**Autocorrelation** measures the degree of similarity between a time series and a lagged version of itself over successive time intervals.

It’s also sometimes referred to as “serial correlation” or “lagged correlation” since it measures the relationship between a variable’s current values and its historical values.

When the autocorrelation in a time series is high, it becomes easy to predict future values by simply referring to past values.

**Autocorrelation in Excel**

There is no built-in function to calculate autocorrelation in Excel, but we can use a single formula to calculate the autocorrelation for a time series for a given lag value.

For example, suppose we have the following time series that shows the value of a certain variable during 15 different time periods:

We can use the following formula to calculate the autocorrelation at lag k =2.

=(SUMPRODUCT(B2:B14-AVERAGE(B2:B16), B4:B16-AVERAGE(B2:B16))/COUNT(B2:B16))/VAR.P(B2:B16)

This results in a value of **0.656325**. This is the autocorrelation at lag k = 2.

We can calculate the autocorrelation at lag k = 3 by changing the range of values in the formula:

=(SUMPRODUCT(B2:B13-AVERAGE(B2:B16), B5:B16-AVERAGE(B2:B16))/COUNT(B2:B16))/VAR.P(B2:B16)

This results in a value of **0.49105**. This is the autocorrelation at lag k = 3.

We can find the autocorrelation at each lag by using a similar formula. You’ll notice that the higher the lag, the lower the autocorrelation. This is typical of an autoregressive time series process.

*You can find more Excel time series tutorials on this page.*