The Day plot (Mrs/Ms versus Hcr/Hc) is widely used by paleomagnetists to estimate the size of ferromagnetic grains and classify them as single-domain (SD), pseudo-single-domain (PSD,) or multidomain (MD). How reliable is this plot? To find out, a numerical micromagnetic model is used to calculate hysteresis loops as a function of grain size for two grain shapes (cube and cuboid with X=1.5Y=1.4Z). Magnetocrystalline anisotropy is ignored. The average Mrs/Ms is calculated for a collection of randomly oriented grains: In the elongated grain it drops from 0.4 to 0.06 over a negligible size range, almost missing the usual PSD range altogether. Other hysteresis parameters, Hc, Hcr, and &khgr;0, can only be calculated for a grain at a time. This is done for two magnetic field directions (close to the longest axis and close to the shortest axis). The single-grain values of Hcr/Hc depend strongly on field direction, but it is clear that the average jumps rapidly from SD to MD values. There are large, rapid fluctuations in Hcr and &khgr;0 associated with changes in remanent states. However, these fluctuations may not be apparent when averaged over a broad size range. This may explain why Hcr and &khgr;0 depend weakly on grain size in real samples. In the size range studied (L≤0.25 μm), hysteresis parameters do not represent a typical grain size. Instead, they depend strongly on the size distribution. ¿ 2000 American Geophysical Union