Fortune’s Shawn Tully recently looked at the Shiller P/E model (developed by Yale University professor Robert Shiller), which calculates the P/E ratio of the market using ten years worth of earnings (vs. just the earnings of the last 12 months). Currently, the market P/E based on the Shiller model is around 15, which is lower than it’s been in 20 years. The bottom line is the lower the P/E is when you invest the higher returns you will most likely get in the future.

On his Yale web page, Shiller has posted the historical data he uses to calculate the market’s 10-year P/E. He periodically updates the calculations, but you can also download the data in Excel spreadsheet form and (with a bit of digging), find and plug in newly available earnings and price data to update it yourself.