How to calculate time series seasonality index in R?

In R, I use the decompose method on my time series object and it gives me seasonal + trend + random component. For seasonal component, it gives me absolute value which is good but I would also like to know the monthly seasonality index as well (like Jan .084, Feb 0.90, Mar 1.12, etc., for example). Is there a quick way to get this seasonality index in R? Also, how is it normally calculated?

• tslm function in forecast package does this automatically. It is wrapper of lm function for time series problems that creates trend and seasonal index automatically. You could try tslm function – forecaster May 5 '14 at 0:45