Is there a formula for calculating the efficiency of a qPCR reaction?

For a singleplex reaction, the efficiency of qPCR is calculated as follows:

Efficiency = 10^(-1/slope) - 1

The slope is derived from a graph of Cycles to Threshold (Ct) values plotted against the Log10 of the template amount. A slope of -3.32 indicates an amplification efficiency of 100%.

Resource: Gene quantification using real-time quantitative PCR: An emerging technology hits the mainstream. David G. Ginzinger. Experimental Hematology 30 (2002): 503-512