The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials.^{[1]} The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with a pressure, which also has units of pascals. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not a pressure.
The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used. When doing the hardness tests the distance between indentations must be more than 2.5 indentation diameters apart to avoid interaction between the workhardened regions.
The yield strength of the material can be approximated as:
.
where c is a constant determined by geometrical factors usually ranging between 2 and 4.
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It was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size; the impression should have welldefined points of measurement; and the indenter should have high resistance to selfdeformation. A diamond in the form of a squarebased pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression was 3/8 of the ball diameter. As two tangents to the circle at the ends of a chord 3d/8 long intersect at 136°, it was decided to use this as the included angle of the indenter. The angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load.^{[2]} Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured. The HV number is then determined by the ratio F/A where F is the force applied to the diamond in kilogramsforce and A is the surface area of the resulting indentation in square millimetres. A can be determined by the formula
which can be approximated by evaluating the sine term to give
where d is the average length of the diagonal left by the indenter. Hence,^{[3]}
The corresponding units of HV are then kilogramsforce per square millimetre (kgf/mm²). To calculate Vickers hardness number using SI units one needs to convert the force applied from kilogramsforce to newtons. To do the calculation directly the following equation can be used:^{[4]}
where F is newtons and d is millimetres.
Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440Hv30/20, where:
Vickers values are generally independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf.^{[5]}
Material  Value 

316L stainless steel  140HV30 
347L stainless steel  180HV30 
Carbon steel  55–120HV5 
Iron  30–80HV5 
