The average internal magnetic field Hh inside a multidomain (MD) grain is the sum of the applied field Ho and a demagnetizing field involving the demagnetizing factor N of the grain. It is shown that NMD governing MD wall displacements is not the same as NSD for single-domain (SD) rotations of the same grain when saturated, as determined by ferromagnetic resonance, for example. NMD can be determined from the measured hysteresis parameters &khgr;0 (initial susceptibility), Hc (coercieve force); Hcr (remanent coercive force), Jrs (saturation remanence), and JD (magnetization when H0=Hcr) by one of three formulas: (1) N=&khgr;0-1 -&khgr;i-1, (2) N=(Hc/Jrs)-&khgr;i-1, and (3) N=JD-1(Hcr-Hc)-&khgr;i-1. These formulas, which resemble equations derived recently by Smith and Merrill, reduce to conventional approximations for N as the intrinsic susceptibility &khgr;i=dJ/dHi→∞. In order to use the exact formulas, &khgr;i must be estimated independently by assuming &khgr;iHc≈constant. Using hysteresis measurements for 2200-¿ cubes of magnetite, which should theoretically have a two-domain or three-domain structure, N is calculated to be about 2, in satisfactory agreement with predicted values of the NMD and about one half NSD=4&pgr;/3 for a cube.