What is Beta?

Beta measures how strongly an investment tends to move when the overall market moves. The market is usually represented by a benchmark, a broad index such as the S&P 500 that serves as the yardstick. A beta of 1 means the investment has historically risen and fallen roughly in step with the benchmark. A beta of 2 means it has tended to move about twice as much in either direction, and a beta of 0.5 about half as much. A beta near zero suggests little connection to the market. A negative beta means it has tended to move the opposite way.

Beta is calculated from past returns. One common way to write it is: beta = correlation × (volatility of the investment ÷ volatility of the market). Correlation is a number between −1 and +1 showing how closely two things move together. Volatility measures how widely returns swing around their average. Say a hypothetical stock has a correlation of 0.8 with the market, a volatility of 30% a year, and the market's volatility is 20%. The ratio is 30 ÷ 20 = 1.5, and 0.8 × 1.5 gives a beta of 1.2. Now say you invest $10,000 in that stock. If the market falls 10% in a month, a beta of 1.2 points to a fall of about 1.2 × 10% = 12%, or $1,200, leaving $8,800. If the market rises 10%, the same beta suggests a gain of about $1,200.

Beta describes the past, and it shifts as companies, sectors and market conditions change. It also captures only the part of risk tied to the market. An investment can have a low beta and still be very risky: a single company facing its own troubles, or an asset that swings for reasons unrelated to stocks, may show a beta near zero while losing a large share of its value. Beta says nothing about expected return, and the figure depends on the benchmark, the period measured and whether daily or monthly data are used.

For a private investor, beta is a quick way to see how a portfolio has tended to behave relative to the broad market. An S&P 500 fund such as SPY has a beta of about 1 against the S&P 500 by construction. Concentrated holdings in technology shares have often shown betas above 1. Bonds have often shown low betas against stocks, though this has varied over time. Knowing a portfolio's overall beta, alongside its drawdown history, helps investors judge whether their level of market exposure matches the losses they are prepared to sit through.

By Dr. Harry Hamann, MBA

What does a beta of 1 mean?
A beta of 1 means an investment has historically moved roughly in line with its benchmark, such as the S&P 500. If the benchmark rose or fell 10%, the investment tended to rise or fall about 10% as well.

Is a high or low beta better?
Neither is better in itself. A high beta means larger swings in both directions relative to the market, and a low beta means smaller ones. Which suits an investor depends on how much market exposure and how large a decline they are comfortable with.

Can beta be negative?
Yes. A negative beta means an investment has tended to move in the opposite direction to the market, rising when the market falls. Few assets show a consistently negative beta, and the relationship can change over time.

Is beta the same as volatility?
No. Volatility measures how much an investment's returns swing overall. Beta measures how much of that movement is linked to the market. A very volatile asset can have a low beta if its swings are unrelated to the market.