How to calculate CFU concentration
A colony-forming unit represents a viable cell or cell cluster capable of forming a visible colony under the selected incubation conditions. Enter the exponent in the dilution (for example, 3 for 10^-3).
Terms and units
- Colonies counted (C)
- The colonies on the plate. For eligible replicate plates, the calculator pools them as ΣC ÷ Σ(d × V).
- Dilution fraction (d)
- The portion of the original sample in the tube you plated: a 10⁻³ dilution is 0.001. A factor of 1,000 is the same dilution as its reciprocal fraction.
- Plated volume (V)
- The volume of that dilution spread or poured onto the plate, in millilitres.
- Sample mass (m)
- Solid samples only: the mass of the sample in grams, before homogenization.
- Homogenization volume (H)
- Solid samples only: the volume, in millilitres, that the full sample mass is dispersed into before further dilution.
Worked example
For 86 colonies from a 10⁻³ dilution with 0.1 mL plated, the estimate is 86 ÷ (0.001 × 0.1) = 8.6 × 10⁵ CFU/mL. A zero-colony plate indicates no colonies were detected under those conditions; it does not prove the sample has zero viable organisms.
How to use this calculator
Enter values from your own measurement or validated laboratory method. Review the units and assumptions shown with each result before using it in a report.
Assumptions and limitations
These tools perform arithmetic from the values you provide. They do not validate a laboratory method or replace your laboratory's procedures. Biological samples and instruments can introduce uncertainty that a calculation alone cannot account for.
Frequently asked questions
How do you calculate CFU/mL from a colony count?
CFU/mL = C ÷ (d × V). For 86 colonies from a 10-3 dilution plated at 0.1 mL: 86 ÷ (0.001 × 0.1) = 8.6 × 105 CFU/mL. The CFU/mL calculation guide walks through the same steps.
Which plates should I count?
Most protocols treat about 30–300 colonies as countable; some use 25–250 (for example certain FDA BAM and USP methods) or 10–150 for yeast and moulds. Plates outside the window are flagged as TFTC or TNTC and omitted from pooled averages. Configure the range under "Countability and replicate plates".
What does a plate with zero colonies mean?
It means no colonies were detected under the incubation conditions used. It does not prove the sample contains zero viable organisms — the concentration may simply fall below what the plated volume can detect.
What's the difference between CFU and cells?
One CFU is whatever produced one visible colony: a single cell, or a cluster of cells that stayed together. CFU/mL therefore estimates viable culturable units, not total cell number. See what one CFU represents.
Guides and references
- How to calculate CFU/mL from colony countsThe formula, a worked example, and the mistakes that shift results by orders of magnitude.
- CFU/mL vs CFU/gSample basis, homogenization volume, and how solid-sample preparation enters the formula.
- What one CFU representsWhy a colony-forming unit is not a cell count, and what the number cannot tell you.
- Calculation methods: CFUThe formulas as implemented, including the pooled-replicate estimator.