Fold Change Expression Calculator

Fold change is the ratio of an expression measurement in a treatment or test condition to the measurement in a control or reference condition. A fold change of 2 means the gene is expressed twice as highly in the treatment; a fold change of 0.5 means half as much. It is the most common way to summarise up- or down-regulation between two conditions.

Log2 fold change is the base-2 logarithm of the fold change. It makes up- and down-regulation symmetric: a 2-fold increase is +1 and a 2-fold decrease is -1, whereas as raw ratios these are 2 and 0.5. This symmetry is why differential-expression tools and volcano plots report results on the log2 scale.

In qPCR, the comparative method gives fold change as 2 raised to the power of negative delta-delta Ct. That is a special case of the general ratio: the normalised expression in the treatment divided by the control. If you already have normalised expression values, you can compute fold change directly as treatment over control without going through Ct values.

A negative log2 fold change means the gene is down-regulated in the treatment relative to the control, because the fold change ratio is less than 1. For example, a fold change of 0.25 gives a log2 fold change of -2, indicating a 4-fold decrease. A log2 fold change of 0 means no change between conditions.

No. Fold change measures the size of the difference, not its statistical significance. A large fold change from noisy or low-count data may not be reliable. Significance requires replicates and a statistical test; fold change and a p-value are reported together, for example in a volcano plot, to judge both magnitude and confidence.