Delta-Delta Ct qPCR Calculator

The delta-delta Ct (ddCt) or comparative Ct method estimates relative gene expression in qPCR without a standard curve. You first normalise each sample by subtracting a reference gene Ct from the target gene Ct (delta Ct), then subtract the control sample's delta Ct from the treatment's (delta-delta Ct). Relative expression is 2 raised to the power of negative delta-delta Ct.

Delta Ct is the cycle-threshold difference between the target gene and a stable reference (housekeeping) gene within the same sample: delta Ct = Ct(target) minus Ct(reference). Normalising to a reference gene corrects for differences in input amount and reverse-transcription efficiency between samples.

qPCR amplification doubles product each cycle under ideal conditions, so a one-cycle difference corresponds to a 2-fold difference in starting template. The base of 2 reflects 100 percent amplification efficiency. Because higher Ct means less starting material, the exponent is negated so that a lower target Ct yields a higher expression value.

It is the fold change in target gene expression in the treatment sample relative to the control sample, after normalisation to the reference gene. A value of 1 means no change, greater than 1 means up-regulation, and less than 1 means down-regulation. A value of 4, for example, means the target is expressed four times higher than in the control.

Yes. The 2^-ddCt method assumes both target and reference amplify with near 100 percent efficiency (a doubling per cycle). If efficiencies differ substantially, an efficiency-corrected method such as the Pfaffl method gives a more accurate ratio. Validate your assay efficiency before relying on the simple 2^-ddCt result.