# Surface roughness

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The surface roughness Rq is used for the calculation of the specific lubricant film thickness L. It is a root mean square roughness Rq or RRMS. According to [Niemann](bibliography.md#niemann_winter_bd1_2005), an approximation is given as Rq ≈ 1.25∙Ra.

According to [Baalmann](bibliography.md#baalmann_1994), an estimate for a usual composite roughness is given by $R_q = 1.2\bullet 10^{-5}\bullet D_{pw}^0.55$ . This formula according to [Baalmann](bibliography.md#baalmann_1994) is used when the sizing button next to the input field is clicked.