Drought Index

Free Drought Index calculator with clear step-by-step results.

About Drought Index

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.

Formulas

Standardised index

Z = (actual rainfall - normal rainfall) / standard deviation

Aridity ratio

Ratio = actual rainfall / potential evapotranspiration

Worked example

With the default values below, Drought Index returns Standardised rainfall index of -1.97. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rainfall in the periodminimum 0 mm: 45 mm
Long-term normal rainfallminimum 0.1 mm: 120 mm
Standard deviation of rainfallminimum 0.1 mm: 38 mm
Potential evapotranspirationminimum 0.1 mm: 150 mm

Results

Standardised rainfall index: -1.97
Drought category: Severe drought
Rainfall versus normal: -62.5% %
Rainfall to evapotranspiration ratio: 0.300

Step by step

1Values used
2Standardised index
3Aridity ratio
4Standardised rainfall index= -1.97
5Drought category= Severe drought
6Rainfall versus normal= -62.5 %
7Rainfall to evapotranspiration ratio= 0.300

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How do the categories map to real indices?

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.

Why include evapotranspiration?

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.