How to Track Partial Options Assignments: A Complete Math Guide

Partial assignments are one of those things you only experience once before you realize how unprepared your tracking setup is for them. You opened 5 contracts. The stock closes just below your strike at expiration. The next morning, 2 contracts have been assigned and 3 have expired worthless. Your spreadsheet doesn't know how to handle this, your broker shows a confusing split confirmation, and your ACB is now a mystery.

Here's the complete guide to recording and calculating partial assignments correctly.

Why Does Partial Assignment Happen?

Partial assignment happens when only some of your contracts are exercised at expiration. This is more common than most traders expect when a stock closes right at the strike price. The OCC (Options Clearing Corporation) uses a random allocation process to distribute exercise notices among short option holders, meaning there's no way to predict which specific contracts will be assigned. The result: you might get assigned on 1 of 5, 2 of 5, or any other combination.

How Do You Split the Premium Between Assigned and Expired Contracts?

The correct method is simple pro-rata allocation. If you collected $3.00/share in cumulative premiums across all 5 contracts and 2 get assigned, you split like this:

Premium allocated to assigned contracts (2 of 5): $3.00 × 200 shares = $600 → reduces your assignment cost basis

Premium allocated to expired contracts (3 of 5): $3.00 × 300 shares = $900 → realized as short-term capital gain (separate from the stock position)

What Is Your ACB on the Assigned Shares?

Using the pro-rata premium from above:

ACB = (Assignment Strike × Assigned Shares − Allocated Premium) ÷ Assigned Shares

ACB = ($150 × 200 − $600) ÷ 200 = $147.00 per share

What Are the Most Common Partial Assignment Tracking Mistakes?

1. Applying all cumulative premiums to the assigned shares only: This understates your ACB and overstates the profitability of the stock position. The premium from expired contracts is separate realized income and should be recorded as such.
2. Forgetting to record the expired contracts as closed trades: Those 3 expired contracts represent $900 in realized short-term gains. They need to be on Schedule D at tax time — they don't just disappear.
3. Tracking the wrong number of shares going forward: After a partial assignment of 2 contracts, your stock position is 200 shares. Not 500. Every subsequent covered call calculation, position size check, and ACB update should be based on 200 shares.

Partial assignments are rare enough that most traders never build the right spreadsheet infrastructure to handle them cleanly. OptionWheelTracker handles pro-rata allocation automatically — log the number of contracts assigned vs expired and the math takes care of itself.