OD600 Cell Density Calculator
Convert OD600 optical density readings to cell density (cells/mL) for E. coli and other bacteria. Account for path length, dilution factor, and instrument calibration.
🦠 Cell Biology📐 cells/mL = OD600 · conversion factor🧬 Biology
OD600 reading
Dilution factor applied
Organism / conversion factor
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| OD600 Cell Density Calculator | — | cells/mL = OD600 · conversion factor | cells/mL |
Step-by-Step Examples
Example 1
E. coli log phase
OD600=0.300, no dilution. E. coli.
- cells/mL = 0.300 × 8×10^8 = 2.4×10^8
- Typical mid-log phase density
- OD600 linear range: 0.05-0.5
✓ 2.4×10^8 cells/mL
Example 2
Diluted sample
Measured OD=0.150 after 1:4 dilution. E. coli.
- True OD = 0.150 × 4 = 0.600
- cells/mL = 0.600 × 8×10^8 = 4.8×10^8
- Dilution needed: OD > 0.5 before dilution
✓ 4.8×10^8 cells/mL (actual culture)
Example 3
Yeast culture
OD600=0.500, S. cerevisiae, no dilution.
- cells/mL = 0.500 × 3×10^8 = 1.5×10^8
- Yeast cells larger than bacteria: fewer per OD unit
✓ 1.5×10^8 yeast cells/mL
Real-World Applications
Microbiology Research
Quick, non-destructive method to monitor culture density for protein expression, fermentation.
Antibiotic Testing
OD600 growth curves reveal MIC by tracking growth inhibition across antibiotic concentrations.
Fermentation Industry
Real-time OD600 monitoring controls fed-batch bioreactors for optimum cell density.
Biosafety
OD600 estimates bacterial load for proper containment and autoclave sterilization.
Common Mistakes to Avoid
⚠️
OD600 is only linear from 0.05 to 0.5
Above OD 0.5: Beer-Lambert law deviates (light scattering). Always dilute concentrated cultures.
⚠️
Conversion factor varies by instrument and organism
The 8×10^8/OD for E. coli is a rough average. Calibrate your specific setup with plate counts.
⚠️
Cells vs biomass
OD600 measures light scattering by ALL cells (live + dead). CFU counts only viable cells. They diverge in sick or stressed cultures.
Frequently Asked Questions
What is OD600? ▾
Absorbance of bacterial suspension at 600 nm. Actually measures light scattering, not true absorbance. Proportional to cell density in linear range.
Why 600 nm? ▾
600 nm is not absorbed by most media or metabolites. Long wavelength minimizes absorption by pigments. Standardized across labs for comparison.
What is the linear range? ▾
OD 0.05-0.5 for typical 1 cm path length cuvette. Outside this: readings are not proportional to cell density. Always dilute to within this range.
Can I measure OD600 in a plate reader? ▾
Yes, but path length differs from cuvettes (usually shorter). Calibrate separately or use path-length correction software.
How do I convert OD600 to CFU? ▾
Plate dilutions and count colonies to establish OD-CFU correlation for your organism and conditions. Can vary 2-3x between labs due to instrument differences.