How OD600 cell density estimates work
Optical density measures light scattering and absorption. Converting OD600 to cells/mL requires an appropriate calibration for the organism, instrument, medium, and optical path length.
Terms and units
- Measured OD600
- The raw optical density reading at 600 nm from your spectrophotometer or plate reader.
- Blank OD600
- The reading of the uninoculated medium in the same cuvette or plate. Subtracting it removes background absorbance and scattering.
- Conversion factor (K)
- Calibration constant in cells/mL per OD unit, measured for your organism, medium, and instrument. Applied as C = corrected OD × K × D.
- Curve parameters (a, b)
- Slope and intercept of a linear calibration (C = a × OD + b), or of a logarithmic model (log₁₀ C = a × OD + b).
- Dilution correction (D)
- The factor for any dilution made between culture and measurement. Enter 1 when the sample was measured undiluted.
Worked example
A conversion-factor calculation assumes a linear relationship through the origin over the valid measurement range. A calibration curve may include a fitted intercept or use a log10 concentration model. The selected model is applied once, then the dilution correction is applied once.
How to use this calculator
Enter values from your own measurement or validated laboratory method. Review the units and assumptions shown with each result before using it in a report.
Assumptions and limitations
These tools perform arithmetic from the values you provide. They do not validate a laboratory method or replace your laboratory's procedures. Biological samples and instruments can introduce uncertainty that a calculation alone cannot account for.
Frequently asked questions
What does OD600 measure?
Light scattering and absorption at 600 nm — a turbidity proxy for biomass. It counts live cells, dead cells, and debris alike, so it is not a viability measurement. See what OD600 measures and how to take a reading.
How do I build an OD600 calibration?
Prepare a dilution series of your culture, measure OD600 and an independent cell count on each dilution, and fit cells/mL against corrected OD. Building an OD600 to cells/mL calibration covers the procedure step by step.
Can OD600 be converted directly to CFU/mL?
Not with a universal constant. The ratio between optical density and colony-forming units shifts with strain, medium, growth phase, and instrument. A conversion needs a calibration built from paired OD600 and plate counts for your own system — see why the relationship needs validation.
Why is my OD600 reading above the linear range?
Standard 1 cm spectrophotometers lose linearity somewhere around OD 0.8–1.0. Dilute the culture into fresh medium, re-blank, and enter the dilution factor in the calculator so the result refers to the undiluted culture.
Guides and references
- What OD600 measures and how to take a readingBlanking, linear range, path length, and the practical steps for reproducible readings.
- OD600 to cells/mL: build and use a calibrationCollecting paired measurements and choosing between a factor, linear curve, or log model.
- OD600 and CFU/mL: why the relationship needs validationWhat shifts the ratio, and how to build a defensible paired calibration.
- OD600 during bacterial growthReading lag, log, and stationary phases from a growth curve.
- Calculation methods: OD600The formulas as implemented, with the dilution correction applied once.