Rmax is the measure of maximum roughness. It is the distance, in height, measured from the top of the highest peak to the bottom of the lowest valley of a surface texture. Rmax is measured in either microinches (one-millionth of an inch ) or microns (micrometers, or one-millionth of a meter).
Rmax is determined by a profilometer, a hand-held device which measures the physical depth of surface irregularities using a brush or diamond stylus. Since maximum roughness is a measure of the extreme depth, not an average, one deep scratch can greatly affect the Rmax.
There are other methods of measuring roughness, such as Ra, Rz, Rp, Rq, Rx and Ry, though the most common method is Ra or roughness average. Rmax only measures the vertical component, or height, of the surface texture, though other parameters, such as waviness, hybrid and spacing measure the surface in two and three dimensions.