The cells-per-OD factor is organism- and instrument-specific; calibrate it for your setup.
About this tool
The OD600 to Cells/mL Converter is a free, in-browser tool that turns an optical-density reading at 600 nm into an approximate cell concentration. Pick an organism preset or enter your own conversion factor, add the dilution used when reading, and it reports the cells per millilitre and, if you enter a culture volume, the total number of cells. Everything runs locally in your browser with no uploads.
The relationship is cells/mL = OD₆₀₀ × factor × dilution. The factor is the cells per mL that correspond to one OD unit — roughly 8 × 10⁸ for E. coli and 3 × 10⁷ for budding yeast — and the dilution factor accounts for reading a diluted sample. For example, an E. coli culture reading OD 1.0 undiluted is about 8 × 10⁸ cells/mL.
You can also enter a target OD and the tool back-calculates the fold dilution needed to bring the culture to that reading. The conversion factor is only approximate and depends strongly on the organism, growth phase, cuvette path length, and spectrophotometer, so the factor field is fully editable and should be calibrated for your own instrument.
Frequently asked questions
What is the formula?
cells/mL = OD₆₀₀ × factor × dilution, where the factor is the number of cells per mL equal to one OD unit and the dilution factor corrects for reading a diluted sample. Multiplying by a culture volume gives the total cell count.
What conversion factors do the presets use?
About 8 × 10⁸ cells/mL per OD for E. coli and 3 × 10⁷ for Saccharomyces (budding yeast). These are typical values only — choose Custom and enter your own calibrated factor for accuracy.
Why is the factor only approximate?
OD measures light scattering, not cells directly, so the cells-per-OD ratio depends on species, cell size and shape, growth phase, clumping, cuvette path length, and the spectrophotometer. Calibrate against plate counts for reliable numbers.
How does the target-OD dilution work?
It takes the undiluted equivalent optical density (your reading times the dilution) and divides by your target OD to give the fold you must dilute the culture to reach that reading, useful for setting up a subculture.
More tools