Hong Life CalculatorsUnderstanding the OD600 to cell density calculator — concept and principle
OD600 is the optical density of a culture measured at 600 nm. Because it takes seconds, microbiology labs use it to follow growth, decide when to induce protein expression, harvest competent cells or dilute cultures to a standard starting density. This calculator converts an OD600 reading into an approximate number of cells per mL using a conversion factor for your organism.
How it is calculated
Cells/mL = OD600 × (cells/mL per OD unit) × dilution factor
- OD600 is the reading from the spectrophotometer, blanked against sterile medium.
- The conversion factor is the number of cells per mL that gives an OD600 of 1.0. A commonly quoted rule of thumb for E. coli is about 8 × 10⁸ cells/mL per OD unit.
- The dilution factor corrects for any dilution made before measuring, for example 10 for a 1:10 dilution.
The principle
Bacterial suspensions look cloudy because cells scatter light. A spectrophotometer cannot tell scattering from absorption, so it reports the lost light as an optical density. At low densities this signal is roughly proportional to the number of cells, so a single factor converts OD to cells. The wavelength of 600 nm is used because common media and cell pigments absorb little there. The factor depends on cell size and shape and on the instrument's optics, which is why it has to be calibrated: larger cells such as yeast scatter more per cell and give far fewer cells per OD unit.
Worked example
- Inputs: OD600 = 0.6, factor 8 × 10⁸ cells/mL per OD, dilution factor 1.
- Cells/mL = 0.6 × 8 × 10⁸ × 1 = 480,000,000 (4.8 × 10⁸ cells/mL).
| OD600 | Cells/mL |
|---|---|
| 0.15 | 120,000,000 |
| 0.3 | 240,000,000 |
| 0.45 | 360,000,000 |
| 0.6 | 480,000,000 |
| 0.75 | 600,000,000 |
| 0.9 | 720,000,000 |
| 1.2 | 960,000,000 |
How to read the result
The value is an estimate of total cells, living and dead. Many E. coli protocols use OD600 around 0.4–0.6 as mid-log phase for induction or for making competent cells, but the exact point depends on strain and medium. Proportionality fails at higher densities, because light is scattered more than once, so readings above the instrument's linear range should be diluted and re-measured. For viable counts, plate serial dilutions and count colony-forming units.
Common mistakes
- Blanking with water instead of the same medium.
- Measuring a dense culture without diluting it into the linear range.
- Using the E. coli factor for another organism or for filamentous cells.
- Comparing ODs from different instruments, cuvettes or plate readers without calibration.
Frequently asked questions
Is OD600 the same as absorbance?
The instrument reports it the same way, but most of the signal comes from light scattering rather than true absorption by molecules.
Why do two spectrophotometers give different OD values for the same culture?
Differences in light path, detector angle and slit width change how much scattered light is collected.
Sources: Koch, A. L., Growth measurement, in Methods for General and Molecular Bacteriology (ASM Press) · Sambrook, J. and Russell, D. W., Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory Press)