How Is Standard Deviation Used to Compare Fund Volatility?
Average return tells you where a fund ended up, but it says nothing about the ride investors took to get there. Standard deviation is the number that fills in that part of the picture, and it can make two funds with nearly identical returns look very different once it’s factored in.
The short answer
Standard deviation measures how much a fund’s returns have historically varied around their own average, over a chosen period. A fund with a higher standard deviation has had more volatile, wider-swinging returns, while a fund with a lower standard deviation has moved in a comparatively narrower, steadier range, even if both funds arrived at similar average returns.
What the number is actually describing
Standard deviation doesn’t say anything about direction — it doesn’t distinguish an unusually good month from an unusually bad one. It simply measures the spread of returns around the average. Two funds could have identical average annual returns over a period, yet one could have gotten there through mild, consistent gains while the other lurched between sharp gains and sharp losses. Standard deviation is what separates those two very different experiences on paper. A fund with a low standard deviation might feel almost uneventful to hold, while a fund with a high standard deviation could produce months that feel dramatically better or worse than the average would suggest.
Why comparing it matters
When comparing funds with similar strategies or in the same category, standard deviation offers a rough sense of which one has historically been the bumpier holding. That matters because a fund with wide swings can be harder to hold onto during a downturn, even for an investor with a long investment horizon, simply because it’s psychologically harder to stay the course through larger drops. Comparing standard deviation alongside asset allocation can also clarify whether a fund’s volatility fits within an intended plan, since a fund that swings more than expected for its category can throw off an otherwise carefully built mix of holdings.
Limitations worth knowing
Standard deviation is calculated from historical data, so it reflects the past, not a guarantee of future behavior — a fund’s volatility can shift as market conditions or the fund’s own holdings change. It also treats upside and downside swings the same way, which is why some investors pair it with other tools, like a Sharpe ratio that adjusts return for risk, or a downside-focused statistic that looks only at how a fund behaved when markets fell. Relying on standard deviation alone can overstate the risk of a fund that swings up more often than it swings down.
Using it in a side-by-side comparison
Standard deviation is most informative when comparing funds that are meant to serve a similar role, such as two options within the same category or two candidates for the same slot in a portfolio. Comparing a fund’s standard deviation against a broad index fund benchmark can also show whether a fund has historically been meaningfully more or less volatile than the market segment it’s often measured against, which is a different question from whether it has performed better or worse.
A practical habit
Looking at standard deviation alongside average return, rather than return on its own, gives a fuller sense of what holding a fund has actually felt like over time. It won’t predict what happens next, but it puts two funds with similar returns on more comparable footing by accounting for how differently they got there.