What is the Shiller P/E Ratio?

To understand the Shiller P/E, we first have to look at the flaw in the standard Price-to-Earnings ratio. A standard P/E looks at the current stock price divided by the earnings of the last 12 months.

Imagine a professional athlete who earns $10 million this year because they had a lucky season, but usually averages $2 million. If you “bought” their future earnings based only on this year, you would be overpaying. The same happens with companies. A single good year—or a single recession—can wildly distort a standard P/E ratio, making the market look cheaper or more expensive than it actually is.

The Shiller P/E, popularized by Yale professor Robert Shiller, solves this by using a 10-year window. It takes the average of the last ten years of earnings and adjusts them for inflation. This “cyclical adjustment” smooths out the peaks of economic booms and the valleys of recessions.

The math behind the ratio is elegant and designed to provide a “sober” look at valuations. The formula is expressed as:

CAPE = Current Price of the Index / (10-Year Average of Inflation – Adjusted Earnings)

By using ten years of data, the ratio covers a full business cycle. This prevents a temporary “earnings crash” (like in 2008 or 2020) from making the P/E ratio look artificially high when prices are actually low. It also prevents a temporary “earnings spike” (like the post-pandemic surge) from making the market look artificially cheap.

Why bother with a 10-year average when we have “Forward P/E” ratios that look at next year’s predicted profits? The answer lies in accuracy versus optimism.

• Standard Trailing P/E: This is a snapshot of the recent past. It is highly sensitive to the “here and now.”
• Forward P/E: This is a guess about the future. Wall Street analysts are notoriously optimistic, often overestimating how much companies will earn.
• Shiller P/E: This is a historical baseline. It doesn’t care about “this time is different” narratives; it only cares about what companies have actually proven they can earn over a decade.

The Shiller P/E acts as the “grown-up in the room,” reminding investors that while earnings can jump 20% in a year, they rarely do so for ten years straight.

As of January 2026, the S&P 500 Shiller P/E ratio is hovering around 39.85. To put that in perspective, we need to look at the historical record:

In 2026, the ratio reached its second-highest level in history, surpassed only by the peak of the Dot-com bubble. This high reading is largely driven by the “AI Supercycle,” where investors are paying massive premiums for tech giants in hopes of a productivity revolution.

Does a reading of 39.85 mean a crash is imminent? Not necessarily. Critics of the Shiller P/E argue that the world has changed since the 1880s, and the ratio may be overly pessimistic today for several reasons:

Modern accounting rules (GAAP) are different than those of the mid-20th century. Some economists, like Jeremy Siegel, argue that these changes make current earnings look lower than they would have in the past, artificially inflating the Shiller P/E.

In the past, companies paid out more in dividends. Today, they spend billions on share buybacks. This reduces the number of shares and increases “Earnings Per Share” in a way the original CAPE model didn’t fully account for.

While interest rates in 2026 have stabilized, they are still lower than the double-digit rates of the 1980s. When interest rates are low, investors are willing to pay more for stocks because bonds offer less competition. This can justify a “new normal” for the Shiller P/E that is higher than the historical 17.3.

If the Shiller P/E is “screaming” that the market is expensive, what should an individual investor do? The goal is not to time the market perfectly, but to manage your risk.

• Don’t Panic Sell: High Shiller P/E ratios are better at predicting returns over the next 10 to 20 years than over the next 10 days. A market can stay overvalued for years before a correction happens.
• Strategic Rebalancing: If your target portfolio is 60% stocks and 40% bonds, but your stocks have grown to 75% due to the 2026 bull run, now is the time to sell some stocks and buy bonds. This naturally forces you to “sell high.”
• Look for Relative Value: While the U.S. S&P 500 might have a CAPE of 39.8, other markets (like Europe or Emerging Markets) or sectors (like Small-Cap Value) often trade at much lower Shiller P/E levels. Diversification is your best defense against an overvalued headline index.

Historically, a ratio around 16–18 is considered “fair value.” Below 10 is considered exceptionally cheap (a generational buying opportunity), and above 25 is considered expensive.

It is not a timing tool. A high Shiller P/E suggests that long-term future returns will likely be lower, but it doesn’t tell you when a crash will happen. The market stayed “expensive” for years in the late 1990s before the bubble burst.

Robert Shiller is a Yale University professor and a 2013 Nobel Prize winner in Economics. He is famous for his work on “Irrational Exuberance” and for predicting both the Dot-com bubble and the 2008 housing crash.

It is called “cyclically adjusted” because it uses a 10-year average to account for the typical business cycle, which usually lasts between 5 and 10 years. This prevents the “noise” of a single bad year from ruining the valuation.

The ratio is typically updated monthly as new S&P 500 price data and inflation (CPI) data become available.

While it was designed for broad indices like the S&P 500, the same principle can be applied to individual “Blue Chip” companies with long earnings histories. However, it is less useful for young growth companies that don’t have ten years of profits yet.

The Shiller P/E ratio is the ultimate “reality check” for investors. In a 2026 market fueled by AI optimism and record-breaking indices, it serves as a reminder that the price we pay for an asset is the single biggest determinant of our long-term success.

While the ratio isn’t a crystal ball that tells you when to sell, its current level suggests that the “easy money” of the last few years may be behind us. By understanding this metric, you can move from being an emotional investor reacting to headlines to a strategic investor who understands the historical tides of value.