Transformation Efficiency Calculator
Calculate transformation efficiency in colony-forming units per microgram of DNA from your plate counts.
How it works
Efficiency (CFU/µg) = colonies ÷ [ (DNA in µg) × (volume plated ÷ total recovery volume) ]
Transformation efficiency normalizes colony counts to the amount of DNA actually spread on the plate, letting you compare batches of competent cells on one scale. Because you usually plate only part of the recovery, the DNA that landed on the plate is the total DNA scaled by the plated fraction. Chemically competent cells typically reach 10⁶–10⁸ CFU/µg and electrocompetent cells 10⁹–10¹⁰; a low number points to poor cells, too much DNA, or salt carried over into an electroporation.
Worked example
Plating 100 µL of a 1,000 µL recovery puts 10% of the reaction on the plate. With 10 ng (0.01 µg) of DNA, that is 0.001 µg plated. If 200 colonies grow, efficiency is 200 ÷ 0.001 = 2 × 10⁵ CFU/µg.
Frequently asked questions
Should I use the total DNA or just the plated fraction?
Just the fraction actually plated. Efficiency is colonies per µg of DNA that had a chance to form a colony on that plate, so scale the DNA by (volume plated ÷ total volume).
What is a good transformation efficiency?
For chemically competent cells, 10⁶–10⁸ CFU/µg is typical; electrocompetent cells reach 10⁹–10¹⁰. Routine cloning of a ligation only needs about 10⁵.
Why is my efficiency unexpectedly low?
Common causes are old or heat-shocked competent cells, too much DNA (which saturates uptake), residual salt during electroporation, or plating on the wrong antibiotic concentration.
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This calculator is for educational purposes. Double-check important results before acting on them.