What Is Standard Deviation in Stocks?
Standard deviation is a statistical measure that quantifies the amount of variation or dispersion in a set of data values. When applied to stock prices, it measures how much a stock's price deviates from its average price over a given period. A higher standard deviation indicates greater price volatility — meaning the stock's price swings more dramatically — while a lower standard deviation suggests the stock trades within a tighter range.
For investors and traders, standard deviation is one of the most widely used metrics for assessing risk. It forms the foundation of modern portfolio theory and is a key input in calculating other important measures like the Sharpe ratio, Bollinger Bands, and Value at Risk (VaR). Understanding a stock's standard deviation helps you make informed decisions about position sizing, stop-loss placement, and overall portfolio risk management.
How to Use This Stock Volatility Calculator
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Search for a Stock
Type a stock ticker symbol (e.g., AAPL, TSLA, MSFT) in the search box. The autocomplete dropdown will help you find the correct symbol across major US exchanges.
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Choose Period & Timeframe
Select the period length for the standard deviation calculation (10, 14, 20, 30, 50, or 100 days) and the timeframe (daily, weekly, or monthly). Shorter periods capture recent volatility changes; longer periods provide a smoother view.
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Analyze the Results
Review the dual chart showing the stock's price history alongside its rolling standard deviation. The summary cards display key volatility metrics including annualized volatility, daily volatility, and a 30-day price range estimate.
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Interpret the Volatility Signal
The data table flags each data point as "Above Avg," "Normal," or "Below Avg" relative to the historical mean. Periods of above-average standard deviation often coincide with market uncertainty or significant news events.
Why Use a Stock Volatility Calculator?
Quantify Investment Risk
Standard deviation provides a concrete number to measure how risky a stock is. Compare volatility across different stocks to build a diversified portfolio that matches your risk tolerance.
Spot Volatility Trends
Track how a stock's volatility changes over time. Rising standard deviation may signal upcoming price breakouts, while declining volatility often precedes consolidation periods.
Optimize Position Sizing
Use volatility data to determine appropriate position sizes. Higher volatility stocks warrant smaller positions to maintain consistent portfolio risk across all holdings.
How Is Stock Standard Deviation Calculated?
The standard deviation of a stock is calculated from its historical closing prices over a specified period. First, the mean (average) closing price is computed. Then, for each day, the squared difference between the closing price and the mean is calculated. The average of these squared differences gives the variance, and the square root of the variance yields the standard deviation. This process is applied on a rolling basis to produce a time series of standard deviation values.
To annualize the volatility, the daily standard deviation of returns is multiplied by the square root of 252 (the typical number of trading days in a year). This annualized figure allows you to compare volatility across different stocks and time periods on a standardized basis. For example, an annualized volatility of 30% means the stock's price is expected to move within approximately 30% of its current price over the next year, assuming a normal distribution of returns.
Interpreting Volatility Levels
Annualized volatility below 20% is generally considered low and is typical of large-cap blue-chip stocks and utilities. Volatility between 20% and 35% is moderate and common among mid-cap growth stocks. Volatility above 35% is high and often seen in small-cap stocks, biotech, and high-growth technology companies. Extremely volatile stocks can exceed 50% or even 100% annualized volatility, particularly during earnings seasons, market corrections, or company-specific events.