SPX market maker exposure and net GEX in dollar gamma

For each strike, gamma is multiplied by open interest, the 100 share contract multiplier, the index level squared and 0.01. Calls count as positive and puts as negative, on the usual assumption that dealers are long the calls and short the puts customers trade. Summing every strike gives net GEX in dollars per 1% move.

Because the index level enters squared, SPX dollar gamma figures are large, often in the billions. Compare a reading with its own history rather than with another ticker.

A large positive reading means dealers are expected to sell into rallies and buy dips, which tends to compress the daily range. A negative reading means hedging is expected to follow the move. The gamma flip is the index level where the estimate changes sign.

Short dated expirations, including 0DTE, carry most of the gamma near spot, so net GEX can change a lot within one session as those contracts age and expire.

Open interest is published once a day and does not say who holds each contract, so dealer positioning is an assumption. Treat net GEX as a map of likely hedging pressure, not a forecast.