OD600 calculator
OD600 to cells per mL, with a dilution factor and volume for a target cell count.
OD600 to cells per mL, with a dilution factor and volume for a target cell count.
Worked example. A culture read at OD600 0.6 with no dilution, converted with the E. coli factor of 8 × 10⁸ cells/mL per OD600 unit, gives 4.80 × 10⁸ cells/mL(4.80 × 10¹¹ cells/L, 4.80 × 10⁵ cells/µL). To pipette 1 × 10⁹ cells from it you need 2.083 mL of culture.
The organism you pick sets the factor: 8 × 10⁸ cells/mL per OD600 unit forE. coli and 3 × 10⁷ for yeast. Both are working averages for cells in exponential growth and both drift with strain, growth phase, medium and the optics of the spectrophotometer, so the result is an estimate, not a count. Pick Custom to type your own factor once you have calibrated the instrument against a plate count. The factor in use is always printed under the result.
A spectrophotometer only reports absorbance accurately while it increases in a straight line with cell density, which holds up to roughly OD600 1.0 on most instruments. Past that point, cells in the light path start shading each other and the reading flattens out, understating the true density. Diluting the culture and multiplying the reading back up with the dilution factor gives an accurate result; reading an undiluted, over-range sample does not.
To follow a culture’s growth over time rather than a single reading, see thebacterial growth calculator or theOD growth calculator. For a direct plate count instead of an optical estimate, use the colony forming units calculator.
The absorbance (optical density) of a liquid culture at 600 nm, a wavelength chosen because it does not damage cells or media. It is a fast proxy for how many cells are in the culture, read straight off a spectrophotometer.
Absorbance only tracks cell density in a straight line up to roughly OD 1.0 on most instruments. Above that, light scattering from crowded cells makes the reading underestimate the true density, so the sample should be diluted and re-read, then the dilution factor entered here.
About 8×10⁸ cells/mL per OD600 unit is a common working value for E. coli in exponential growth, and about 3×10⁷ for yeast. Both vary with strain, growth phase and the instrument, so pick Custom and enter your own factor, calibrated against a plate count (see the colony forming units calculator), whenever the exact number matters.
If you diluted the culture before reading it, for example 1:10, enter 10 as the dilution factor. The calculator multiplies your OD600 reading by it before converting to cells/mL, giving the concentration of the original, undiluted culture. Leave it blank for an undiluted reading.
Enter the number of cells you want under the result, written as 1e9 or 1,000,000,000. The calculator divides it by the cells/mL it just worked out, giving the volume of culture that contains that many cells.