  Carbon atoms are counted at near atomic-level precision using a scanning
transmission electron microscope calibrated by carbon nanocluster mass
standards. A linear calibration curve governs the working zone from a few
carbon atoms up to 34,000 atoms. This linearity enables adequate averaging of
the scattering cross sections, imparting the experiment with near atomic-level
precision despite the use of a coarse mass reference. An example of this
approach is provided for thin layers of stacked graphene sheets. Suspended
sheets with a thickness below 100 nm are visualized, providing quantitative
measurement in a regime inaccessible to optical and scanning probe methods.
