🔬 OD600 to Cell Density Calculator
Estimate cell density (cells/mL) from an OD₆₀₀ reading, using a conversion factor you can set for your organism.
Cell density
OD₆₀₀ × factor × dilution
4.000e+8 cells/mL
Per litre
4.000e+11 cells/L
How the od600 to cell density calculator works
Cell density ≈ OD₆₀₀ × conversion factor × dilution. The conversion factor is organism- and instrument-specific: roughly 8×10⁸ cells/mL per OD for E. coli and about 3×10⁷ for budding yeast are common starting points. Enter the factor for your organism (in units of ×10⁸) and any dilution you made before reading.
The organism factor is a published approximation, and instruments differ — so it is an input you control, not a hidden constant. For accuracy, calibrate the factor against a colony count on your own spectrophotometer.
Frequently asked questions
How do you convert OD600 to cells/mL?
Multiply the OD₆₀₀ reading by an organism-specific conversion factor (cells per mL per OD unit) and by any dilution factor. For E. coli a factor near 8×10⁸ cells/mL per OD is commonly used.
What is the OD600 conversion factor for E. coli?
Roughly 8×10⁸ cells/mL per OD₆₀₀ unit is a common rule of thumb, but it varies with strain, growth phase and spectrophotometer. Calibrate against plate counts for accuracy.
What about yeast?
Budding yeast (S. cerevisiae) is often taken as about 3×10⁷ cells/mL per OD₆₀₀, though this too is instrument-dependent. Enter the factor that matches your organism.
Why is the factor an input rather than fixed?
Because it depends on the organism, strain, growth phase and the specific spectrophotometer. Making it an input keeps the tool accurate and free of stale assumptions — you set the value that applies to you.
Should I dilute before reading OD?
Spectrophotometers are linear only up to about OD 0.8–1.0. Dilute dense cultures into range, read, then enter the dilution factor here to recover the true density.
Is OD600 an exact measure of cell number?
No — it measures light scattering, which correlates with cell number only within a calibrated range and organism. Treat the result as an estimate and calibrate for precision.