How close to the short strike is too close on the last day?

A band, not a point

Pin risk does not need the stock to close exactly on the strike. What counts is the distance to the short strike compared with the move one more day can bring. That one-day, one-standard-deviation move is the price × IV × the square root of (1 ÷ 365). When the distance is a small fraction of it, the model puts the odds of the short option finishing in the money at about even, whichever side of the strike the price sits on. The probability-of-profit calculator, set to a short put or call with 1 day to expiration, shows the same picture.

Why waiting for the close settles nothing

The holder can still decide after the final close, within cut-off times that differ by product, exchange and broker; the expiration page covers who decides, and your broker’s rules are the ones that bind. Once the options have stopped trading, you can no longer close the spread, while the price behind that decision can keep moving. The last moment at which you choose the outcome yourself is therefore before the close, not after it.

What deciding late costs

Closing has a price you can see: the short option’s remaining value, less what the long leg still fetches, plus part of the gap between bid and ask. Near the strike with a day left, that value is almost pure time value and can be a large share of the credit. Letting it run has a price you cannot see yet: an assignment you did not plan, on 100 shares per contract, after the long leg has expired. The choice is between paying a known amount for certainty and leaving the outcome to a coin toss.

The common misreading

Pin risk is often treated as the rare case of a close exactly on the strike, and traders relax once the stock sits a few cents away. With a day left, a few cents are a small fraction of the one-day move, so the odds of finishing in the money barely differ from a close right on the strike. Measure the distance in multiples of the one-day move and decide while you can still trade, not by the cents on the screen.

Worked example

Hypothetical numbers for illustration, not a market quote and not a recommendation.

An example bull put spread: short 95 € put, long 90 € put, 1.50 € credit. A day before expiration the stock is at 95.10 €, implied volatility 30 %. The one-day move is 95.10 × 0.30 × √(1/365) ≈ 1.49 €, so 0.10 € above the strike is about 7 % of it. As a model value (lognormal, rate zero), the 95 € put has about a 48 % chance of finishing in the money and is still worth about 0.55 € (55 € per contract), over a third of the credit; the 90 € put is worth almost nothing.

Both expiring at a 95.10 € close keeps the planned 150 €. If the 95 € put is exercised after a drop after the close, you buy 100 shares for 9,500 €: at a next-session open of 93 € that is −200 € on the shares, −50 € with the credit; at 85 € it is −850 €, beyond the 350 € maximum loss the credit spread calculator shows, as the 90 € put has expired. Fees are ignored.

Strategies where the term matters

• Short Iron Condor — Is the range you expect genuinely narrower than the move the market has priced in?
• Bull Put Spread — Do you want a credit against an obligation - or would you rather pay a debit?
• Covered Call — Are you willing to hand over the shares at the call strike - even if they keep running afterwards?

Calculators for this term

• Probability of Profit Calculator
• Credit Spread Calculator

Track it in your journal

• Options trading journal for spreads and multi-leg strategies

Related terms

• Expiration
• Assignment
• Expected move

Every term in one place: the options glossary.