National Drought Mitigation Center

Standardized Precipitation Index


The Standardized Precipitation Index (SPI), developed by T.B. McKee, N.J. Doesken, and J. Kleist and in 1993, is based only on precipitation. One unique feature is that the SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications.

View climate division based 1-, 2-, 3-, 6-, 9-, 12-, and 24-month, SPI maps going back to 1896. The data used to produce these maps are available from the National Centers for Environmental Information.

Archived Maps

Calculating the SPI

Map Interpretation

A discussion of time steps and how each is used.

SPI Program

Download a program which will allow you to calculate SPI values from a custom dataset.