Gamma exposure, or GEX, is an estimate of how much stock options market makers would need to buy or sell to stay hedged as the underlying price moves. Positive GEX means their hedging tends to lean against moves and dampen volatility; negative GEX means it tends to go with moves and amplify them.
How it works
An option's delta tells you how many shares it behaves like; its gamma tells you how fast that delta changes when the stock moves $1 (see options Greeks). A market maker who keeps a book delta-neutral must re-hedge every time delta changes, and gamma sets the size of those adjustments.
- Dealer long gamma. As the stock rises, the book's delta rises, so the dealer sells shares; as it falls, the dealer buys. Hedging leans against the move.
- Dealer short gamma. As the stock rises, the book becomes shorter, so the dealer buys; as it falls, the dealer sells. Hedging goes with the move.
GEX adds this up across every listed contract. A common way to express it in dollars of hedging per 1% move is:
dollar gamma = gamma × open interest × 100 × spot² × 0.01
Here 100 is the contract multiplier and spot² × 0.01 turns gamma per $1 into a dollar change in delta for a 1% move. Net GEX is the sum across calls minus the sum across puts.
The sign convention is an assumption
Open interest shows how many contracts are open, not who is long or short. To assign a sign, a widely used convention, popularized by public research on index options, assumes customers sell calls and buy puts, so dealers are long calls (counted positive) and short puts (counted negative). That broadly fits index options, where investors buy puts for protection and sell calls for income. It fits less well in single stocks where speculators buy calls heavily, where dealers may actually be short calls. Some providers also quote GEX per one-point move (gamma × open interest × 100 × spot), so figures from different sources are not directly comparable.
How to read it
- Sign. Positive net GEX tends to coincide with narrower ranges and mean reversion; negative net GEX with wider ranges and trending moves.
- Size relative to the stock. A hedging need of $50 million matters little in a stock that trades billions of dollars a day and a great deal in one that trades $100 million.
- Where it changes sign. Recomputing GEX at hypothetical prices shows where it would turn from positive to negative: the gamma flip.
- Where it concentrates. GEX by strike shows where hedging is heaviest; those strikes often line up with the call wall and put wall.
- Time to expiry. Near-dated options carry most of the gamma, so GEX can change sharply after a large expiry.
Worked example (illustrative numbers)
Take a stock at $200 with open interest at only two strikes.
| Contract | Open interest | Gamma | Dollar gamma per 1% | Sign |
|---|---|---|---|---|
| $210 call | 20,000 | 0.020 | $16 million | + |
| $190 put | 30,000 | 0.015 | $18 million | − |
For the call: 0.020 × 20,000 × 100 × 200² × 0.01 = $16 million. For the put the same arithmetic gives $18 million, counted as negative. Net GEX is about −$2 million per 1% move. Read literally, if the stock rises 1% to $202, dealers would need to buy about $2 million of stock to stay hedged, and if it falls 1% they would need to sell about $2 million. Whether that matters depends on the stock's daily dollar volume.
Limitations and common mistakes
- Positioning is estimated. If customers in a stock are mostly buying calls, the convention gets the sign wrong.
- Open interest is end-of-day. Same-day options opened and closed within the session never appear in open interest, so their hedging is invisible to an open-interest model.
- Model inputs move. Gamma depends on implied volatility and time, both of which change during the day.
- Dealers do not hedge mechanically. They net positions across books, hedge with futures or other options, and tolerate some risk.
- It is context, not a signal. Earnings, macro releases and news can overwhelm hedging flows in either regime.
In Clear Tape
On the Options page, the free SPY, QQQ and IWM cards show net dealer gamma as a signed dollar figure and a regime label: positive gamma (range-bound) or negative gamma (trending). Pro members can analyze any stock with listed options, with dollar gamma by strike, a profile of net dealer gamma from 15% below to 15% above the current price, and plain-language notes on the regime. Clear Tape uses the formula above with calls counted positive and puts negative, so positioning is an estimate, not reported data. Members see each session's closing data once the next session has opened. See pricing for plan details.
For educational purposes only. Not investment advice.