X-ray ptychographic computed tomography has recently emerged as a nondestructive characterization tool for samples with representative sizes of several tens of micrometers, yet offering a resolution currently lying in but not limited to the 100-nm range. Here we evaluate the quantitativeness of this technique using a model sample with a known structure and density, and we discuss its sensitivity as a function of resolution. Additionally, we show an example application for the determination of the mass density of individual 2-μm-sized SiO₂ microspheres with a relative error of 2%. The accuracy and sensitivity demonstrated in this paper will enable quantitative imaging, segmentation, and identification of different phases in complex materials at the nanoscale.