Microorganisms are a major component of Earth’s biosphere, and their activity significantly affects the biogeochemical cycling of bioavailable elements. To correctly determine the flux of carbon and energy in the environment, reliable estimates of microbial abundances and cellular carbon content are necessary. However, accurate assessments of cellular carbon content and dry weight are not trivial to obtain. Here we report direct measurements of cell dry and carbon mass of environmentally relevant prokaryotic microorganisms using a microfluidic mass sensor. We show a significant nonlinear relationship between carbon mass and cell volume and discuss this relationship in the light of currently used cellular mass models.