Microbiology calculators for CFU, OD600, and serial dilution
Estimate cell density from OD600, calculate CFU/mL and CFU/g from plate counts, and plan serial dilutions. Every result is computed from the values you enter and shown with its formula.
Each tool applies a fixed formula to the values you enter, then shows the terms, the substituted numbers, and the assumptions behind the result. Start with the calculation methods for the full formulas, or read the guides for worked explanations.
OD600 to cells/mL
How optical density maps to a cell estimate, and why your own calibration matters.
Pick the tool that matches the data you have and the quantity you need to estimate.
Use CFU when you have plate counts
A CFU calculator converts colony counts using the dilution fraction and plated volume. Liquid samples are reported as CFU/mL; solid samples as CFU/g, with sample mass and homogenization volume included. For example, 86 colonies from a 10-3 dilution plated at 0.1 mL gives 8.6 × 105 CFU/mL. The CFU calculator shows the formula with your values substituted.
Use OD600 for an optical density estimate
OD600 measures culture turbidity at 600 nm; it does not distinguish viable cells. Converting OD600 to cells/mL requires a calibration for the organism, medium, instrument, and measurement range. The OD600 cell density calculator applies a factor or calibration curve and displays the model used.
Use serial dilution planning for stepwise transfers
A serial dilution repeats a measured transfer into diluent. The cumulative factor is the product of each step factor; two 1:10 steps give a cumulative factor of 100. The serial dilution planner calculates transfer and diluent volumes and tracks each tube.
Use calculations with the method that produced your data
These tools apply formulas to your inputs; they cannot validate your protocol or calibration. Check units, dilution conventions, instrument settings, and assumptions before interpreting a result. See calculation methods for equations and microbiology guides for examples.
What a calculator result can—and cannot—tell you
A calculator makes a formula easier to apply consistently, but its output depends on the measurements and assumptions behind it. CFU concentration describes colonies recovered under particular growth conditions; it is not always a count of individual cells. OD600 is a turbidity estimate that needs a relevant calibration to estimate cells per millilitre. Converting OD600 to CFU also requires a separately validated relationship.
Enter the dilution of the aliquot actually plated, include further sample-preparation dilutions, and use the plated volume in millilitres. In a serial dilution, keep the step ratio distinct from the cumulative dilution. Reviewing these details helps catch factor-of-ten and unit errors.
Use the tools alongside your laboratory's established procedures. Check the formula, substituted values, units, and warnings before interpreting a result. The guides provide examples; the methods reference lists each equation.
FAQ
Frequently asked questions
Short answers about the calculators and the formulas behind them.
How can I calculate CFUs online?
Open the CFU calculator and enter the colonies counted, the dilution exponent (for example 3 for a 10-3 dilution), and the volume plated. The result is calculated in your browser from those values and shown with the formula, so you can check every step. Open the CFU calculator.
How do you calculate CFU/mL from colony counts?
CFU/mL = colonies ÷ (dilution fraction × plated volume in mL). The dilution fraction is the portion of the original sample in the tube you plated: 10-3 dilution = 0.001. For 86 colonies from a 10-3 dilution plated at 0.1 mL, that is 86 ÷ (0.001 × 0.1) = 8.6 × 105 CFU/mL. The CFU calculator applies this formula and shows the substituted values.
What is the difference between CFU/mL and CFU/g?
The unit follows the sample basis. CFU/mL reports viable organisms per millilitre of a liquid sample; CFU/g reports them per gram of a solid sample, which first has to be homogenized into a known volume. The calculator divides by the sample mass and multiplies by the homogenization volume for solid samples. Switch the sample type in the CFU calculator to see which inputs change.
How do I calculate cell density from OD600?
Take an OD600 reading, subtract the blank, and multiply by a calibration that converts absorbance to cells per mL for your organism and instrument. For example, cells/mL = corrected OD × K, where K is a calibration factor such as 8.0 × 108 cells/mL per OD. The OD600 calculator applies your factor, curve, or log model and shows the result.
Can OD600 be converted directly to CFU/mL?
Not reliably on its own. OD600 measures light scattering, not viability, and the ratio between optical density and colony-forming units shifts with strain, medium, growth phase, and instrument. A conversion needs a calibration built from paired OD600 and plate-count measurements for your own system. See why the OD600 to CFU/mL relationship needs validation.
What is a serial dilution used for?
It reduces a concentrated sample step by step until the dilution plated yields countable colonies, while keeping each transfer volume large enough to measure accurately. The serial dilution planner works out the transfer and diluent volumes for each tube and tracks the cumulative dilution factor.
What does 10 CFU per mL mean?
It means the sample is estimated to contain ten colony-forming units in each millilitre under the conditions used to grow and count them. A CFU may come from one cell or a cluster, so this is a viable, culturable-unit estimate rather than a total cell count. Use the CFU calculator with your plate count, dilution, and plated volume.
How many bacteria are in one CFU?
A colony-forming unit can begin with one bacterium or with a group of cells that remain together. A plate count therefore does not always equal the number of individual cells in the original sample. Read more in what one CFU represents.
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Check the working. Results include formulas, substituted values, and important limitations so you can review each calculation.
