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Convert OD600 to CFU/mL via a calibration slope with organism presets, blank correction, dilution handling, and linearity warnings.
Execution
Fill in the form, run the tool, and review the result in one place.
Samples
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Tool usage guide
Convert a spectrophotometer OD600 reading into estimated CFU/mL using a calibration slope. Subtract the medium blank, apply an organism preset (E. coli, B. subtilis, S. cerevisiae) or your own CFU-per-OD factor, then scale by the fold-dilution used before the measurement. Optionally enter culture volume to get total CFU. The calculator flags net OD values above about 0.6 as outside the typical linear range.
Background
A flask of E. coli in LB is being followed on a spectrophotometer. The undiluted sample reads 0.45 against a medium blank of 0.02.
Problem
Convert the reading to CFU/mL using the standard E. coli slope without plating.
How to use
Enter Measured OD600 0.45, Blank OD600 0.02, Organism E. coli, and Sample dilution 1.
Outcome
Net OD 0.43 × 1×10⁹ yields 4.3×10⁸ CFU/mL, inside the linear range with no warning.
Background
A 25 mL Bacillus subtilis culture is too dense to read undiluted. A 10-fold dilution reads OD600 0.85 with no blank.
Problem
Recover original cell density, check linearity, and estimate total CFU in the flask.
How to use
Enter OD600 0.85, blank 0, organism B. subtilis, sample dilution 10, and culture volume 25 mL.
Outcome
4.25×10⁹ CFU/mL (total ~1.06×10¹¹ CFU). Net OD 0.85 triggers a warning to dilute further and remeasure.
CFU/mL = (measured OD600 − blank OD) × calibration factor × sample dilution. The factor is the CFU/mL expected at OD600 = 1 for the chosen organism or your custom slope.
They are order-of-magnitude slopes: E. coli ≈ 1×10⁹, B. subtilis ≈ 5×10⁸, and S. cerevisiae ≈ 1×10⁷ CFU per OD unit. Real values depend on strain, medium, and spectrophotometer path length; calibrate with plate counts when accuracy matters.
Select Custom calibration factor and enter the CFU/mL you measured at OD600 = 1 from your own standard curve. The presets are only approximate.
OD600 is typically linear only up to about 0.2–0.6. Higher readings underestimate density; dilute the sample, remeasure, and enter the new dilution fold.
If you enter volume in mL, the tool also reports total CFU in that culture (CFU/mL × volume). Leave it empty to skip the total.
Background
A lab has determined that its S. cerevisiae strain and spectrophotometer give 8×10⁷ CFU/mL at OD600 = 1.
Problem
Apply that slope instead of the built-in 1×10⁷ yeast preset.
How to use
Select Custom calibration factor, enter 80000000 (or the lab value), plus the measured OD600, blank, and dilution.
Outcome
CFU/mL uses the entered factor × net OD × dilution; volume can still be added for total CFU.