What volatility actually is
Volatility is simply a measure of how much a price moves. In markets it is usually expressed as an annualized percentage: a stock with 20% volatility is expected to move, up or down, by roughly 20% over a year (and proportionally less over shorter windows). It says nothing about direction — only about the size of the swings.
There are two kinds, and the difference is the whole game. Realized (or historical) volatility looks backward: it measures how much a price actually moved over some past period. Implied volatility looks forward: it is the volatility the options market is currently pricing in for the future. You cannot observe the future, but you can observe what people are paying for options — and from those prices you can back out the market’s expectation.
Implied volatility matters more to traders because it is the market’s live, money-backed forecast. It is what you pay when you buy an option and what you collect when you sell one, and it tends to move ahead of events rather than after them. The two measures are also linked in a way that is itself tradeable: implied volatility usually sits a little above the volatility that subsequently gets realized. That persistent gap — the reason option sellers get paid on average — only becomes visible once you can read implied and realized side by side.
A worked example
Suppose VolDex® on a broad ETF reads 13. Divide by about 16 and you get a rough daily expected move of ~0.8% — a calm market. Weeks later the same VolDex® reads 22: now the market is pricing ~1.4% of daily movement. Same ETF, same index, very different regime.
Now add realized volatility. If the ETF has actually been moving at a 10% pace while implied sits at 15%, options are expensive relative to what is happening — the kind of gap a premium seller looks for. Read either number alone and you would miss it; the edge is in the comparison.
Common pitfalls
Confusing volatility with direction. A high reading means bigger expected moves — up or down. It is not a bearish signal on its own.
Reading a raw level with no context. A volatility of 13 is low for a single biotech and high for a short-term Treasury ETF. A number only means something against its own history.
Assuming implied equals reality. Implied volatility is an expectation, and expectations are often wrong — which is exactly why the gap between implied and realized can be traded.
One practical intuition ties it together: volatility clusters. Quiet days tend to follow quiet days, and violent days cluster into violent stretches, so a sharp jump in implied volatility often marks the start of a noisier regime rather than a one-off. That is another reason the change in the number frequently carries more information than the level itself.
What to do with this
Anchor on one clean, constant measure of implied volatility rather than eyeballing option quotes, then always read it three ways: against realized volatility (is it expensive?), against its own history (is this level extreme?), and against yesterday (is it rising or falling?). The next lessons build each of those skills, starting with why the at-the-money option is the right place to measure from.
Next lesson · continue the courseWhy at-the-money options matter most →
VolDex® is the cleanest live read on implied volatility — the at-the-money level at a constant 30-day horizon. It is free to watch.