Free Drought Index calculator with clear step-by-step results.
Expresses a rainfall shortfall in standard deviations rather than millimetres, which is what makes drought comparable between a wet region and a dry one. The aridity ratio adds the demand side - rainfall against potential evapotranspiration - because 45 mm means something very different in a cool month than a hot one.
Standardised index
Z = (actual rainfall - normal rainfall) / standard deviation
Aridity ratio
Ratio = actual rainfall / potential evapotranspiration
They follow the convention used by standardised precipitation indices: below -1 moderate, below -1.5 severe and below -2 extreme, with the same thresholds mirrored on the wet side.
Rainfall alone ignores how fast water leaves the soil. Two months with identical rainfall can be drought and non-drought if potential evapotranspiration differs by a factor of two.