You'll need to know the density of your cells to perform this conversion. You can determine it experimentally using various methods like gravimetric method, by optical density + buoyant density method, by Transmission Electron Microscopy + densitometric image analysis... or try to find the value in a database/texbook. An estimation could be applied, depending on the microorganism/cell type.
Bruce E. Rittmann; Perry L. McCarty. Environmental Biotechnology: Principles and Applications. Empirical Formula for Microbial Cells, Chapter (McGraw-Hill Education: New York, Chicago, San Francisco, Athens, London, Madrid, Mexico City, Milan, New Delhi, Singapore, Sydney, Toronto, 2001). https://www.accessengineeringlibrary.com/content/book/9781260440591/toc-chapter/chapter2/section/section3