Summary. Gamma exposure, usually shortened to GEX, estimates how much option dealers must trade in the underlying to stay hedged as its price moves. Its sign shows whether that hedging leans against moves or follows them. This guide defines GEX, shows how it is calculated, explains the levels derived from it and sets out its limits.
1. The short definition
Option dealers, the market makers on the other side of most customer option trades, aim to carry little directional risk, so they hedge each option's delta in the underlying, usually with futures or shares. Gamma is the rate at which delta changes as price moves, so it sets how much that hedge must be adjusted after every move.
Gamma exposure adds this up across the chain, scaled into dollars of the underlying that dealers would trade for a 1% move. It estimates forced, mechanical flow, not sentiment, and expresses no view on direction.
The sign depends on who holds what. If customers buy calls, dealers are short them and short gamma: as price rises the calls gain delta, so dealers must buy the underlying, and as price falls they sell. If customers sell calls, as overwriting programs do, dealers are long the calls and hedge the opposite way.
2. Why GEX moves price, and why it can pin it
The sign of net dealer gamma sets the direction of hedging:
- Positive gamma (dealers long gamma). Dealers sell into rallies and buy into dips. This tends to compress realized volatility and can hold price near heavily traded strikes, particularly close to expiration.
- Negative gamma (dealers short gamma). Dealers buy into rallies and sell into declines, adding to moves in either direction. Realized volatility tends to expand.
Research supports the distinction. Ni, Pearson, Poteshman and White find lower stock volatility when option hedgers are net long gamma and higher volatility when they are net short, and Barbon and Buraschi link negative dealer gamma to intraday momentum and more frequent flash crashes. Ni, Pearson and Poteshman document pinning near strikes on expiration dates, attributing it partly to market makers' hedging.
The spot level at which net dealer gamma changes sign is the gamma flip, or zero-gamma level. Above it hedging is stabilizing; below it, amplifying. The flip comes out of a calculation, so it usually falls between strikes rather than on one. Many traders treat it as the main regime line for an index, with the caveats in Section 6.
3. How GEX is calculated
Gamma for each contract comes from an option pricing model, usually Black-Scholes, using spot, strike, time to expiry, implied volatility and the interest rate. Ignoring dividends,
where is the standard normal density. That gamma is multiplied by:
- the contract's open interest, the number of contracts outstanding;
- the contract multiplier , 100 for US equity and index options;
- spot squared times 0.01, which expresses the result in dollars per 1% move;
- a sign for the dealer's side. The common convention, from the SqueezeMetrics white paper, treats dealers as long calls and short puts, because index customers tend to sell calls and buy puts for protection. Calls add positive GEX and puts negative.
With for calls and for puts, net dealer gamma exposure is
Suppose net GEX is +$2 billion. Dealers would then sell roughly $2 billion of the underlying if it rose 1% and buy about that much if it fell 1%. Some tools quote dollars per one-point move instead, replacing with ; the two scales should not be mixed. The same calculation strike by strike gives the GEX-by-strike profile.
4. Call walls, put walls and the gamma flip
- Call wall. The strike with the largest positive net GEX. Under the usual convention it concentrates dealer-long call gamma, so hedging sells as price rises toward it, which tends to slow advances. A sustained break generally needs demand from outside the hedging flow.
- Put wall. The strike with the largest negative net GEX. Dealers are assumed short puts there, which makes them short gamma: their hedge sells as price falls toward it. When the put wall acts as support, it is mainly because put holders take profits near the strike, which lets dealers buy back their short hedges. When it gives way, the short gamma adds to the decline, so breaks below a put wall often come with a rise in volatility.
- Gamma flip. The price at which net GEX, recalculated at hypothetical spot levels, crosses zero.
The flip is not where a running total of per-strike GEX turns positive. That shortcut uses gamma at today's spot only. The correct calculation re-evaluates every option's gamma over a grid of possible spot levels, holding positions fixed, and finds where the total crosses zero.
5. Why traders care
GEX does not forecast direction. It describes the likely character of a move once one starts: whether hedging will absorb it or extend it.
In a strongly positive-gamma regime, intraday moves have tended to reverse and realized volatility to run below implied, so traders who use GEX often expect range-bound conditions. In a negative-gamma regime, moves have tended to persist and ranges to widen, so the same traders allow for trends and wider swings when sizing positions. These are tendencies, not rules. News or large non-dealer flow can outweigh hedging on any day.
GEX is computed from positions that already exist, before price confirms the regime. It is a map of where forced hedging flow is likely to sit.
6. Limitations of GEX
- The sign convention is an assumption. It is broadly consistent with evidence on customer activity in index options, but dealers run mixed books, and heavy customer call buying can leave them short calls at specific strikes.
- Open interest is a daily snapshot. The Options Clearing Corporation publishes it once per day, so intraday positions appear only in the next report. Same-day volume fills part of the gap, but volume does not show whether a trade opened or closed a position, or which side the dealer took.
- Other Greeks drive flow too. Implied volatility moves delta through vanna and time moves it through charm. Around large expirations these flows can matter as much as gamma. See vanna vs charm.
- Same-day options distort the picture. Gamma near the strike rises roughly in proportion to as expiry approaches, so 0DTE contracts can make the profile look extreme on expiration day. See 0DTE gamma.
- Model inputs move the levels. The flip and the walls shift when implied volatility or time change, even if positions do not.
The DataFirst terminal shows net GEX, the walls and the flip; see options flow. Each level is covered in GEX levels explained, and the hedging behind them in dealer positioning.
For educational and informational purposes only. Not financial advice. Options trading involves substantial risk of loss. The numbers in the examples are hypothetical.