The grading equation is applicable to other granular soils of various types, including of course weathered residual soil like decomposed granite soil. However, the proposed equation is the grading equation of discrete distribution type. Therefore, in this study, an attempt was made to transform the equation from the discrete type to the continuous distribution type, and a grading equation of continuous type was derived. By comparing the continuous type equation with the main grading equations, it was found that this equation had a greater scope of application.