Abstract: Provided herein is technology relating to quantifying the concentration of nucleic acids and particularly, but not exclusively, to methods for modeling data from a single quantitative amplification reaction, e.g., a PCR assay, to determine the initial concentration of a DNA target. In alternative embodiments, additional unknowns may be used, e.g., parameters for Bn, which is the concentration of probe at cycle n (and which changes in TAQMAN assays due to cleavage of the probe), EC for efficiency of probe cleavage, PDC for apparent probe dissociation constant, etc.