).
Is there more than one Rockwell scale?
Yes. The ASTM standard defines thirty, split into regular and superficial categories by force level (NIST SP 960-5, section 2.3).
Does the Rockwell number have units?
No. It is a hardness number reported without units (NIST SP 960-5, section 2.4).
How often should a hardness tester be verified?
NIST recommends at minimum each day tests are made, per scale used, and again after changes to indenter, anvil or test force (NIST SP 960-5, section 6.2).
Sources
- S. R. Low, Rockwell Hardness Measurement of Metallic Materials, NIST Special Publication 960-5, January 2001, nvlpubs.nist.gov (read 2026-11-07).
- NIST, hardness standardization and Standard Reference Materials, nist.gov (read 2026-11-07).
