Cosmological redshift is calculated by (1) figuring out how much the observed wavelength is shifted from the rest wavelength, and (2) expressing that shift as a ratio of the rest wavelength. Although it sounds complicated, it really is not. It turns out that this redshift number is very useful when deriving properties of distant galaxies, such as age and distance.

Here is an example for illustration. Say an astronomer is measuring the spectrum of a distant galaxy. If the unredshifted rest wavelength of a spectral feature is one hundred nanometers, but for this galaxy the feature appears at two hundred nanometers, then the measured redshift is one. If the feature appears at three hundred nanometers, the redshift is two; if it is at four hundred nanometers, the redshift is three; and so on.