Bacterial Transformation Efficiency Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/3/2026
Transformation efficiency is found by estimating the total number of colonies (colonies counted × total volume ÷ volume plated), then dividing that number by the amount of DNA transformed in micrograms. Example: 50 colonies counted on 100 µL plated (out of 1000 µL total) with 1 ng of DNA gives an efficiency of 500,000 CFU/µg.
Explanation
Transformation efficiency measures a bacterial strain's competence to take up foreign DNA (typically a plasmid), expressed in colony-forming units per microgram of DNA (CFU/µg) — that is, the number of transformed bacteria that would theoretically be obtained using exactly 1 µg of DNA. In practice, the entire transformed culture is rarely plated onto a single dish: only a small fraction is plated (for example 100 µL out of a 1 mL recovery volume), and colonies are counted on that fraction. The calculation therefore happens in two steps: first extrapolate the colony count from the plated fraction to the total volume (by multiplying by the dilution factor, total volume ÷ volume plated), then divide that estimated total by the amount of DNA actually used in the transformation. Typical values vary by the competent-cell preparation method: roughly 10⁵ to 10⁶ CFU/µg for the classic calcium chloride chemical method, 10⁷ to 10⁹ CFU/µg for optimized chemical methods, and 10⁹ to 10¹⁰ CFU/µg for electroporation. This measurement is used to compare the quality of different batches of competent cells, or to check that a transformation protocol is working as expected before using it for an important cloning step. For the melting temperature of primers used upstream in a PCR cloning protocol, see our primer melting temperature calculator; for the number of copies obtained after amplification, our PCR amplification calculator.
Example: 50 colonies on 100 µL plated (1 mL recovery, 1 ng of DNA)
Inputs
Colonies counted: 50. Total volume: 1000 µL. Volume plated: 100 µL. DNA transformed: 0.001 µg (1 ng).
Calculation
Estimated total colonies = 50 × (1000 ÷ 100) = 50 × 10 = 500. Efficiency = 500 ÷ 0.001 = 500,000 CFU/µg.
Result
The transformation efficiency is 500,000 CFU/µg, an order of magnitude consistent with the standard calcium chloride chemical method.
Frequently asked questions
Why not simply divide the counted colonies by the DNA plated?
That's mathematically equivalent, as long as the same dilution factor is applied to the DNA rather than the colonies: dividing the counted colonies by the DNA actually plated (total amount × volume plated ÷ total volume) gives exactly the same result as the method used here. This calculator extrapolates the colonies to the total volume rather than the reverse, since that's the more commonly published convention in lab protocols.
What transformation efficiency counts as good?
This depends entirely on the method used to prepare the competent cells: an efficiency of 10⁵ to 10⁶ CFU/µg is normal for the classic chemical method (calcium chloride), while optimized chemically competent cells reach 10⁷ to 10⁹ CFU/µg, and electroporation can exceed 10⁹ to 10¹⁰ CFU/µg. Always compare your result to the method you actually used, not to a universal number.
What if no colonies grow on the plate at all?
A total absence of colonies can have several causes: non-functional or expired competent cells, poor-quality or insufficient DNA, an error in the selection antibiotic used, or a poorly executed heat-shock step. A positive control (a transformation with a control plasmid known to work) helps determine whether the problem lies with the competent cells themselves or with the protocol followed.