How SPX and SPY gamma exposure differ

Two books on one index

SPX options are written on the S&P 500 index itself. SPY options are written on shares of the SPDR S&P 500 ETF Trust, a fund that tracks the index. Both follow the same 500 stocks, but they are separate option books with separate contracts and separate open interest.

A gamma exposure model runs on one book at a time. An SPX wall and an SPY wall are two calculations on two sets of contracts. Neither is a copy of the other.

Prism by GEXfreak models the SPY, QQQ and IWM books. It does not model SPX. Any SPX level you see elsewhere comes from a different book and another provider’s method.

Sources: Cboe: S&P 500 Index Options Product Suite Comparison.

The contract terms side by side

The two contracts differ in exercise, settlement and size. Those terms shape what stays in each book and when it leaves.

Sources: Cboe: S&P 500® Index Options Product Specifications; Cboe: Options on Exchange Traded Products (ETPs) Product Specifications; Cboe: S&P 500 Index Options Product Suite Comparison.

About ten times the size, not exactly ten

Cboe describes an SPY option as about one-tenth the size of an SPX option. In its own example, with the S&P 500 at 4,500, one SPX contract carries about $450,000 of notional value and one SPY contract about $45,000.

The ratio is approximate. SPY’s share price is not held at exactly one-tenth of the index level. When you need the ratio, divide the two prices you see at the same moment.

Size matters for exposure too. Under the dollar convention in our gamma exposure guide, one SPX contract carries roughly ten times the dollar gamma of an SPY contract with the same moneyness, time and volatility, because it controls roughly ten times the notional. Compare the two books in dollars per 1% move, not in contract counts.

Sources: Cboe: S&P 500 Index Options Product Suite Comparison.

Why the walls can sit at different prices

A wall is the strike where one book’s modeled exposure concentrates. Scale an SPY wall by ten and it will not always land on the SPX wall. Several differences can pull them apart.

• Separate positions. Contracts in each book are opened and closed separately, so open interest in one says nothing about the other.
• Different strike grids. A scaled SPY strike can fall between listed SPX strikes.
• Expiry timing. Standard third-Friday SPX series stop trading the day before and settle on opening prices. Same-date SPY series trade until the close, so that expiry leaves the two books at different times.
• Early exercise. SPY options can be exercised before expiration, and exercised contracts leave open interest. SPX options cannot be exercised early.
• Different models. Each provider picks its own sign assumption, expiry windows and wall rule.

Sources: Cboe: S&P 500® Index Options Product Specifications; Cboe: Options on Exchange Traded Products (ETPs) Product Specifications; Options Industry Council (OCC): Open Interest: Why It Matters.

Converting a level needs care

A fictional example with round numbers: at the same moment, the S&P 500 is at 6,000.00 and SPY is at 598.00. The ratio is 6,000.00 ÷ 598.00, about 10.03.

An SPY wall at the 600 strike converts to about 6,020 index points at that ratio. A flat multiply-by-ten puts it at 6,000, twenty points away. Now suppose the SPX book’s own wall is at 6,050. The 6,000 figure is a conversion error. The 6,020 and 6,050 figures are two readings from two books, and neither overrides the other.

A converted level is still an SPY level, drawn in index points. Label it that way.

• Use a ratio from two prices taken at the same time, not a remembered ten.
• Recompute it each session instead of carrying an old ratio forward.
• Keep the expiry window the same on both sides of any comparison.
• Treat a gap between a converted SPY wall and an SPX wall as two readings, not an error to average away.

Where Prism fits

Prism draws the SPY book on SPY prices. It does not convert SPY levels into SPX points, and it does not publish SPX walls. If you trade SPX, treat Prism’s SPY levels as related context from a separate book, and read SPX levels from a source that models the SPX chain.

The same logic applies to ES futures. Our guide to ETF context for futures traders covers reading SPY beside ES.

How this sits in a plan you already have

Pick the instrument you trade and read levels from that instrument’s own book. Use the other book as context, with its price scale labeled. Neither book chooses an entry, a stop or a direction.

• Wait for price at a level from your own instrument’s book instead of a converted one.
• Place the stop beyond structure on the chart you execute on, in that chart’s prices.
• When the two books disagree, record both instead of picking the one that fits your idea.
• If the data clock is not current, stand down. A demo or stale book is not a live book.

What remains unknown

Open interest in either book does not show who holds each contract or how it is hedged. Hedges need not sit in the same instrument as the options, so hedging for one book can trade in another market.

Cboe’s study of same-day SPX options found buying and selling largely balanced in its sample, with market-maker net gamma small next to S&P futures liquidity. That finding covers SPX 0DTE contracts in its sample period. It does not describe SPY, and it sets no rule for either book.

A level in either book can fail. Check price structure, liquidity and event risk before applying your own risk plan.

Sources: Options Industry Council (OCC): Open Interest: Why It Matters; Cboe — Mandy Xu: Much Ado About 0DTEs: Evaluating the Market Impact of SPX 0DTE Options.