Biology
Colony Forming Unit Dilution Calculator
Convert colony counts back to the original sample concentration and show the dilution path, plate-level estimate, replicate average, and countability warning.
CFU per mL represented by selected plate-
Inverse cumulative dilution factor-
Modeled colonies across accepted replicates-
Colonies above lower countable limit-
Colonies below upper countable limit-
Original-sample equivalent plated volume-
Estimated CFU in one original-sample mL-
Log10 CFU per mL-
Decision view
Dilution tube to colony-plate path
Dilution tube to colony-plate pathThe cumulative dilution and plated aliquot connect the original sample to the observed colonies and reconstructed CFU concentration.
Exact scenario comparisonCounted colonies on selected plate changes while all other entered assumptions remain constant.
| Counted colonies on selected plate | CFU per mL represented by selected plate | Inverse cumulative dilution factor | Modeled colonies across accepted replicates | Colonies above lower countable limit | Colonies below upper countable limit | Original-sample equivalent plated volume | Estimated CFU in one original-sample mL | Log10 CFU per mL |
|---|
Period-by-period detail
CFU calculation audit
How to use Colony Forming Unit Dilution Calculator
- Record the exact dilution plated.
- Convert plated volume to the reported volume unit.
- Use laboratory countability and replicate rules.
Calculator guide
Understanding Colony Forming Unit Dilution Calculator
A CFU estimate depends on counted colonies, plated volume, cumulative dilution, replicate plates, and the countable-range decision.
Calculation method
How the calculation works
Convert a colony count, plated volume and cumulative dilution into the estimated original-sample CFU concentration while preserving countability references. CFU per unit volume equals colonies divided by plated volume and cumulative dilution; replicate estimates are averaged only when the plates are considered usable.
Dilution-to-plate path
Trace the counted colonies back to the original sample
Tubes show cumulative dilution before a plated aliquot produces the observed colony count.
Worked situations
Practical examples
- 85 colonies from 0.1 mL of a 10⁻⁴ dilution corresponds to 8.5 million CFU/mL.
- Too-numerous-to-count plates should not be treated as exact counts.
- Replicate disagreement may indicate mixing or plating variability.
Better inputs
Useful tips
- Mix each dilution before sampling.
- Record zero-count plates rather than silently omitting them.
- Report the dilution and plated volume with the result.
Before relying on the result
Limitations and common mistakes
- CFU measures colonies under the chosen culture conditions, not all viable organisms.
- Clumping, recovery, incubation, media selectivity, counting rules, contamination, and sampling uncertainty are not modeled.
- Use validated microbiology procedures and biosafety controls.
Reference
Key terms
- CFU
- Colony-forming unit inferred from growth on a culture plate.
- Cumulative dilution
- Overall dilution relative to the original sample.
- Plated volume
- Diluted sample volume applied to the plate.
Important note
Calculated from the entered values using the displayed biological or statistical model. Study design, sampling, measurement quality, and biological variation affect interpretation.
Frequently asked questions
Can a zero-count plate be used?
It may support a detection-limit statement, but not a positive point estimate.
Should replicate counts be averaged?
Only under the laboratory's acceptance rules.
Does one colony equal one cell?
Not necessarily; clumps can form one colony.