Cost basis after assignment: count each premium once
An assigned put changes share basis; an exercised call changes sale proceeds. Use one position's transaction history to separate those adjustments from expired premium and avoid counting the same credit twice.
By OptionsKing. Publisher and editorial methods.
What you will learn
By the end of this lesson, you should be able to:
- Compute share basis from the assigned put's strike and premium.
- Keep expired-option income separate from stock tax basis.
- Recognize double-counting in both tax reporting and performance tracking.
Keep tax basis separate from a strategy's cumulative cash break-even. For a standard assigned equity put, the credit on that specific contract reduces the cost basis of the acquired shares. Premium from earlier expired puts or later expired calls is a separate realized option result, not another reduction in those shares' tax basis.
The two rules, verbatim
Put exercised: "Reduce your basis in the stock you buy by the amount you received for the put."
Call exercised: "Increase your amount realized on the sale of the stock by the amount you received for the call."
And the third case, which is where people go wrong: option expires: "Report the amount you received for the put as a short-term capital gain." Not a basis adjustment. A standalone gain, and it never touches the basis of anything.
The full chain on one position
INTC from the running book, which was assigned in cycle 3 and still held at year end, having had five covered calls written against it in between.
| Cycle | Event | Premium | Basis after | Why |
|---|---|---|---|---|
| 1 | $21.50 put expired | $46 | no shares yet | short-term gain, standalone |
| 2 | $22 put expired | $41 | no shares yet | short-term gain, standalone |
| 3 | $22.50 put assigned | $43 | $22.07 | strike $22.50 less $0.43 |
| 4 | $23 call expired | $44 | $22.07 | unchanged, standalone gain |
| 5 | $24.50 call expired | $36 | $22.07 | unchanged, standalone gain |
| 6 | $28 call expired | $42 | $22.07 | unchanged, standalone gain |
| 7 | $30 call expired | $43 | $22.07 | unchanged, standalone gain |
| 8 | $31 call expired | $46 | $22.07 | unchanged, standalone gain |
$341 of premium collected on this position across the year, and exactly $43 of it touched the basis. The other $298 was taxed as it was earned and left the basis alone.
The mistake, and what it costs
The comparison below isolates one error: subtracting $211 of expired call premium from the correct $22.07 share basis. That gives $19.96, while the $211 has already been reported as option gain. Subtracting all $341 of historical premium would instead give $19.09 and would double-count still more previously reported income.
The intuitive move is to treat every dollar of premium as lowering your cost. It feels right: you paid $22.50 and collected $3.41 of premium along the way, so surely you are in at $19.09.
Economically, that is a reasonable way to think about your break-even. For tax it is wrong, and it is wrong in a direction that costs you money.
| Correct | The mistake | |
|---|---|---|
| Basis | $22.07 | $19.96 |
| Proceeds at $29.40 | $2,940 | $2,940 |
| Reported gain | $733 | $944 |
| Premium already reported as income | $298 | $298 |
| Total reported | $1,031 | $1,242 |
| Tax at an assumed 32% | $330 | $397 |
You would pay tax on $211 you already paid tax on. The expired call premium was income when it expired; folding it into the basis as well reports it twice, and both times against you.
The mirror image, in your performance ledger
The same premium, the same $588 across the book's five assignments, causes the opposite error in the other direction when it lands in a spreadsheet.
If your ledger records the credit as income, and then measures the assigned shares against the reduced tax basis, the premium has been counted twice in your favour. On the running book that inflated the year by exactly $588: $278 on AMD, $215 on MU, $43 on INTC, $40 on KMI, $12 on WBD.
So the same figure produces two opposite mistakes depending on which document you are filling in. The rule that resolves both:
- For tax: the assigned put's premium reduces initial share basis. Other tax adjustments, including a disallowed wash-sale loss, may also apply; unrelated expired premium does not reduce that basis.
- For performance: one consistent method measures share moves against the assignment strike and records premium separately. An adjusted-basis method can also work if the same credit is not counted again. The ledger page has the reconciliation.
Two numbers, two purposes, and neither is a corrected version of the other.
The call side, and the bit people miss
When a call is exercised, the premium increases the amount realized rather than reducing the basis. AMD from the running book: called away at $123, and the $227 call premium made the proceeds $12,527 rather than $12,300.
The consequence is that the call premium inherits the share's holding period. The publication: on exercise, "The gain or loss is short-term or long-term depending upon the holding period of the stock." On a stock you had held for two years, that premium is taxed at long-term rates. On the 92-day AMD position it was short term, along with everything else. The holding period page covers why 92 days is the norm here.
Three practical notes
- Reconcile the broker's 1099-B with your transaction records. Publication 550 provides an adjustment procedure when exercised-call premium is not reflected in reported sale proceeds; that instruction does not establish how often a mismatch occurs. Check the basis on any lot that arrived by assignment.
- Record the assignment strike and the credit, per lot, on the day. Reconstructing $22.07 from a year-old statement is painful and reconstructing it across two assignments on the same ticker is worse.
- A disallowed wash-sale loss also adds to basis. If any of this position was subject to a wash sale, that adjustment stacks on top of the premium reduction.
This page explains mechanics and quotes the IRS publication it takes them from. It is not tax advice, it does not know your bracket, your state, your filing status or your other positions, and one sentence in your own situation can change the answer. Take the worked examples to whoever prepares your return.
Check your understanding
A $30 put is assigned after you received a $1 premium. Later calls expire for another $2 per share of total premium. What is the shares' tax basis before any other adjustments?
Show the explanation
$29 per share: $30 minus the assigned put's $1 premium. The later $2 of expired-call premium is separate short-term option gain, not another basis reduction. A cumulative strategy break-even may incorporate those cash flows, but it is not the tax basis.
Common questions
What is my cost basis after a put is assigned?
The strike less the premium you received for the put. A $22.50 put sold for $0.43 gives a basis of $22.07 per share, or $2,207 for the lot.
Does covered call premium reduce my cost basis?
An expired call's premium is separate short-term gain. An exercised call increases stock sale proceeds rather than reducing basis. In this example only the assigned put's $43 credit changes initial share basis.
What happens if I subtract all my premium from my cost basis?
You can report previously taxed income twice. The table isolates subtracting $211 of expired calls again, producing $19.96 rather than $22.07. Subtracting all $341 of historical premium would produce a different, also incorrect, tax basis.
Why does my spreadsheet show a bigger gain than my account?
Probably because the premium is counted twice: once as income and again inside a reduced cost basis. On the worked book that inflated the year by exactly $588, the put premium on the five assigned positions.
Sources
Rules and thresholds above were checked against these documents on August 4, 2026. Exchange and broker rules change. Confirm anything you are about to act on with your own broker.
Keep reading
Do the math on your own trade
Every price in this article is an illustrative worked example, not a quote. Read Taxes for sellers for the rest of the series, and the disclaimer before you act on any of it. Selling options carries real risk of loss, and the loss can be far larger than the premium you collected.