The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale. Larger numbers correspond to harder materials.
 When testing metals, indentation hardness correlates linearly with tensile strength.
 The differential depth hardness measurement was conceived in 1908 by Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, "the cone test"). The differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections. The Brinell hardness test, invented in Sweden, was developed earlier – in 1900 – but it was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive.
 Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) from Connecticut in the United States co-invented the "Rockwell hardness tester," a differential-depth machine. They applied for a patent on July 15, 1914. The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races.  The application was subsequently approved on February 11, 1919, and holds U.S. Patent 1,294,171. At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT. New Departure was a major ball bearing manufacturer which in 1916 became part of United Motors and, shortly thereafter, General Motors Corp.
 After leaving the Connecticut company, Stanley Rockwell, then in Syracuse, NY, applied for an improvement to the original invention on September 11, 1919, which was approved on November 18, 1924.  The new tester holds U.S. Patent 1,516,207. Rockwell moved to West Hartford, CT, and made an additional improvement in 1921. Stanley collaborated with instrument manufacturer Charles H. Wilson of the Wilson-Mauelen Company in 1920 to commercialize his invention and develop standardized testing machines.  Stanley started a heat-treating firm circa 1923, the Stanley P. Rockwell Company, which operated until 2012.  The building, which still stands, was empty in 2016.  The later-named Wilson Mechanical Instrument Company has changed ownership over the years, and was acquired by Instron Corp. in 1993.
 The Rockwell hardness test can be conducted on several various hardness testers. All testers, however, fall under one of three categories. Bench model hardness testers can be found either in a digital or analog model. Digital bench models utilize a digital display and typically take more technical training to be able to operate, whereas the analog models are simpler to operate as well as very accurate and display results on a dial on the front of the machine. All bench model testers are usually found within a workshop or laboratory setting. Other testers are portable, and all portable testers will come in a digital model including a digital results screen similar to that of the bench digital model.  Portable testers are  practical and easy to use.[citation needed]
 The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load.  The minor load establishes the zero position.  The major load is applied, then removed while still maintaining the minor load. The depth of penetration from the zero datum is measured from a dial, on which a harder material gives a lower measure.  That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques. 
 The Rockwell test is very cost-effective as it does not use any optical equipment to measure the hardness based on the small indention made, rather all calculations are done within the machine to measure the indention in the specimen, providing a clear result in a manner in which is easy to read and understand once given. This also prevents any reworking or finishing needing to be done to the specimen both before and after testing. However, it is critical to double check specimens as the smallest indentions made from testing could potentially result in incorrect measurements in hardness, leading to catastrophe. After time, the indenter on a Rockwell scale can become inaccurate as well and need replacing to ensure accurate and precise hardness measurements.
 The equation for Rockwell Hardness is 
  
    
      
        H
        R
        =
        N
        −
        h
        d
      
    
    {\displaystyle HR=N-hd}
  
, where d is the depth in mm (from the zero load point), and N and h are scale factors that depend on the scale of the test being used (see following section).
 It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.
 Legacy Rockwell hardness testers operation steps:
 In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. Also, readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used if the hardness of a convex surface is to be measured.
 There are several alternative scales, the most commonly used being the "B" and "C" scales. Both express hardness as an arbitrary dimensionless number.
 The superficial Rockwell scales use lower loads and shallower impressions on brittle and very thin materials.  The 45N scale employs a 45-kgf load on a diamond cone-shaped Brale indenter, and can be used on dense ceramics.  The 15T scale employs a 15-kgf load on a 1⁄16-inch-diameter (1.588 mm) hardened steel ball, and can be used on sheet metal.
 The B and C scales overlap, such that readings below HRC 20 and those above HRB 100, generally considered unreliable, need not be taken or specified.
 Typical values include:
 Several other scales, including the extensive A-scale, are used for specialized applications. There are special scales for measuring case-hardened specimens.