Calculate bacterial transformation efficiency. Enter the number of colonies on the plate, plus the DNA concentration and DNA volume used (to get the total DNA mass). If you plated only a fraction of the recovered transformation (or plated a dilution), you can optionally enter those values to correct the efficiency to the full transformation.

Transformation Efficiency Calculator

Enter any 3 of the 4 main fields (colonies, DNA concentration, DNA volume, efficiency) to calculate the missing variable.

Optional correction (recommended if you plated only part of the transformation or plated a dilution). Leave blank to assume you plated all recovered cells, undiluted.

Transformation Efficiency Formula

Transformation efficiency describes how many successful transformants are produced per amount of DNA used in a bacterial transformation. In practice, it is usually reported as colony-forming units (CFU) or transformants per nanogram or per microgram of DNA, with optional correction for the plated fraction and any dilution.

TE = \frac{N \cdot F}{m}

When DNA is entered as concentration and volume, the DNA mass is:

m = C \cdot V

If you plated only part of the recovered transformation, or you plated a diluted sample, use this correction factor:

F = \frac{V_r}{V_p} \cdot D

To convert a result from transformants/ng to transformants/µg:

TE_{\mu g} = 1000 \cdot TE_{ng}

Variable Meanings

  • TE = transformation efficiency
  • N = number of colonies counted on the plate
  • m = total DNA mass used in the transformation
  • C = DNA concentration
  • V = DNA volume added to the transformation mix
  • F = correction factor for partial plating and dilution
  • Vr = total recovery volume after transformation
  • Vp = volume plated
  • D = dilution factor of the plated sample

How to Calculate Transformation Efficiency

  1. Count the number of colonies on the selective plate.
  2. Determine the DNA concentration and the volume of DNA added.
  3. Calculate total DNA mass from concentration × volume, making sure the units match.
  4. If needed, correct for the fraction plated and any dilution.
  5. Divide the corrected transformant count by the DNA mass.

If the entire recovered transformation was plated without dilution, the correction factor is effectively 1, so the calculation becomes simply the colony count divided by DNA mass.

Rearranged Forms

Because this calculator can solve for a missing input, these equivalent forms are also useful:

N = \frac{TE \cdot m}{F}
m = \frac{N \cdot F}{TE}
C = \frac{m}{V}
V = \frac{m}{C}

Example Calculation

Assume you count 250 colonies, use DNA at 10 ng/µL, add 1 µL of DNA, recover the cells in 1000 µL, and plate 100 µL of an undiluted sample.

m = 10 \cdot 1 = 10 \text{ ng}
F = \frac{1000}{100} \cdot 1 = 10
TE = \frac{250 \cdot 10}{10} = 250 \text{ transformants/ng}
TE_{\mu g} = 250 \cdot 1000 = 2.5 \times 10^5 \text{ transformants/}\mu g

This means the transformation produced the equivalent of 250 transformants for every nanogram of DNA used, or 250,000 transformants per microgram.

How to Interpret the Result

  • Higher efficiency means the competent cells and protocol produced more transformants from the same DNA input.
  • Lower efficiency can result from poor cell competency, degraded DNA, large plasmid size, ineffective recovery, strong selection pressure, plating error, or incorrect dilution correction.
  • Best comparisons are made only when strain, plasmid, antibiotic, transformation method, and recovery conditions are similar.

Input Guidance

Input What to Enter
Colonies The count from a readable selective plate. Avoid plates that are overgrown or confluent.
DNA Concentration The concentration of the DNA stock used for transformation, using the correct unit selected in the calculator.
DNA Volume Only the volume of DNA solution added to the transformation, not the total reaction volume.
Total Recovery Volume The full post-recovery liquid volume before any plating step.
Volume Plated The actual amount spread onto the plate.
Dilution Factor Use 1 for undiluted plating, 10 for a 1:10 dilution, 100 for a 1:100 dilution, and so on.

Common Unit Notes

  • 1 µg = 1000 ng
  • 1 mL = 1000 µL
  • If concentration is entered in µg/µL, convert carefully before comparing results with values reported in transformants/ng.
  • If your output is desired in transformants/µg, multiply the transformants/ng value by 1000.

Common Mistakes

  • Forgetting to account for plating only a fraction of the recovered cells.
  • Ignoring dilution when the plated sample was not taken directly from the recovery tube.
  • Mixing ng and µg units.
  • Using colony counts from plates that are too crowded to count accurately.
  • Entering the total transformation volume instead of the DNA volume.

Why Transformation Efficiency Matters

Transformation efficiency is a practical way to judge the quality of competent cells and the effectiveness of a transformation workflow. It helps compare protocols, troubleshoot low colony counts, estimate expected clone yield, and decide whether a transformation should be repeated before moving on to downstream screening or sequencing.