How is the S/N ratio calculated?

The calculation of the S/N is based on the range of the noise in a chromatogram obtained after injection of a blank, observed over a distance equal to 20 times the width at half-height of the peak (20 × W_h) in the chromatogram obtained with the reference solution.

It should be emphasised that this requirement was included in the Ph. Eur. general chapter from the early days until 2009, when it was revised and reduced to a range of (5 × W_h).
It is noteworthy that the revised general chapter published in 11.0 includes the possibility to calculate the noise based on the (5 × W_h) approach when the (20 × W_h) approach is not feasible (because of peaks due to solvents or reagents, or arising from the mobile phase or the sample matrix, or due to the gas chromatography temperature programme).