Microbiology · Culture density
OD600 to cells/mL calculator.
Estimate cell density from OD600, or the OD to expect at a target density, with an editable cells-per-OD factor.
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How OD600 relates to cell number
OD600 is the optical density of a culture at 600 nm. Cells scatter light, so the reading rises with cell density. Sigma-Aldrich gives OD600 = 1.0 as roughly 8 x 108 E. coli cells/mL, and stresses that this is only a rough estimate because cell size, shape and the spectrophotometer's optics all matter. Other published figures differ, which is why the factor is editable here.
Formula. cells/mL = OD600 x dilution factor x cells per OD. Worked example. A reading of 0.6 with the default factor gives 0.6 x 8 x 108 = 4.8 x 108 cells/mL. If you diluted 1 in 10 to get a reading of 0.3, the culture is 0.3 x 10 x 8 x 108 = 2.4 x 109 cells/mL.
For a real calibration, plate dilutions of the culture at several ODs and use the CFU per mL calculator to get the ratio for your own strain and instrument.
Frequently asked questions
How many cells are in 1 mL at OD600 = 1?
About 8 x 10^8 for E. coli is a commonly quoted rough figure (Sigma-Aldrich). Other organisms and instruments differ a lot, so calibrate if the number matters.
Why does my reading above 1 look wrong?
At high density the relationship between OD and cell number is no longer linear. Dilute the culture, re-read, and enter the dilution factor.
Does OD600 count live cells only?
No. It also reflects dead cells and debris. Use plate counts (CFU/mL) if you need viable cells.
Do I subtract the blank?
Yes. Zero the instrument on sterile medium and enter the blank-subtracted reading.
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Research use and scope
This is arithmetic for research-bench planning, not clinical, diagnostic or safety advice. Conversion factors are standard approximations; strain, instrument, reagent and protocol differences matter. Check results against your own protocol, kit manual and lab supervisor, and follow your institution's biosafety rules.