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Reading the Implied Volatility Surface
Implied volatility is the single most important input in option pricing, and it is different for almost every strike and expiration. The implied volatility surface brings all of those numbers into one picture, showing how the market prices risk across both price levels and time.
1. What implied volatility is
An option pricing model such as Black-Scholes needs five inputs: the underlying price, the strike, time to expiration, interest rates and volatility. All but volatility can be observed directly. Implied volatility (IV) is the volatility value that makes the model price equal to the option's actual market price. In other words, it is the volatility the market is implicitly charging for.
IV is quoted as an annualized percentage. An IV of 16% on an index implies a one-standard-deviation move of roughly 16% over a year, or about 1% per trading day (16 ÷ √252 ≈ 1). Higher IV means more expensive options and a larger expected range of outcomes.
2. Strikes, forwards and moneyness
Comparing IV across expirations requires a common scale. Raw strikes do not work well because the expected future price differs by expiration once interest rates and dividends are taken into account. This site therefore uses moneyness, defined as strike divided by the forward price for that expiration.
The forward is solved from put-call parity: for a European option, call price minus put price at the same strike equals the discounted difference between the forward and the strike. Using several strikes near the money gives a robust estimate. A moneyness of 100% is at the money; 90% is a strike 10% below the forward, and 110% is 10% above it.
On each side of the forward, the surface uses out-of-the-money options: puts below the forward and calls above it. These are the most liquid contracts and their prices are dominated by volatility rather than intrinsic value, which makes their implied volatilities more reliable.
3. The smile and skew
If Black-Scholes were a perfect description of markets, every strike would share the same IV and the curve would be flat. In practice, plotting IV against moneyness for one expiration produces a curve called the volatility smile.
For equity indices the curve is lopsided: IV is much higher for low strikes than for high strikes. This tilt is called skew. It exists because investors pay up for downside protection, because markets historically fall faster than they rise, and because volatility tends to rise when prices fall. Out-of-the-money puts are therefore priced with a premium.
Skew is not fixed. It tends to steepen when demand for protection rises and flatten when investors chase upside, for example through heavy call buying. Single stocks often show more symmetric smiles, and around events such as takeovers the shape can change dramatically. Commodities and some volatility products, including VIX options, often show the opposite tilt, with higher IV on upside strikes.
4. The term structure of implied volatility
Holding moneyness fixed and plotting IV against time to expiration gives the term structure. When markets are calm, short-dated IV is usually below long-dated IV, so the curve slopes upward, much like VIX futures in contango. During stress, near-term IV jumps above longer-term IV and the curve inverts.
Scheduled events leave visible marks. An expiration that falls just after a central bank meeting, an inflation report or a major earnings release often carries a higher IV than its neighbors, creating a bump in the term structure. After the event passes, that extra premium usually disappears, which is often described as a volatility crush.
5. Putting it together: the surface
The implied volatility surface combines both views: one axis is moneyness, one is time to expiration, and the height is IV. Each slice across moneyness is a smile; each slice across time is a term structure.
A few features show up on most equity index surfaces. Skew is steepest in the shortest expirations and flattens with time. The front of the surface reacts most to news, while the back moves more slowly. When the market sells off, the whole surface usually lifts, and the front lifts the most.
The surface also has to be internally consistent. If longer-dated options were priced too cheaply relative to shorter-dated ones, or if call prices failed to decline as strikes rose, there would be risk-free arbitrage. Professional systems fit smooth models to the raw quotes to enforce these constraints; the surface on this site shows market IV values directly, so small irregularities can appear where quotes are sparse.
6. Summary metrics: ATM IV, skew and slope
| Metric | Definition on this site | How to read it |
|---|---|---|
| 30D ATM IV | IV at 100% moneyness, interpolated to 30 days | The overall price of near-term volatility, comparable to the VIX for the S&P 500 |
| 30D skew (90%−110%) | IV at 90% moneyness minus IV at 110%, at 30 days | Larger values mean puts are relatively more expensive than calls, a sign of stronger demand for protection |
| Term slope (90D−30D) | ATM IV at 90 days minus ATM IV at 30 days | Positive is a normal upward slope; negative means the term structure is inverted |
7. Reading the Vol Surface page on this site
- 3D surface: drag to rotate, scroll to zoom and double-click to reset. Colors run from green for low IV through yellow to red for high IV. Hover over a point, or tap it on mobile, and the two grids behind the surface display the volatility smile for that expiration and the term structure for that moneyness, with dashed lines connecting them to the point.
- Heatmap: the same data flattened into a grid of expiration by moneyness. It is the fastest way to spot where IV is concentrated.
- Volatility smile: smiles for several expirations, shortest to longest, so you can see how skew flattens with time.
- ATM term structure: IV at 100% moneyness for each expiration, where event bumps and inversions are easiest to see.
- Controls: the expiry buttons limit the surface to 30 days, 90 days, one year or all expirations, and the strike range setting widens or narrows the moneyness window.
8. Limitations
- Deep out-of-the-money strikes and long expirations often have wide bid-ask spreads, so their IVs are less reliable. Contracts without a bid are excluded.
- IV values are taken as provided in the delayed data feed and are not smoothed by an arbitrage-free model.
- For VIX options, the underlying is the VIX future for each expiration rather than the VIX index, so moneyness is measured against each expiration's own forward.
Continue with How to Read Gamma Exposure or VIX Term Structure Explained.