Enter the total precipitation for a chosen period of time and the average precipitation for the same period over a long-term record, as well as the standard deviation of precipitation for the same period over a long-term record into the calculator to determine the Standardized Precipitation Index.

Standardized Precipitation Index Formula

The following formula is used to calculate the Standardized Precipitation Index (SPI). 

SPI = (P - Pavg) / σ

Variables:

  • SPI is the Standardized Precipitation Index
  • P is the total precipitation for a chosen period of time (mm)
  • Pavg is the average precipitation for the same period over a long-term record (mm)
  • σ is the standard deviation of precipitation for the same period over a long-term record (mm)

To calculate the Standardized Precipitation Index, subtract the average precipitation for the same period over a long-term record from the total precipitation for a chosen period of time. Then, divide the result by the standard deviation of precipitation for the same period over a long-term record.

What is a Standardized Precipitation Index?

The Standardized Precipitation Index (SPI) is a meteorological tool used to measure and quantify the severity of a drought or an excess of rainfall in a specific region over a specific period of time. It is calculated based on the probability of recording a certain amount of precipitation, considering the climatological probability. The SPI can be calculated for different time scales, reflecting the impact of drought on the availability of different water resources. Negative SPI values indicate less precipitation than average (drought), while positive values indicate more precipitation than average (wetness).

How to Calculate Standardized Precipitation Index?

The following steps outline how to calculate the Standardized Precipitation Index (SPI).

  1. First, determine the total precipitation for a chosen period of time (P) in millimeters (mm).
  2. Next, determine the average precipitation for the same period over a long-term record (Pavg) in millimeters (mm).
  3. Next, determine the standard deviation of precipitation for the same period over a long-term record (σ) in millimeters (mm).
  4. Next, gather the formula from above: SPI = (P – Pavg) / σ.
  5. Finally, calculate the Standardized Precipitation Index (SPI) using the formula.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Total precipitation for a chosen period of time (P) = 120 mm

Average precipitation for the same period over a long-term record (Pavg) = 80 mm

Standard deviation of precipitation for the same period over a long-term record (σ) = 20 mm