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Gamma risk: why the last week can bite an option seller

Gamma is how fast an option's delta changes when the stock moves. Near expiry it gets large, so a short option can go from "probably fine" to "probably assigned" on a small move. Here's a weekly put that does exactly that, worked out day by day.

Options guide · 9-minute read · Updated 2026-10-27

The short answer

Gamma measures how fast Delta changes when the stock moves $1. Low gamma means your position's risk changes slowly. High gamma means it can change in hours.

Gamma is highest on options near the strike and close to expiry. That's why weekly options and expiration week are where sellers get surprised. The fast Theta you collect in the last days comes with fast-changing risk. Sellers call that Gamma risk.

Delta is the speed; gamma is the acceleration

Think of delta as how fast your position gains or loses as the stock moves. Gamma is how fast that speed itself changes.

A short put with a delta of 0.25 loses about $25 per contract for a $1 drop. If gamma is low, the next $1 drop costs about the same. If gamma is high, the next drop might cost $45, then $65. The losses speed up as the stock falls toward your strike.

For a seller, gamma always works against you. You're short gamma: every move away from you makes the next one hurt more.

A worked example: two puts, same stock, same drop

This is an illustration, not a trade idea. A made-up stock, XYZ, trades at $50 on Monday. Implied volatility is 30%. The prices are rough model numbers.

You look at two puts with about the same delta:

• A weekly $49 put, 5 days left, for $0.30. Delta 0.27. Gamma about 0.19.
• A 45-day $47 put for $0.80. Delta 0.25. Gamma about 0.06.

The weekly's gamma is roughly three times bigger. Same odds today, very different behaviour tomorrow.

Wednesday: XYZ drops $2 to $48

A 4% drop took the weekly from a 27% shot at ending in the money to about 77%. It's now behaving like 77 shares of stock. The 45-day put barely moved by comparison: it went from 25 shares' worth to 38.

The weekly's loss, −$85, is almost three times the $30 you collected. The 45-day put's loss is smaller than its premium.

Friday: XYZ closes at $46

The weekly put, every outcome

Here's the weekly on its own at Friday's close, so you can see the full spread of results.

Every row below the break-even loses more than the trade could ever make. Each $1 below the break-even costs more than three trades' worth of premium.

Why the last week works this way

Near expiry, there's little time for the stock to come back. So the option's value hinges almost entirely on which side of the strike it ends.

• Stock a little above the strike: the put is worth pennies.
• Stock a little below: the put is worth the full gap.

The price flips between those two states quickly. That flip is gamma. On expiration day, a stock bouncing around your strike can swing delta from 0.20 to 0.80 and back in an afternoon. See pin risk explained for what that does to assignment.

What the weekly seller could have done on Wednesday

At $48 with three days left, there were three choices. None was free.

1. Close for $1.15. Take the −$85 loss and walk away. The cleanest choice if you didn't want the shares at $49.
2. Roll to a later expiry and lower strike for a small credit. This buys time but keeps you in a falling stock. Rolling options explained covers the math.
3. Hold and accept assignment. Fine only if you'd truly buy 100 shares at $49 and the $4,900 fits your plan.

What the seller should not do is "hope it bounces by Friday." With gamma this high, hoping is a real position.

The rule for next time

• If you sell weeklies, size them smaller than you'd size a monthly. The swings per dollar are bigger.
• Decide your exit before expiration week. Many sellers close or roll by about 21 days left, or at a set loss, so they're not holding into the steepest gamma.
• Don't sell near-the-money weeklies before news. Earnings, a Fed day or a big data release can move a stock more than the week's premium ever paid for.

In the wheel strategy, gamma is the reason many sellers prefer 30 to 45 days. You give up some daily decay, and in return the position changes slowly enough to think. 7 DTE vs 30 DTE compares the two.

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