OD600 calculator
Convert optical density to cell density and plan culture dilutions.
Exactly what the instrument displayed.
1 if you read it neat, 10 for a 1 in 10.
1 cm unless you are using a short-path cuvette.
Commonly reported between 2 × 10⁸ and 2 × 10⁹.
Optional, for a total cell count.
Culture density
Corrected to a 1 cm path and back to the undiluted culture.
Cells per mL
3.2 × 108
Beam saw 0.4 per cm · culture OD 0.4
About Escherichia coliThe 8 × 10⁸ figure assumes exponential growth in rich medium on a 1 cm benchtop spectrophotometer. Slow-growing cells are smaller, so the same OD represents more of them.
OD_culture = OD_read ÷ ℓ(cm) × D; cells/mL = OD_culture × F; ℓ_well = V ÷ πr²- Cells per millilitre at OD600 = 1.0 — BioNumbers, Nucleic Acids Research, 2010
- Path length and scattering in optical density measurements — BMC Biophysics, 2013
- Non-linearity of optical density in microplate readers — Scientific Reports, 2016
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OD600 measures scattered light, not absorption, so the cells-per-OD factor belongs to an instrument as much as to an organism, and it drifts with cell size as growth rate changes. Treat the count as an estimate good to a factor of about two unless you have calibrated against plate counts or a haemocytometer on the instrument you are using.
About this tool
Turn an OD600 reading into cells per millilitre, correcting for the dilution you read on and the path length of the cuvette or plate well, and work out the volumes to dilute a culture to a target density. The linearity of the reading is checked against what the instrument actually saw rather than the corrected figure.
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Science last reviewed .