Diatom-rich sediments are common in several oceanic regions, especially the Southern Ocean. Some of these are strongly affected by bottom currents and are expected to be sorted by the flow. Examination of data on diatoms' response by instruments commonly used for size measurement is presented here. Diatoms are silt- to fine sand-size, filigree silica structures of many shapes with high porosity, thus both the bulk density and shape strongly influence the “size” that is calculated from measurements. We document the particle size of diatoms measured by instruments based on settling velocity (Sedigraph), electrical resistance pulse counting (Coulter counter), and laser diffraction (Malvern laser sizer). The Malvern laser consistently measures the largest diameters, followed by the Coulter counter and then settling-based techniques. Relationships between these inferred sizes (all expressed as quartz-equivalent spherical diameters) have implications for the physical properties of diatom tests. Earlier work has demonstrated that laser diffraction responds to the particles' external projected area. The effects of both low effective density and irregular shape of diatoms (compared with terrigenous grains) on their settling velocity causes the Sedigraph to indicate relatively small diameters. Shape effects are less pronounced for quasi-spherical diatom species. The Coulter counter records the diatoms' solid volume and is relatively unaffected by their density or shape. The measurement of different physical parameters by these instruments offers a basis for estimation of diatom porosity (fluid-occupied volume within the test). Measurements of the external diameter and solid volume allow estimates of porosity Φ. For (laser) sizes > 20 μm this is Φ = 75–95%, which can be quite well modeled as a perforated spherical shell of wall thickness ~ 1–1.5 μm. For paleocurrent interpretations, a settling-based technique makes Sedigraph measurements of particle size the most useful, but the visual size is best given by laser.