Bone crystals are particularly small and hence estimating their sizes have proved to be difficult and values obtained inconsistent. Here we use rat bone crystals of different ages, as well as different synthetic carbonate apatite crystals, to compare two methods commonly used for determining bone crystal sizes. One method involves direct measurement of crystal lengths and widths, but not thicknesses, from transmission electron microscope (TEM) photographs of dispersed crystals. The second method utilizes X-ray diffraction line width broadening to estimate the average length of crystals. We conclude that line width broadening values tend to reflect crystal coherence lengths rather than the physical dimensions of the whole crystal. TEM measurements provide reliable estimates of average crystal lengths and widths and their ranges. Sample preparation procedures, however, cause breakage of the fragile crystals, which probably results in underestimates of in vivo crystal sizes.