What is gamma exposure (GEX)?

How Owl Mastery measures gamma exposure

Delta is how many shares of the underlying an option moves like. Gamma is how much that delta changes when the underlying moves by $1. Dealers who take the other side of customer options trades commonly hedge their delta by trading the underlying, and gamma tells them how much that hedge has to change as price moves.

Gamma exposure scales this up to the whole options market for one stock or index. For each strike, Owl Mastery combines gamma with the open interest at that strike, which is the number of contracts still open. It states the result in dollars of delta exposure per $1 move in the underlying and sums each strike across all expiries.

• Call GEX counts as positive.
• Put GEX counts as negative.
• Net GEX is call GEX plus put GEX.

The signs follow a simplifying assumption. The model takes dealers to be, on balance, long the calls and short the puts that their customers trade. Open interest does not show who holds each side, so real positioning can differ.

A worked example

The numbers below are illustrative, not live data. Exposure is in millions of dollars per $1 move.

Strike	Call GEX	Put GEX	Net GEX
95	+5	-30	-25
100	+20	-15	+5
105	+35	-5	+30

Across these three strikes, net GEX adds up to +$10 million per $1 move. Read through the model, a $1 rise would leave dealers about $10 million longer, so they would sell about that much to stay hedged, and a $1 fall would have them buy about that much. Gamma changes as price moves, so this holds only for small moves.

From exposure to levels

The daily levels come from the same per-strike numbers, computed over every strike. In the table above, 105 would be call resistance and 95 put support.

• Call resistance is the strike with the largest net GEX, and put support is the strike with the most negative net GEX.
• The HVL is the strike where the running sum of net GEX, added up from the lowest strike, is lowest.
• The gamma wall is the strike with the largest gamma times open interest, puts counted as negative. It is the same strike as call resistance.

The methodology page lists every formula. With a free account, the dashboard also charts net, call and put GEX by strike within 10% of the last close.

Why dealer hedging can damp or amplify moves

A dealer who is long gamma gets longer as price rises and shorter as it falls. To stay hedged, they sell into rallies and buy into dips. That flow opposes the move, so when net GEX is positive, swings tend to be smaller and price tends to stay in a range.

A dealer who is short gamma gets shorter as price rises and longer as it falls. To stay hedged, they buy as price rises and sell as it falls. That flow adds to the move, so when net GEX is negative, moves tend to run further and volatility tends to be higher.

	Positive net GEX	Negative net GEX
Assumed dealer position	Long gamma	Short gamma
Hedging as price rises	Sell	Buy
Hedging as price falls	Buy	Sell
Tendency	Moves damped, narrower ranges	Moves amplified, wider ranges

These are tendencies, not rules. Hedging is one flow among many, and news, earnings or a broad sell-off can outweigh it. The positive vs negative gamma article walks through the hedging step by step.

How to read gamma exposure

• The sign near the price. Check whether net GEX around the current price is positive or negative. That sign suggests whether range-bound trading or bigger swings are more likely.
• The HVL. Below the HVL, net GEX is negative on balance, and above it, positive on balance. So the side of the HVL the close is on gives a rough read of the regime.
• The largest strikes. Strikes with large positive GEX, such as call resistance, are places where rallies may stall. Strikes with large negative GEX, such as put support, are places where declines may slow, or speed up once price breaks through.
• The distance. How far the close sits from each level, in percent, shows how much room price has before it reaches that level. The one-day range shows how much of that room a typical day covers.

The SPX page lists each level and its distance from the latest close, and the guide to reading gamma levels goes through them in order.

Limits of gamma exposure

• Once a day. Open interest is reported at the end of each day, so the levels are computed after each US close and stay fixed through the next session. Positions opened and closed within one day, including many same-day expiry trades, never show up in open interest.
• Assumed positioning. The signs assume who holds each side. When customers buy calls or sell puts in size, the real dealer position can differ from the assumption.
• A snapshot. Gamma shifts as price, time to expiry and implied volatility change, so the exposure at the close can differ from the exposure during the next session.
• A tendency, not a forecast. Hedging competes with every other buyer and seller. GEX does not say which way price moves.
• No history. Owl Mastery keeps the latest snapshot only, so it cannot show how the levels have shifted over time.