How is OD600 converted to cells per mL?

Within the linear range, cell density is proportional to absorbance, so cells/mL equals the blank-corrected, dilution-corrected OD600 multiplied by a conversion factor. For E. coli a common factor is about 8 x 10^8 cells per mL at OD600 of 1 on a 1 cm pathlength.

Why subtract a blank?

Media, salts, and the cuvette itself absorb and scatter light even with no cells present. Subtracting a media-only blank removes that background so the remaining absorbance reflects cell mass rather than the broth.

What is the linear range of OD600?

Most spectrophotometers stay linear only between roughly OD 0.1 and 0.8. Above that, cells shadow one another and the reading underestimates density, so dilute the sample into that window and apply the dilution factor.

Why can I change the conversion factor?

The cells-per-OD factor depends on the organism, cell size, and instrument geometry, so a yeast or mammalian culture differs greatly from E. coli. Calibrate the factor against plate counts or a haemocytometer for your own strain and reader.

Is OD600 the same as CFU/mL?

Not exactly. OD600 measures total cell mass including dead and non-dividing cells, whereas CFU/mL counts only viable colony-forming units. The two correlate but the ratio is strain and condition specific, so calibrate if you need true viable counts.

Cell Density from OD600 Calculator

Optical density at 600 nm is the fastest proxy for how many cells are in a culture. This calculator turns a raw OD600 reading into an estimated cell density by correcting for the blank, applying your dilution, and multiplying by a conversion factor you can tune for your organism.

How it works

Within the linear range of the spectrophotometer, absorbance is proportional to cell mass, so the conversion is a single multiplication after corrections:

corrected OD = raw OD − blank OD
true OD      = corrected OD × dilution factor
cells/mL     = corrected OD × conversion factor × dilution factor

The default conversion factor of 8 × 10⁸ cells/mL per OD unit applies to E. coli at OD600 = 1 on a 1 cm pathlength. The tool flags readings whose corrected OD falls outside the roughly 0.05–0.8 linear window.

Tips and example

A raw reading of 0.40 against a 0.02 blank with no dilution gives a corrected OD of 0.38, or about 0.38 × 8 × 10⁸ = 3.0 × 10⁸ cells/mL. If your culture reads above OD 0.8, dilute it tenfold, re-read, and enter a dilution factor of 10. Because the cells-per-OD value is strain and instrument specific, calibrate it against plate counts before trusting absolute numbers.