OD600 to Cell Count Calculator
Optical density at 600 nm is the fastest routine measure of how dense a liquid microbial culture is, but a raw OD reading is not a cell count. Within the linear range of your spectrophotometer the relationship is approximately proportional, so cells per millilitre equals OD600 times a conversion factor. Because that factor depends on your strain, instrument, and cuvette, there is no universal value. This calculator converts your reading using a factor you calibrate yourself, corrects for dilution, and returns cells per mL and total cells in your culture volume.
OD600 conversion formula
Undiluted OD600 = measured OD600 * dilution factor
Cells per mL = undiluted OD600 * calibration factor
Total cells = cells per mL * culture volume (mL)
Million cells per mL = cells per mL / 1,000,000
The calibration factor has units of cells per mL per OD600 unit and must be measured for your strain and instrument. Keep readings within the spectrophotometer's linear range, diluting dense cultures first.
Best practice
- Calibrate the factor against a direct count for your specific organism and machine; never assume a textbook value.
- Read within the linear range (typically below OD600 of about 0.8) and dilute denser cultures.
- Blank the spectrophotometer against the same sterile medium used for the culture.
- Path length matters: most factors assume a 1 cm cuvette.
- Total cells scales directly with culture volume, useful for planning inoculations and harvests.
OD600 to cell count: frequently asked questions
How does OD600 relate to cell count?
Within the linear range of the spectrophotometer, optical density at 600 nm is approximately proportional to cell concentration. The conversion is cells/mL = OD600 times a conversion factor that you must calibrate for your own strain, instrument, and cuvette path length. There is no universal constant; the factor differs between organisms and machines, so this calculator treats it as a user-editable input.
Why is the conversion factor not a fixed number?
Cell size, shape, refractive index, instrument optics, and cuvette path length all affect light scattering at 600 nm. A figure that is sometimes quoted for E. coli is on the order of 8e8 cells/mL per OD600 unit, but the true value for your setup can differ severalfold. The only defensible approach is to calibrate against a direct count, so we require you to enter your own factor.
What is the linear range of OD600?
Most spectrophotometers are linear only up to roughly OD600 of 0.4 to 0.8. Above that, multiple scattering causes the reading to underestimate true density, so dense cultures must be diluted before measuring. Always read within the validated linear range of your instrument, then multiply back by the dilution factor.
How do I account for dilution?
Enter the dilution factor you applied before reading. If you diluted 1 part culture into 10 parts total, the dilution factor is 10. The calculator multiplies the measured OD600 by this factor to recover the OD600 of the undiluted culture before converting to cells per mL.
How do I calibrate my own conversion factor?
Grow a culture, measure its OD600 in the linear range, then determine the true cell concentration by an independent method such as a haemocytometer count, flow cytometry, or colony-forming-unit plating. Divide the measured cells/mL by the OD600 to get your factor. Repeat across several densities to confirm linearity, then use the slope.
Official sources
- National Institute of Standards and Technology: NIST spectrophotometry and measurement standards.
- U.S. National Library of Medicine: NCBI microbiology methods literature.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.