Anyone who has held an at-the-money option on expiry day has seen it: the premium doubles, then halves, then doubles again within an hour. The reason is gamma, the Greek that measures how quickly an option's delta changes as the underlying moves.
Gamma is small for options with weeks to run and very large for at-the-money options in their final days. This article explains what that means, works through figures, and sets out why gamma risk is the main danger for option sellers near expiry and the main attraction for some buyers.
Delta, and how gamma changes it
Delta is roughly how much an option's premium moves for a one-point move in the underlying. An at-the-money call option has a delta of about 0.5: if the index rises 10 points, the premium rises about 5.
Gamma is how much delta itself changes for a one-point move. If gamma is high, a modest move in the index can turn an option that behaves like half the underlying into one that behaves almost like all of it, or almost like nothing.
Think of delta as speed and gamma as acceleration. With weeks to expiry, an option's speed changes gently as the market moves. In the final days, at-the-money options can accelerate or brake sharply on small moves.
Gamma is concentrated at the money. Deep in-the-money options already behave almost like the underlying, with delta close to 1, and far out-of-the-money options barely respond, with delta close to 0. Neither changes much on a small move, so both have low gamma. The strikes nearest the current price, where the outcome is most uncertain, carry the most gamma, and that concentration sharpens as expiry approaches.
Worked figures: two weeks versus one day
Illustrative figures for the 24,000 call option, with the index starting at 24,000, under ordinary volatility.
| Index moves to | 14 days left: premium (delta) | 1 day left: premium (delta) |
|---|---|---|
| 23,900 | 123 (0.41) | 12 (0.19) |
| 24,000 | 169 (0.50) | 45 (0.50) |
| 24,100 | 224 (0.60) | 112 (0.81) |
| 24,200 | 288 (0.68) | 202 (0.96) |
With two weeks left, a 100-point rise lifts the premium from 169 to 224, about a third, and delta moves from 0.50 to 0.60.
With one day left, the same 100-point rise lifts the premium from 45 to 112, nearly two and a half times, and delta jumps from 0.50 to 0.81. A 100-point fall takes it from 45 to 12, losing almost three quarters.
That is gamma at work. The same index move produces far larger percentage swings when expiry is close and the option is near the money. For anyone holding several such options, the combined position can change character within minutes.
What it means for sellers
A seller of the at-the-money option with one day left collects about 45. If the index stays put, they keep it. If the index rises 200 points, the option is worth about 202, a loss of about 157, three and a half times the premium collected.
With two weeks left, the same 200-point rise takes the option from 169 to 288, a loss of 119, about 0.7 times the premium.
So near expiry the trade-off becomes extreme: decay is fastest, which is why short-dated selling is popular, but the exposure to a moderate move is largest. A seller is effectively collecting small, frequent amounts in exchange for occasional large losses on ordinary-sized moves, not just on rare crashes.
This is gamma risk in plain terms: the risk that delta changes against you faster than you can react. A position that looked nearly flat in the morning can carry a large directional exposure by afternoon.
What it means for buyers
For buyers, high gamma is the attraction of expiry-day options. A cheap at-the-money option can multiply in value on a moderate move, as the table shows.
But the same table shows the cost: a moderate move the wrong way, or no move at all, wipes out most of the premium within hours. Time decay and gamma are two sides of one coin. The high gamma that creates large gains on a move is paid for by the rapid decay when the move does not come.
Buyers who choose near-expiry options are making a bet on a quick, sizeable move. That can be reasonable for a small, defined amount, but losing many such bets in a row is the normal outcome when moves fail to arrive, so position size must assume a string of total losses.
Managing gamma risk
Use defined-risk structures when selling. Credit spreads and iron condors cap the loss even if gamma turns a short option against you. The bought wing limits how far delta can work against you.
Close or roll before the final day. Many sellers take most of the available decay earlier and close positions before expiry day, when gamma is at its peak, rather than holding for the last few rupees.
Size for the move, not the premium. Estimate the loss on a plausible move, such as a typical day's range or larger, and size so that loss is acceptable.
Be wary of short options close to the money near expiry. The nearer the strike to the price, the larger the gamma.
Watch events. A result or policy announcement on or near expiry combines high gamma with a likely move.
Use stop rules that can actually execute. Near expiry, prices move fast and spreads widen; a stop-loss may fill well beyond its trigger.
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Frequently Asked Questions
What is gamma in options?
The rate at which an option's delta changes as the underlying moves. High gamma means delta, and so the premium's sensitivity, can change sharply on small moves.
Why is gamma highest near expiry?
For at-the-money options, little time remains for the price to move away, so a small move decides whether the option ends with value or worthless, making delta swing sharply.
What is gamma risk for option sellers?
The risk that a moderate move turns a short option's delta strongly against the seller, producing losses several times the premium collected, especially near expiry.
How do traders manage gamma risk?
With defined-risk spreads, closing before the final day, sizing for plausible moves rather than premium, and avoiding short options near the money into expiry or events.
Is high gamma good for option buyers?
It can multiply a cheap option's value on a quick move, but the same options lose most of their value quickly if the move does not come.
Related Reading
- How big a move the market is pricing
- Decay, the other side of gamma
- Delta, gamma, theta and vega overview
- The realities of expiry day
- Selling premium with a capped loss
- Sizing for the bad day
Disclaimer: TheFinBaba provides educational content only - this is not investment advice. Trading involves risk of loss.
