Transformation efficiency tells you how good a batch of competent cells is and whether a transformation worked as expected. This calculator returns the standard figure, colony-forming units per microgram of DNA, correcting for the fact that you rarely plate the whole reaction.
How it works
The efficiency is colonies divided by the DNA mass on the plate. The DNA on the plate is the DNA used in the reaction scaled by the fraction of the recovery volume you actually spread:
DNA on plate (ug) = DNA used (ug) x (volume plated / total volume)
efficiency = colonies / DNA on plateFor example, 250 colonies from a reaction using 0.1 ng of DNA, recovered in 1,000 µL with 100 µL plated, gives 0.0001 ug x (100 / 1000) = 0.00001 ug on the plate and 250 / 0.00001 = 2.5e7 CFU/ug.
Interpreting the result
The efficiency number tells you whether your cell preparation is adequate for your cloning task:
- Below 10^5 CFU/µg: Very poor. Suitable only for retransforming intact, high-copy plasmids. A ligation would likely yield few or no colonies.
- 10^6 to 10^7 CFU/µg: Routine quality. Adequate for cloning typical ligation products from clean DNA ends and efficient ligations.
- 10^8 CFU/µg: High efficiency. Reliable for most challenging ligations — blunt-end ligations, low-quality PCR products, or directional cloning with mismatched efficiency between sites.
- 10^9 CFU/µg and above: Library-grade cells. Required for constructing cDNA or genomic libraries where coverage depends on transforming millions of distinct ligated clones.
Using a control plasmid
The efficiency figure only means something when you know what DNA you used. The standard benchmark uses a purified, supercoiled control plasmid — pUC19 is the most common reference. Supercoiled DNA transforms 3 to 10 times more efficiently than relaxed or linear DNA, and a ligation product transforms far less efficiently than supercoiled plasmid because the ligation is never 100% efficient and the nicked product is a poor transformation substrate.
If you calculated efficiency using a ligation reaction, your number will not compare directly to a manufacturer’s spec that used supercoiled control DNA. Always run a parallel control transformation with your reference plasmid to get a comparable efficiency figure.
Why the plating fraction matters
The scaling step is the most error-prone part of the calculation in practice. The total reaction is recovered in some volume (often 1,000 µL of SOC broth after outgrowth), but you only spread a small fraction of this on each plate. If you plate 100 µL from 1,000 µL total, you plated one-tenth of the reaction and observed one-tenth of the colonies. The calculator scales your DNA mass by the same fraction, so the resulting efficiency correctly reflects the total reaction and can be compared across experiments with different plating volumes.
If you plate the entire reaction onto one plate, the plating fraction is 1 and no scaling is needed.
Troubleshooting low efficiency
Heat shock temperature or timing: Too hot or too short is one of the most common causes. Verify your heat block is accurate.
Cell thaw method: Thaw competent cells on ice, never at room temperature. Even a few minutes of room-temperature exposure can halve efficiency.
Recovery media temperature: Cold SOC delays recovery. Pre-warm to 37°C before use.
DNA quality: High salt, DMSO residue from gel extraction, or damaged ends from PCR all reduce efficiency. Purify DNA with a clean column step before transformation.
Tips and notes
Always count a plate in the 30 to 300 colony range so counting error stays small and colonies do not merge. Benchmark cells with a pure supercoiled control plasmid rather than a ligation, because ligated DNA transforms far less efficiently and would understate cell quality. If efficiency drops over time, check that competent cells were kept at minus 80 degrees and thawed on ice, and that no heat-shock or recovery step was skipped.