What is Standard Deviation?
A measure of how much a series of returns spreads out around its average — the most common proxy for volatility.
Educational — not investment advice
Standard deviation captures typical dispersion. A fund with a 5% standard deviation of monthly returns bounces around far less than one at 15%. In finance it is the workhorse definition of volatility, and it feeds into the Sharpe ratio, VaR and portfolio optimization.
Roughly, in a normal distribution about two-thirds of outcomes fall within one standard deviation of the average, and ~95% within two.
The trap most people miss
It assumes a bell-curve world. Real markets have fat tails — extreme moves happen far more often than a normal distribution predicts — so standard deviation understates true crash risk. It also treats upside and downside as equally "risky."
Frequently asked about Standard Deviation
Is standard deviation the same as volatility?
In practice, yes — volatility is usually measured as the standard deviation of returns.
Does low standard deviation mean safe?
It means low typical dispersion, but not safe from rare extremes. Fat-tailed events live outside what standard deviation captures.
Related terms
In Tradune: see volatility and VaR on your portfolio.
Track what you actually own
Tradune puts every holding in one place — no brokerage login — and measures your return against a buy-and-hold SPY twin funded on your own dates.
Start freeFree for everyone right now · no card · nothing to buy
This definition is for educational purposes only and is not investment, tax, or financial advice. Concepts are explained in general terms; how they apply to your situation can differ. Always do your own research.