Conversion between the different hardness scales is not exact since the definition of hardness varies with the type of test used. It is a system property not an intrinsic material property like modulus.

The Vickers hardness number HV (or H_V) is the test force divided by the sloping (i.e. surface) area of the indenter up to a depth where it has the same cross-sectional area as the residual indent:-

H_V = constant x test force/surface area of indentation.

In contrast, indentation hardness, H_IT, is defined as the test force divided by the projected area of contact:-

H_IT = F/A(h_c)

The projected contact area A(hc) is calculated from knowledge of the geometry of the indenter and stiffness of the contact

From ISO/DIS 14577-1.2 (draft indentation standard):-

The ratio of surface area (A_s) to projected area (A_p) is a constant for a perfect Vickers or modified Berkovich indenter:-

A_s/A_p = 26.43/24.5 = 1.08

The diagonal length measured in conventional Vickers tests is related to Ap by:-

d² = 2 A_p

which leads to:-

H_V = 0.0945 x H_IT (in MPa)

BUT, it is also necessary to check units. The above equation converts a Vickers hardness number to a H_IT value in MPa (as kg/mm² = 10⁶ N/m).

(note: it is common practice for HV to be quoted in MPa and HIT in GPa – so the above equation becomes:-

H_V = 94.5 x H_IT (in GPa)

 Example:- HV 700 is approximately equivalent to H_IT of 7.5 GPa)

Note that this correlation does not allow for tip blunting at small depths and the two scales diverge as the indentation contact becomes more elastic.