Standard Deviation
Standard deviation measures how widely an investment's returns are spread around their average. A low standard deviation means returns have been steady and predictable. A high standard deviation means returns have swung sharply, both up and down, so any single year could land far from the average.
In simple terms: two funds can both average 12% a year, but the one with the higher standard deviation gets there on a much bumpier ride. On fund factsheets it is usually shown as an annualised percentage based on the last three years of returns.
How standard deviation is calculated
Standard deviation (SD) takes each period's return, measures how far it sits from the average, squares those gaps so negatives do not cancel positives, averages them, and then takes the square root.
SD = √ [ Σ (R − R̄)² ÷ (n − 1) ], where R is each period's return, R̄ is the average return, and n is the number of periods.
Fund houses and research platforms usually calculate it from monthly or daily returns and then annualise it, so the figure you see on a factsheet is already expressed on a yearly basis.
A worked example: same average, very different ride
Here are five years of returns for two hypothetical funds. Both average exactly 12% a year.
| Year | Fund A | Fund B |
|---|---|---|
| Year 1 | 10% | −5% |
| Year 2 | 12% | 30% |
| Year 3 | 14% | 8% |
| Year 4 | 11% | 25% |
| Year 5 | 13% | 2% |
| Average return | 12% | 12% |
| Standard deviation | About 1.6% | About 15% |
Fund A's returns stayed within a few points of the average every year. Fund B delivered the same average, but an investor who needed money at the end of Year 1 would have been sitting on a loss. That uncertainty is exactly what standard deviation captures.
How to read standard deviation
If returns follow a roughly normal pattern, about two out of three periods land within one standard deviation of the average, and about 19 out of 20 land within two. A fund averaging 12% with an SD of 15% could reasonably return anywhere from −3% to 27% in a typical year.
Equity funds usually show double-digit standard deviation, while liquid and overnight funds are typically well below 1%. Comparing a small-cap fund with a debt fund on SD tells you nothing useful.
A low SD means a steadier ride, not necessarily a better outcome. Pair it with return measures such as the Sharpe ratio, which divides excess return by standard deviation.
Measures spread
Shows how far returns typically stray from their average, in either direction.
Feeds other ratios
It is the risk input behind the Sharpe ratio and many other risk-adjusted measures.
Category context
Only meaningful when comparing funds of the same type over the same period.
Limitations of standard deviation
- Treats upside as risk: A sudden 20% gain raises SD just as much as a 20% fall. The Sortino ratio addresses this by counting only downside deviation.
- Assumes a normal pattern: Markets have fatter tails than a bell curve suggests. Crashes happen more often than SD alone implies, which is why investors also look at maximum drawdown.
- Backward looking: It describes the past three or five years. A fund that had a calm period can still fall sharply in the next one.
- Says nothing about direction: SD tells you how bumpy the ride was, not whether the fund made money.
How to use standard deviation in fund selection
- Match it to your risk tolerance. If a 15% swing in a single year would make you sell, a high-SD fund is the wrong fit, whatever its average return.
- Use it alongside beta, Sharpe ratio and rolling returns rather than on its own.
- For short-term goals, prefer lower-SD options so the money is less likely to be down when you need it.
- Look at it at portfolio level too. A sensible asset allocation can bring overall SD down even if some holdings are volatile.
Frequently asked questions
What is a good standard deviation for a mutual fund?
There is no single good number. It depends on the category. Equity funds typically show double-digit standard deviation, while liquid funds are usually below 1%. Compare a fund's SD with others in the same category over the same period, and judge it alongside its returns.
Is high standard deviation bad?
Not necessarily. High standard deviation means returns have been more volatile, which can come with higher long-term returns. It becomes a problem when the swings are larger than you can stay invested through, or when your goal is close.
What is the difference between standard deviation and beta?
Standard deviation measures a fund's total volatility on its own. Beta measures how much a fund moves relative to its benchmark. A fund can have high standard deviation but a beta close to 1 if its benchmark is also volatile.
Where can I find a fund's standard deviation?
Most fund factsheets list standard deviation in the risk ratios section, usually calculated over three years. Research platforms and AMC websites also publish it.