What this is for: Approximate CFU/mL for McFarland standards, OD600 → cells/mL with your calibration factor, and doubling time from two OD readings.
The formula: doubling time = elapsed time ÷ log2(OD2 ÷ OD1)
How to use:
- To use turbidity by eye, pick your McFarland standard and read the approx. density in CFU/mL.
- To use a spectrophotometer, enter the OD600 reading, the cells/mL per 1.0 OD600 factor for your organism, and any dilution factor, then read the cell density.
- To get doubling time, enter the OD and time in min at two points during exponential growth.
- Read the generations elapsed and the doubling time in min.
McFarland Standard → Approx. Density
McFarland values are the standard approximations for E. coli-like organisms.
Approx. density (CFU/mL)
| McFarland standard | Approx. CFU/mL |
|---|---|
| 0.5 | 150,000,000 |
| 1 | 300,000,000 |
| 2 | 600,000,000 |
| 3 | 900,000,000 |
| 4 | 1,200,000,000 |
OD600 → cells/mL
Cell density (cells/mL)
Doubling Time from two OD readings
Doubling time assumes exponential growth between the two OD readings.
min
min
Generations elapsed
Doubling time (min)
Prefer to work offline or keep a copy? The download link is right below.
Download the Bacterial Growth: McFarland, OD600, Doubling workbook (.xlsx)
Download the Bacterial Growth: McFarland, OD600, Doubling workbook (.xlsx)
Notes & limits (from the workbook): McFarland values are the standard approximations for E. coli-like organisms. The OD600 conversion factor is organism-dependent (E. coli ≈ 8e8 cells/mL per OD600) — edit it. Doubling time assumes exponential growth between the two OD readings.