Beta explained: measuring a stock's sensitivity to the market

Beta is a single number that summarizes how strongly a stock's returns have tended to move with the market as a whole. It is used mainly to estimate the cost of equity in valuation, and it is often quoted as a quick gauge of "risk". It captures only one kind of risk, though, and it is an estimate that changes with how you measure it.

What beta is

Beta is the slope of a line fitted through a stock's returns plotted against the market's returns, usually using the S&P 500 as the market.

Beta = covariance(stock returns, market returns) / variance(market returns)

By construction the market has a beta of 1, and a risk-free asset has a beta of 0.

How to read it

Example (illustrative numbers)

Take a stock with a beta of 1.5. In a month when the market rises 4%, the stock would be expected to rise about 6% (1.5 x 4%). In a month when the market falls 4%, it would be expected to fall about 6%. For a stock with a beta of 0.5, the same market moves would correspond to about plus or minus 2%.

"Expected" matters here. Beta describes an average relationship. In any individual month the stock can do something quite different, because company-specific news also moves the price.

How beta is estimated

There is no single official beta. Estimates vary with:

• The window. Common choices are about five years of monthly returns or two years of weekly returns.
• The return frequency. Daily, weekly and monthly data can give different answers.
• The market index. A broader or different index changes the result.
• Adjustments. Some providers shrink estimates toward 1, because extreme values tend to move toward the middle over time.

Because it is a statistical estimate, beta has a margin of error. Looking at how well the market explains the stock's returns (the R-squared of the regression) shows how much to trust it. For many individual stocks, the market explains only a modest share of the variation.

Industry betas and levering

A single stock's beta can be noisy, so analysts often start from the average beta of similar companies. Debt raises the risk to shareholders, so betas must be adjusted for leverage. A common approach is:

Levered beta = unlevered beta x (1 + (1 - tax rate) x debt/equity)

Illustration: an unlevered beta of 0.9, debt-to-equity of 0.5 and a tax rate of 25% gives 0.9 x (1 + 0.75 x 0.5) = 0.9 x 1.375, about 1.24. This "bottom-up" approach reduces noise and recognizes that a company's beta reflects both its business risk and its financial leverage.

What beta does and does not tell you

It tells you how a stock has co-moved with the market. That is the portion of risk that diversification cannot remove, which is why the CAPM uses it to set the cost of equity.

It does not tell you:

• Total volatility. A stock can have a very high price volatility but a low beta if most of its swings are unrelated to the market. Beta is not standard deviation. See volatility and the VIX.
• Company-specific risk. A single-product biotech firm can have a low beta and still carry a large risk of failure. That risk is real; it is simply what diversification across many stocks is for. See diversification.
• Downside versus upside. Beta treats big gains and big losses symmetrically.
• Business quality, valuation or future returns. A low beta does not make a stock a good investment.
• Stability. Beta changes as business mix, leverage and market conditions change.

Typical patterns

Rules of thumb, not guarantees. Cyclical, highly leveraged and fast-growing companies tend to have higher betas. Businesses with steady demand, such as utilities and consumer staples, tend to have lower ones. Industry averages are a better guide than any single company's number.

Common mistakes

• Reading beta as a measure of how "safe" a stock is.
• Using a beta from a different time window or index without checking.
• Using a beta estimated when the company had a very different mix of businesses or debt.
• Forgetting that correlation matters; see correlation between stocks for the related concept.
• Treating a precise-looking figure like 1.37 as more exact than it is.

Where to see it on StocksXray

StocksXray's data includes Damodaran datasets for betas and for the equity risk premium. See the methodology page for how valuation is built. For how beta combines with other inputs, see the equity risk premium.

Key takeaways

• Beta is the sensitivity of a stock's returns to the market's returns; the market's beta is 1.
• It is an estimate that depends on the window, frequency and index.
• Beta is not total volatility and ignores company-specific risk.
• In valuation, beta feeds the CAPM cost of equity.