CAPE Ratio: Reading the Market's Long-Term Weather, Not Its Daily Forecast
A single year of corporate earnings can be inflated by a boom or crushed by a recession, making a standard price-to-earnings ratio noisy exactly when investors most want clarity. The CAPE ratio smooths that noise over a decade, but it has repeatedly punished investors who treated it as a timing signal rather than a slow-moving gauge.
The core principle
The CAPE ratio, short for cyclically adjusted price-to-earnings ratio and also known as the Shiller P/E after the economist who popularized it, is calculated as CAPE = real price / average real earnings over the trailing 10 years. "Real" here means inflation-adjusted: both the price and each year of historical earnings are converted to today's dollars before the average is taken, so a decade-old earnings figure is not unfairly diluted by inflation.
The purpose of averaging ten years, roughly a full business cycle, is to prevent a single unusually strong or weak year from distorting the ratio. A standard trailing P/E ratio calculated on the most recent year alone can swing wildly during a recession, when earnings collapse faster than prices, making stocks look artificially expensive right when they may actually be cheap. CAPE dampens that effect by anchoring the denominator to a longer, steadier average.
It helps to be explicit about what CAPE is not measuring. It says nothing about interest rates, nothing about corporate profit margins relative to their own history, and nothing about the composition of the index being measured, all of which have shifted meaningfully across different eras. A market composed heavily of capital-light technology companies with high, durable margins can plausibly sustain a structurally higher average CAPE than a market composed mostly of capital-intensive industrial and manufacturing companies, simply because the underlying businesses have different reinvestment needs and different justified valuation multiples. This is one reason serious users of CAPE tend to compare a market's current reading to its own rolling history over recent decades, rather than to a single fixed number pulled from a much earlier era with a very different economy underneath it.
How the math works
Example 1: computing CAPE from scratch. Suppose a broad market index trades at a real (inflation-adjusted) price level of 5,200. Real earnings per share over the trailing ten years, in today's dollars, averaged $182. CAPE is 5,200 / 182, or approximately 28.6. The long-run historical average CAPE for the US market since the early 20th century is close to 17, so a reading of 28.6 sits well above the historical norm, roughly 68% higher.
Example 2: comparing two points in time to see what CAPE does and does not predict. Assume at Time A the index has a CAPE of 15, below the long-run average of 17, and at Time B, twelve years later, CAPE has risen to 32, well above average. Historically, starting valuation has correlated with subsequent ten-year real returns: elevated starting CAPE readings in the high 20s and above have often preceded average annualized real returns in the low single digits over the following decade, while below-average starting CAPE readings have often preceded returns well above the market's long-run historical average of roughly 6% to 7% real. Applying that historical tendency, a portfolio starting at Time A's CAPE of 15 might reasonably expect stronger real returns over the next ten years than a portfolio starting at Time B's CAPE of 32, but the actual range of realized outcomes at any single starting CAPE level has historically been wide enough that this is a probabilistic tilt, not a formula that pins down a specific return.
Example 3: translating CAPE into a rough expected return, and why it is only a starting point. One common, simplified approach treats the inverse of CAPE, the earnings yield, as a rough anchor for long-run expected real returns. At a CAPE of 28.6, the earnings yield is 1 / 28.6, or approximately 3.5%. At a CAPE of 15, the earnings yield is 1 / 15, or approximately 6.7%. Historically, realized long-run real equity returns have often landed somewhat above the earnings yield implied by the starting CAPE, since this simplified approach tends to understate the contribution of dividends and real earnings growth over time, but the directional relationship, lower starting CAPE associated with a higher subsequent return anchor, has held up reasonably well as a first approximation, even if the precise number should never be treated as a guarantee.
How it shows up in real portfolios
A long-term index investor with a 20 to 30 year horizon uses CAPE mainly to calibrate expectations, not to time entries and exits. Seeing a CAPE well above its historical average is a reason to plan for somewhat lower expected returns over the next decade, perhaps saving a bit more each year, rather than a reason to sell out of stocks entirely.
A retiree building a withdrawal plan uses CAPE differently, sometimes as one input into a variable, valuation-aware withdrawal strategy that trims spending slightly in years when starting valuations are elevated and the sequence-of-returns risk is higher, and allows somewhat more spending when valuations are closer to their historical norm.
An investor who tried to time markets using CAPE alone over recent decades has a mixed record at best. CAPE has spent long stretches, sometimes a decade or more, above its historical average without a crash arriving on schedule, and investors who moved to cash the first time CAPE crossed 25 or 30 have in some periods missed years of further gains while waiting for a correction that arrived much later than expected, if at all in that stretch.
A financial planner working with a high-earning client on a retirement projection sometimes uses a valuation-aware return assumption, quietly lowering the assumed future equity return a percentage point or so when current CAPE is well above its historical average, rather than defaulting to the market's long-run historical average return regardless of the current starting point. This is a modest, defensible adjustment to a financial plan's assumptions, distinct from making an outright bet on market direction, and it tends to produce more conservative, more resilient savings and spending targets for the client without requiring anyone to guess when a correction will actually arrive.
Actionable breakdown
- CAPE smooths 10 years of inflation-adjusted earnings.
- Long-run US average CAPE is roughly 17.
- High CAPE has historically correlated with lower, not zero, future returns.
- The relationship is decade-scale, not a short-term timing signal.
- Use CAPE to set return expectations and savings rates.
- Compare current CAPE to its own long-run history, not a fixed cutoff.
- Never use CAPE alone to justify exiting the market entirely.
Common pitfalls
- Treating CAPE as a precise timing signal. A high reading says little about what happens over the next month, quarter, or even several years.
- Selling everything on a high reading, then missing further gains as valuations stay elevated for years, sometimes erasing the very benefit the exit was meant to capture.
- Ignoring structural shifts, such as changes in accounting standards, sector composition, or interest rate regimes, that can push the "normal" CAPE level structurally higher or lower across different eras, making a strict comparison to a century-old average less meaningful.
- Applying US CAPE norms to other markets, when different countries have different long-run average CAPE levels shaped by their own accounting rules and growth profiles.
- Forgetting the inflation adjustment. Comparing a nominal price-to-earnings ratio to a historical CAPE reading, which is inflation-adjusted, is an apples-to-oranges comparison that will systematically understate how expensive a market looks.
Related concepts
CAPE is one member of a broader family covered in our valuation ratios guide, and its historical track record is best understood alongside our market history guide. See also risk premium for the return investors demand for holding stocks at all, and dividend yield for a complementary valuation signal.
The bottom line
A high CAPE ratio is a reason to temper long-term return expectations and adjust a plan modestly, never a standalone signal to abandon a long-term investing strategy.