The Kendrick mass defect is defined as the difference between the exact Kendrick mass and the nominal Kendrick mass (integer):

The Kendrick scale effectively converts the mass of CH2 from 14.01565 to exactly 14. Thus, homologous series (namely, compounds with the same constitution of heteroatoms and number of rings plus double bonds, but different numbers of CH2 groups) will have an identical Kendrick mass defect. The Referenced Kendrick mass-defect (RKMD) employs additional steps to remove the mass defect of a lipid core/backbone of interest and normalizes these values such that homologous species with saturated chains have a RKMD of 0 and species with varying degrees of unsaturation have integer RKMD values of -1, -2, -3 and so on (see example of a RKMD plot below).

(where 0.013399 is the mass defect contribution of 2H)