Transformation efficiency is defined as the number of colony forming units (cfu) which would be produced by transforming 1 µg of plasmid into a given volume of competent cells. The term is somewhat misleading in that 1 µg of plasmid is rarely actually transformed. Instead efficiency is routinely calculated by transforming 100 pg-1 ng of highly purified supercoiled plasmid under ideal conditions. If you plan to calculate efficiency to compare cells or ligations, keep in mind the many variables which affect this metric.

Transformation efficiency (TE) equation:
TE = Colonies/µg/Dilution
Colonies = the number of colonies counted on the plate
µg = the amount of DNA transformed expressed in µg
Dilution = the total dilution of the DNA before plating

TE calculation example:
Transform 2 ul (100 pg) of control pUC19 DNA into 50 ul of cells, outgrow by adding 250ul of NEB 10-beta/Stable Outgrowth Medium and dilute 10 ul up to 1 ml in NEB 10-beta/Stable Outgrowth Medium prior to plating 30 µl. If you count 150 colonies on the plate, the TE is:
Colonies = 150
µg DNA = 0.0001
Dilution = 10/300 x 30/1000 = 0.001
TE = 150/0.0001/0.001 = 1.5 x 109 cfu/µg