I am having troubles understanding the correlation between OD measurements and CFU/ml. What is the conversion factor between the two units, and does this conversion factor somehow compensate for dead cells OD might be reading? (Also lets say the OD is set at the standard 600nm).


1 Answer 1


The optical density measurement is the quantity of living and non-living cells in a sample. The colony forming unit measures only the viable cells in a sample. An OD 600nm is equivalent to approximately 8 x 10^8 CFU/ml. However, this is only an approximation and cannot fully take into account the dead cells the OD may be reading, it also depends on the microorganism that is being researched. A standard curve can be calculated from data, but it is quite difficult to attain an exact result. A very useful article on the correlation between the two measurements can be found here (from the Brazilian Journal of Microbiology).

  • $\begingroup$ Welcome to biology SE - links to external material that allow further reading or offer support to content are strongly encouraged here $\endgroup$
    – rg255
    Commented Jun 10, 2016 at 19:52

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .