Predicts secondary structural content from a circular dichroism (CD) spectrum. K2D takes advantage of a recent notable development in the field of CD spectra estimation from structural data, DichroCalc, and of the high number of proteins in the Protein Data Bank (PDB), which covers a large range of structural configurations. When the size of the protein is included in the algorithm, K2D produces results comparable to other methods in terms of performance for the 190–240 nm interval and improves the performance for the 200–240 nm interval. K2D represents an improvement mostly in the predictions of beta-sheet content. Thus, we recommend using K2D if the size of the protein is known, especially if the CD spectrum is restricted to the 200 nm and up wavelength interval.