How is PCR efficiency calculated?

PCR efficiency is calculated from slope of standard curve. Plot the Ct values against corresponding log10(target concentration), get the slope of the linear regression line, and:

efficiency = (10^(-1/slope)-1)*100%

An efficiency between 90 and 110% is generally considered acceptable.

What does PCR efficiency > 100% mean?

In above figure, the blue dots are from the PCR reaction with 100% efficiency, every doubling in the template concentration produces 1 Ct decrease; the red dots represent the PCR reaction that has efficiency at 123.6%. It tilts up at higher concentration end. It implies the inhibion of the template solution, the dilution lessens the inhibiton, and see the less delay in Ct at the low concentration end, that makes the curve slope increase and results in PCR efficiency greater than 100.