What Is Gamma in Options? Convexity, Dealer Hedging and Expiration Pinning

On expiration Fridays, heavily traded stocks have a habit of closing almost exactly on a round-number strike. It is not a coincidence. In a study of US expiration dates, Ni, Pearson and Poteshman found that optionable stocks cluster at strike prices on expiry, with returns shifted by an average of at least 16.5 basis points, and traced part of the effect to market makers rebalancing their hedges. The Greek behind that hedging is gamma. With same-day options now a record 66.2% of SPX volume (Cboe, July 2026), gamma is a market-moving force, not a footnote. For the four Greeks together, see the options Greeks explained.

What is gamma in options?

Gamma is the change in an option’s delta for a $1 move in the underlying stock. If delta is how many shares your option behaves like right now, gamma is how many shares that number gains or loses per dollar of movement. It is the option’s convexity, the curvature that makes an option’s payoff bend rather than run in a straight line like stock.

A worked example with a 30-day, 100-strike call on a $100 stock at 30% implied volatility:

1. Delta is 0.53 and gamma is 0.046.
2. The stock rises to $101. Delta rises by about the gamma, to roughly 0.58.
3. Your one contract now behaves like 58 shares instead of 53. On the way up you gained exposure; had the stock fallen $1, delta would have dropped to about 0.49 and you would have lost exposure.

That asymmetry is why buyers like gamma: exposure grows when you are right and shrinks when you are wrong. Gamma is the same for a call and a put at the same strike and expiry, and it is always positive for the owner.

Why does gamma spike near expiration?

Gamma spikes because an at-the-money option’s delta has to resolve to either 0 or 1 at expiry, and as time runs out it must make that jump over a narrower and narrower price range. Far from expiry, delta changes gently; on the final day, a $2 move can swing it from 0.10 to 0.90.

Notice the other side of the chart: away from the strike, short-dated gamma collapses to almost nothing. A 3-day option $10 out of the money has essentially no gamma, because it is almost certainly going to expire worthless and nothing short of a big jump will change that. Gamma is concentrated, in both price and time, exactly where the outcome is still undecided.

What is the difference between long and short gamma?

Long gamma means you profit from movement; short gamma means you profit from stillness. Every option position sits on one side, and theta is the price that changes hands between them.

• Long gamma: long calls, long puts, straddles, protective puts. Big moves in either direction add to your P&L faster than a straight line would. You pay theta every day you wait.
• Short gamma: covered calls, cash-secured puts, credit spreads, iron condors, short straddles. You collect theta, but losses accelerate as the stock moves through your strike.

A concrete trade-off: a 7-day short straddle at the 100 strike collects about $3.31 per share and earns roughly $0.24 a day of theta. A 5% move by expiry costs $5.00, a net loss of about $1.69, wiping out the equivalent of a week of decay and then some. That is short gamma in one sentence: small, steady gains and occasional sharp losses.

How does dealer gamma hedging move the market?

Market makers don’t want directional risk, so they delta-hedge: they buy or sell the underlying to offset the delta of the options they hold. Gamma decides how much they have to trade as prices move, and their net gamma decides which direction that trading pushes the market.

• Dealers long gamma (customers have sold them options, as with heavy covered-call and overwriting flows): as the market rises, their delta rises, so they sell to rebalance; as it falls, they buy. Hedging leans against the move and dampens volatility.
• Dealers short gamma (customers have bought options, as with a rush into puts or short-dated calls): as the market rises, they must buy; as it falls, they must sell. Hedging chases the move and amplifies volatility.

The same open interest creates far more hedging as expiry nears: 1,000 at-the-money contracts require re-hedging about 4,600 shares per $1 with 30 days left and about 25,400 shares per $1 on the final day. Scale that to the index level and you see why analysts track aggregate dealer gamma, measured as gamma exposure (GEX), to anticipate whether the coming week should be calm or jumpy.

A “gamma squeeze” is the short-gamma case in a single stock: heavy call buying leaves dealers short gamma, so a rally forces them to buy shares, which pushes the stock up, which raises delta and forces more buying. It ends when the calls expire, are sold, or the open interest is rolled.

What is pinning, and why does it happen on expiration day?

Pinning is the tendency of a stock to close at or near a heavily traded strike on expiration day. When dealers are long gamma around a big strike, every move above it triggers selling and every move below it triggers buying, so the hedging acts like a magnet.

The evidence is solid. Ni, Pearson and Poteshman (2005) showed closing prices of optionable stocks cluster at strikes on expiration dates, shifting returns by at least 16.5 basis points on average, about $9 billion of market value per expiry in their sample. They attribute it to market-maker hedge rebalancing and, in some cases, proprietary traders with large option positions.

Pinning also creates a practical problem for covered-call writers. If a stock closes within cents of your short strike, you may not know until Saturday whether you were assigned, because after-hours moves can change exercise decisions. See options expiration (OPEX) for how pin risk and late exercise work.

Should you be long or short gamma?

Choose your side of gamma based on whether you expect the move to be larger or smaller than the option market is pricing, not on direction alone.

In practice most investors with stock portfolios are best served by being modestly short gamma in their income core (covered calls, cash-secured puts at 30–45 days) and long gamma in their hedge layer (puts or put spreads sized to the risk they can’t afford). The options portfolio structure guide lays out that split.

How do you manage gamma risk on short options?

Manage short gamma by controlling when you hold it: the danger is concentrated in the final week, so most of the risk can be avoided by closing or rolling before then.

• Close early. Taking profits at 50% of the premium on a 45-day sale usually exits before the steep part of the gamma curve.
• Roll before the final week. If a short call is near the money with 7 days left, rolling up and out swaps high gamma for lower gamma and fresh time value.
• Define the risk. A credit spread caps the loss that runaway gamma can cause; a naked short option doesn’t.
• Size by the tail. Ask what a 2-standard-deviation move does to the position, not what a normal day does.
• Mind the index. In 0DTE-heavy markets, intraday swings near large strikes can be exaggerated by dealer hedging in the final hours.

What should you check about gamma before a trade?

1. Note the position’s gamma and multiply by 100 × contracts: that is how many shares of exposure you add or lose per $1.
2. Check days to expiry. Under 10 days, at-the-money gamma is large enough to dominate the trade.
3. Check where the biggest open-interest strikes sit relative to the price and your strikes.
4. Check whether dealers are likely long or short gamma in the index this week (a GEX reading).
5. If you are short gamma, set the profit-take and roll date before you enter.
6. If you are long gamma, compare the theta you pay per day with the move you need, and be honest about the odds of getting it in time.

Frequently asked questions

Key takeaways

1. Gamma is the change in delta per $1 move: about 0.046 for a 30-day at-the-money option on a $100 stock at 30% IV.
2. It is highest at the strike and explodes into expiry: the same option’s gamma is about 0.25 with one day left.
3. Long gamma (owning options) profits from big moves and pays theta; short gamma (selling options) earns theta and loses on big moves.
4. Dealers who are long gamma sell rallies and buy dips, which dampens volatility and can pin prices to large strikes on expiry day.
5. Dealers who are short gamma hedge in the direction of the move, which amplifies volatility; this is the mechanism behind a “gamma squeeze”.
6. Income sellers should cut or roll short options before the final week, when gamma risk is largest relative to the theta left.

Sources and method

1. Ni, Pearson and Poteshman (2005), Stock Price Clustering on Option Expiration Dates, Journal of Financial Economics 78(1) — pinning to strikes on expiry and its 16.5 basis-point average effect
2. OCC, Characteristics and Risks of Standardized Options — exercise, assignment and expiration mechanics
3. Cboe, SPX Options Specifications — AM and PM settlement and daily expirations
4. Cboe Global Markets, Trading Volume for July 2026 — 0DTE share of SPX options volume
5. Black and Scholes (1973), The Pricing of Options and Corporate Liabilities — the model behind every gamma figure here

Gamma, delta and price figures use Black–Scholes for a non-dividend stock at $100, 30% implied volatility and a 4% rate unless stated; Nvidia examples use the October 6, 2026 close of $240.20 and an assumed 40% implied volatility. Dealer-hedging examples are simplified: real market makers hedge portfolios, not single strikes. Examples are illustrative.