REFERENCE
Calculation methods and assumptions
The calculators apply the equations below to the values entered by the user. Each result is an arithmetic estimate: it does not verify that the input measurements, sample preparation, calibration, or laboratory method are suitable. Check the displayed units and assumptions alongside your established procedure.
OD600 cell density
Optical density at 600 nm is a turbidity measurement. To estimate cells per millilitre, the measured optical density is corrected for the blank and passed through a calibration model. If the culture was diluted before reading, the resulting concentration is multiplied by that dilution correction factor.
Factor: C = ODcorrected × K × D
Linear: C = (a × ODcorrected + b) × D
Log: C = 10^(a × ODcorrected + b) × D
C is the estimated cells/mL; K is a conversion factor; a and b are calibration parameters; and D is the dilution correction factor. The selected calibration must match the organism, strain, medium, instrument, path length, and useful measurement range. The calculator does not make OD600 a viable count: an OD600-to-CFU/mL estimate requires paired measurements and a separately validated relationship.
For example, if corrected OD is 0.25 and a suitable factor is 8.0 × 108 cells/mL per OD unit, the factor model estimates 2.0 × 108 cells/mL before any dilution correction. See the OD600 cell density calculator and the calibration guide.
CFU/mL and CFU/g
Colony-forming units per millilitre or gram are estimated from the colonies observed, the fraction of original sample represented on the plate, and the plated volume. Use the dilution fraction, rather than its reciprocal factor, in the denominator.
Solid: CFU/g = C × H ÷ (m × d × V)
Pooled replicates: ΣCi ÷ Σ(di × Vi)
C is colonies counted, d is the dilution fraction, V is plated volume in millilitres, H is homogenization volume, and m is sample mass in grams. For a solid sample, this equation assumes the full mass is homogenized into H before further dilution. For eligible replicate plates, the pooled estimate combines colony counts and their represented sample volumes; it does not simply average the individual concentrations when dilution or plated volume differs.
Example: 86 colonies from a 10−3 dilution plated at 0.1 mL gives 86 ÷ (0.001 × 0.1) = 8.6 × 105 CFU/mL. A zero-colony plate means no colonies were observed under the selected conditions, not that the original sample is proven to contain none. See the CFU calculator for liquid or solid samples.
Serial dilution
For each step, the final tube volume is divided by the transfer volume to get the step dilution factor. The remaining volume is diluent. The cumulative factor is the product of the step factors up to that tube; the cumulative fraction is its reciprocal.
Transfer volume = final volume ÷ step factor
Diluent volume = final volume − transfer volume
Cumulative factor = factor₁ × factor₂ × …
Cumulative fraction = 1 ÷ cumulative factor
For a 1:10 step with 10 mL final volume, transfer 1 mL into 9 mL diluent. After four identical steps, the cumulative factor is 10,000 and the cumulative fraction is 0.0001. A step ratio such as 1:10 describes the preparation; the factor 10 is the multiplier used in the cumulative product. Mix each tube according to your laboratory procedure before transferring to the next. The serial dilution planner calculates variable as well as repeated step factors.
Interpretation and laboratory use
Precision in a displayed result does not remove uncertainty in counting, pipetting, sampling, instrument response, or biological variation. Countability ranges, detection limits, growth conditions, calibration ranges, and acceptance criteria depend on the selected method. Use this page to understand the arithmetic, and follow your laboratory's validated procedures when collecting and interpreting measurements.
Read the calculation guides for additional worked examples, or return to the tools index to choose a calculator.