Cell density converter
Convert optical density to cells per milliliter. Pick the organism: an OD of 1 means eight hundred million E. coli, but only thirty million yeast.
That's 2× as dense as a culture in mid-log phase, while a culture in stationary phase is 2.5× denser. A 10 mL culture at that density holds about 8×10⁹ cells.
What it means
How dense is that culture?
Conversion factors are rules of thumb for a 1 cm path length. They differ between spectrophotometers, strains and growth phases, so calibrate your own.
Every unit
Your density in every unit
Learn
Three things worth knowing
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OD 1 ≈ 8 × 10⁸ E. coli per mL
For yeast the same OD is about 3 × 10⁷, nearly thirty times fewer, because yeast cells are much larger and scatter much more light each.
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Scattering, not absorbing
600 nm is chosen because nothing in the cell absorbs there. The reading comes almost entirely from light scattered out of the beam, which is why it is called optical density and not absorbance.
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Above OD 1 it stops being linear
Dense cultures scatter light back into the beam, so a reading of 2 may be four times as many cells. Dilute into the linear range and multiply back.
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Popular density conversions
Background
About OD600
Optical density at 600 nm measures how much light a culture scatters, which tracks how many cells are in the way. It is fast and non-destructive, which is why every bench uses it — but it is a proxy, and the number of cells behind an OD of 1 depends entirely on the organism and its size.