How to Read a Volatility Surface

A volatility surface separates three signals, the general level, the skew across strikes and the term structure across expiries, and reading them in order is what a surface question tests.

A volatility surface plots implied volatility against strike and expiry for one underlying, and reading it means separating three pieces of information that sit on top of each other: the general level, the skew across strikes at a fixed expiry, and the term structure across expiries at a fixed strike. Treating the surface as a single number is the mistake that shows up fastest in an interview.

The level is the at-the-money implied volatility for the expiry a trader cares about, usually the near-dated one, and it is the closest single number to a market-implied estimate of how much the underlying will move over that horizon. Skew is the variation in implied volatility across strikes at that same expiry, and on equity index options it is persistently downward sloping: out-of-the-money puts trade at higher implied volatility than out-of-the-money calls of equivalent distance from spot. The mechanism behind that slope, rather than only its shape, is explained in the drivers of the volatility smile.

Term structure is the variation in implied volatility across expiries, holding moneyness fixed, usually read at the at-the-money point. A surface with rising implied volatility from one month to three months to twelve months is in contango, the normal resting state for a calm index; a surface where near-dated implied volatility sits above longer-dated implied volatility is in backwardation, which shows up when a specific event is priced into the near expiry or when the whole market is under stress.

The standard way to quantify skew is the risk reversal: take the implied volatility of a 25-delta put and subtract the implied volatility of a 25-delta call at the same expiry. On S&P 500 index options this number is typically negative and worth a few volatility points in calm markets, commonly in a 3 to 6 point range, and it widens further, sometimes sharply, once realised moves turn negative and demand for downside protection increases faster than dealers can supply it.

Single-name skew behaves differently from index skew because single stocks carry upside jump risk from acquisition rumors and earnings beats that the index, an average across hundreds of names, does not carry in the same way. A stock’s skew can flatten or even invert around a binary event, which is one reason skew on a single name cannot be read using the intuition built on index options.