Volatility

Two investments can finish a period at the same value after taking very different paths. Volatility includes upward and downward swings; it is not another name for losses. A historical volatility figure summarizes those movements over a stated period and at a stated frequency, such as daily or monthly. It does not predict the direction of the next price move.

In a sentence

• "Both investments returned 8%, but the second had much higher volatility."
• "I compare volatility over the same period before comparing two funds."
• "Volatility tells me how much returns fluctuate, not how much I earned."

How it works

Historical volatility is commonly measured using the standard deviation of returns, a statistic that describes how widely returns vary around their average. A higher figure means greater variation in the observations being measured, not necessarily a lower overall return.

1. Choose the investment and the measurement window, such as a fund over the past year.
2. Use observations at consistent intervals. Daily returns and monthly returns describe different sets of movements, even when they cover the same year.
3. Separate investment performance from deposits and withdrawals. Account for distributions such as dividends when measuring total returns, rather than treating every change in an account balance as a market move.
4. Measure how much the returns vary around their average. Both unusually positive and unusually negative returns contribute to the calculation.
5. Compare figures calculated on the same basis. Check the observation frequency, date range, and whether the figure is expressed on an annualized basis before comparing two published percentages.

A smooth-looking monthly chart can hide sharp movements within each month. Historical volatility also changes with the window you choose, so a quiet recent period does not establish how an investment will behave in the next one.

An example

You start with $1,000 in each of two hypothetical investments and check their values at the end of each month. Assume no deposits, withdrawals, distributions, or fees, so every change comes from the investment's price.

Three-month return = ($1,080 − $1,000) ÷ $1,000 × 100 = 8%

The larger-swing investment moves up $200, down $300, then up $180, while the other rises in smaller steps. Both earn the same final return, but the larger-swing investment has much greater variation in its monthly returns.

Why it matters

Volatility matters when the date you need money may arrive during a decline. In the example, selling the larger-swing investment at the end of month 2 produces $900, not the $1,080 available a month later. The later recovery is part of this hypothetical example, not something volatility guarantees.

Investment volatility also helps explain changes in net worth. Market values can move your total even when your saving habits are unchanged. Separating market-driven changes from money you save or spend keeps a volatile month from looking like a sudden improvement or deterioration in those habits.

Volatility versus investment risk

Volatility measures variability, while investment risk also includes possibilities such as permanent loss, difficulty selling when needed, and failing to preserve purchasing power. An investment with little visible price movement is not necessarily safe. Its price may be observed infrequently, or it may carry risks that have not yet appeared in its historical returns.

Liquidity is a separate question about how readily you can turn an asset into spendable cash without a substantial loss in value. An investment can trade readily and still be highly volatile. Whether its volatility is acceptable depends partly on when you need the money and how much loss you can absorb.

Common questions

Is volatility the same as rate of return? No. A rate of return measures the gain or loss over a period relative to the starting investment, including income where applicable. Volatility describes variation along the way, so identical ending returns can conceal very different experiences.

Does high volatility mean an investment is falling? No. Sharp upward movements contribute to volatility too. A volatile investment can finish a period higher, lower, or near where it started.

Does low volatility mean an investment is safe? No. Low historical volatility describes the observed returns during the measurement period. It does not rule out future losses or risks that the statistic does not capture.

Can volatility predict the next market move? A volatility figure does not tell you whether the next move will be up or down. Historical volatility summarizes past observations, and future movements can differ substantially.

Go deeper

• How to track net worth when your investments swing every day separates money you contribute from changes driven by the market.
• How to read your net worth trend puts individual monthly movements into a longer view and looks for the events behind them.
• The Compound interest calculator shows how a starting balance and monthly contribution grow at a given return over the years; treat that chosen return as an assumption, not a prediction of market swings.