Implied Volatility

Implied volatility (IV) is the market's expectation of how much a stock's price will move over a given period, expressed as an annualized percentage. It is derived from option prices — when options are expensive, IV is high; when options are cheap, IV is low. IV does not predict direction, only the magnitude of expected movement.

Why It Matters

IV is the single most important factor in determining whether an option is cheap or expensive. Two options on different stocks with the same delta, same days to expiration, and same stock price can have wildly different premiums — the difference is IV. A stock with 60% IV will have options priced roughly twice as expensively as a stock with 30% IV, all else equal.

For sellers, high IV means richer premiums to collect. For buyers, high IV means you are paying more for the option and need a bigger move to profit. Understanding IV helps you decide not just what to trade, but when to trade it — and whether to be a buyer or a seller.

How It Works

IV is calculated by working the Black-Scholes (or similar) pricing model backward. Instead of plugging in volatility to get a price, you plug in the market price to solve for the volatility that justifies it.

What IV tells you: An IV of 30% means the market expects the stock to move within a range of roughly 30% (annualized) over the next year, with 68% probability (one standard deviation). To estimate the expected move over a shorter period:

Expected move = Stock price x IV x square root of (days / 365)

IV expansion and contraction:

• IV rises when uncertainty increases — before earnings, during market selloffs, around major events
• IV falls when uncertainty resolves — after earnings (IV crush), during calm markets
• IV tends to be mean-reverting: extreme highs come down, extreme lows rise back up

IV and the VIX: The VIX is the implied volatility of S&P 500 options. It is often called the "fear gauge." When the VIX spikes, it means the market is pricing in large expected moves. Individual stock IVs tend to correlate with the VIX but can diverge significantly around stock-specific events.

IV across expirations: Different expirations can have different IVs (this is the term structure). An earnings date in two weeks might inflate IV for the nearest monthly expiration while leaving longer-dated expirations relatively unchanged.

Quick Example

Stock VWX trades at $100 with IV at 40%. The expected one-month move (30 days) is:

$100 x 0.40 x sqrt(30/365) = $100 x 0.40 x 0.287 = $11.46

The market expects VWX to stay within roughly $88.54 to $111.46 over the next month (68% of the time). An ATM straddle should cost approximately $11.46 if priced efficiently.

If IV were only 20%, the expected move drops to $5.73. Options would cost roughly half as much.