Beta measures a specific thing — how much a stock's price has historically moved relative to the broader market, not a prediction of future performance.
The Formula and Scale
Beta = covariance of the stock's returns and the market's returns ÷ variance of the market's returns, typically calculated against the S&P 500 as the benchmark. A beta of 1.0 means the stock has historically moved in line with the market — a 10% market move corresponding to a roughly 10% stock move. A beta above 1 signals higher historical volatility than the market; below 1 signals lower; a negative beta means the stock has historically moved opposite the market.
Beta is calculated from historical data — it describes past correlation, not a guaranteed future relationship, and a stock's beta can shift meaningfully after a business model change or major event, so treat a beta reading as one input among several, not a fixed permanent label.
A Risk-Averse Investor: Favor lower-beta stocks for reduced relative volatility versus the broader market.
Someone Wanting to Amplify Market Moves: higher-beta stocks move more sharply in both directions — a genuine double-edged trade-off.
Use Beta the Way
- Treat beta as a historical measure, not a guaranteed future correlation.
- Check beta alongside sector and fundamentals, not alone.
- Reassess beta periodically since it can shift after major business changes.
See high-risk vs. low-risk stocks for the fuller risk framework.
