Compliance

Form 3921: Every ISO Exercise Puts Your 409A Number on an IRS Return

Every ISO exercise creates a Form 3921, and its box 4 reports the fair market value your 409A set. The deadlines, the penalties, and what payroll misses.

By 409.AI Team - 2026-09-25

# Form 3921: Every ISO Exercise Puts Your 409A Number on an IRS Return

Fourth quarter is when option exercises cluster at private companies. People leave and their exercise windows start running. Employees who have been sitting on vested incentive stock options decide to pull the trigger before year end for tax reasons. A tender offer closes and suddenly forty people exercise in the same three weeks.

Each one of those exercises creates a filing obligation most startups don't think about until late January, when someone in finance discovers that the company owes the IRS a separate information return for every single ISO exercise that happened during the year. The form is Form 3921, and box 4 asks for something specific: the fair market value per share on the date the option was exercised.

For a private company, that number doesn't come from a stock ticker. It comes from your 409A valuation. Which means every ISO exercise quietly converts your valuation into a figure the IRS has in writing, tied to a date, signed by your company.

What Section 6039 actually requires

[Section 6039(a)](https://www.law.cornell.edu/uscode/text/26/6039) of the Internal Revenue Code requires every corporation that transfers a share of stock to a person pursuant to that person's exercise of an incentive stock option under Section 422(b) to file a return with the IRS. [Section 6039(b)](https://www.law.cornell.edu/uscode/text/26/6039) requires the same corporation to furnish a written statement to each person named in that return.

The filing is per exercise, not per employee and not per year. If one engineer exercises three separate times in 2026, that's three Forms 3921. [Treasury Regulation 1.6039-1(a)](https://www.law.cornell.edu/cfr/text/26/1.6039-1) sets out what each one has to carry: the transferring corporation's name, address and EIN, the employee's name, address and identifying number, the grant date, the exercise date, the exercise price per share, the fair market value per share on the exercise date, and the number of shares transferred.

Two details catch companies out. First, the obligation attaches to the corporation, not to payroll, and it doesn't care whether the person still works for you. A departed employee who exercises in November is a Form 3921 you owe in January. Second, the return is required only with respect to the first transfer of legal title to the shares, which includes an immediate deposit into a brokerage account following exercise. Later transfers of the same shares don't generate new returns.

Form 3921 covers ISOs only. Employee stock purchase plan transfers under Section 423 go on Form 3922, which almost no private company needs.

Three deadlines, and the one you can't extend

The [IRS instructions for Forms 3921 and 3922](https://www.irs.gov/instructions/i3921) point to the general information return rules for timing, and those give you three dates for a calendar year of exercises:

The written statement to the employee is due by January 31 of the following year, under [Section 6039(b)](https://www.law.cornell.edu/uscode/text/26/6039) and [Regulation 1.6039-2](https://www.law.cornell.edu/cfr/text/26/1.6039-2).
Copy A filed with the IRS on paper is due by February 28.
Filed electronically, it's due by March 31.

For 2026 exercises, both of the first two dates land on a Sunday in 2027, so [Section 7503](https://www.law.cornell.edu/uscode/text/26/7503) pushes them to the next business day. Check the calendar rather than assuming, because this shifts every year.

You can request an extension of the IRS filing deadline on [Form 8809](https://www.irs.gov/forms-pubs/about-form-8809). It does nothing for the January 31 statement to the employee. That one is the hard deadline, and it's also the one employees care about, because they need the numbers to file their own returns.

Then there's the electronic filing threshold, which is where small companies get surprised. The [Form 3921 instructions](https://www.irs.gov/instructions/i3921) confirm the e-file threshold dropped to 10 returns, calculated by aggregating all information returns, for returns required to be filed on or after January 1, 2024. Aggregating means your 1099-NECs to contractors, your W-2s and your Forms 3921 all count toward the same 10. A twelve-person startup with four contractors and six ISO exercises is an electronic filer.

Box 4 is your 409A, on the record

Here is where the form stops being routine paperwork.

Box 3 is the exercise price, which is fixed and sitting in your cap table. Box 5 is the share count. Box 4 is the fair market value per share on the exercise date, and for a company with no public market there is exactly one defensible source for it: the 409A valuation in effect on that date.

Say an employee exercises 40,000 shares at a $0.42 strike. The 409A in effect says common stock is worth $2.10. The spread is 40,000 times $1.68, or $67,200, and that's what box 4 drives.

Now assume a Series B closed six weeks before that exercise and the board never refreshed the valuation. The supportable fair market value on the exercise date was $3.80. The real spread is $135,200, roughly double what you reported. You've filed an incorrect information return, and the employee has computed a tax position off a number you gave them.

This is the mechanism that turns a valuation timing problem into a reporting problem. A 409A is a point-in-time opinion, and [the trigger events that require a refresh](https://409.ai/articles/409a-valuation-frequency-how-often-should-you-get-one) don't wait for the twelve-month mark. A priced round, a tender offer, a major secondary, a big change in results: any of those can make the number you're about to write in box 4 indefensible.

It also cuts the other way. [Safe harbor under Section 409A](https://409.ai/articles/409a-safe-harbor-price-vs-qualified-appraiser) makes your valuation presumptively reasonable, and the IRS has to show it was grossly unreasonable to dislodge that. If you've been putting FMV figures on information returns for three years against independent appraisals, you have a coherent record. If you've been using a board estimate or a stale report, you've handed over a series of dated, self-reported numbers that [an examiner can line up against each other](https://409.ai/articles/irs-audit-409a-valuation-document-request).

Why payroll won't catch these

NSO exercises run through payroll. The spread is wage income, it's subject to withholding, and your payroll provider produces the W-2 without anyone thinking about it.

ISO exercises don't work that way. [Section 3121(a)(22)](https://www.law.cornell.edu/uscode/text/26/3121) excludes from FICA wages any remuneration on account of a transfer of stock pursuant to the exercise of an incentive stock option, or any disposition of that stock. The spread at exercise isn't W-2 income either. So an ISO exercise generates no payroll event at all, and nothing in your normal year-end process trips over it. The only record is in the cap table.

Two rules decide what actually belongs on a Form 3921, and both of them are easy to get wrong.

The $100,000 limit splits a single exercise

Under [Section 422(d)](https://www.law.cornell.edu/uscode/text/26/422), to the extent the aggregate grant-date fair market value of stock with respect to which ISOs are exercisable for the first time in any calendar year exceeds $100,000, the excess isn't an ISO.

Take a grant with a $2.50 grant-date FMV. That caps ISO treatment at 40,000 shares becoming first exercisable in a year. If acceleration makes 60,000 shares first exercisable in 2026, then 40,000 are ISO shares and 20,000 are NSO shares. One exercise of all 60,000 produces a Form 3921 for 40,000 shares and W-2 income for the other 20,000. [How a higher 409A pushes more of a grant past the ISO limit](https://409.ai/articles/iso-100k-limit-409a-grant-date-fair-market-value) is worth understanding before you try to reconstruct the split in January.

The three-month rule quietly converts options

[Section 422(a)(2)](https://www.law.cornell.edu/uscode/text/26/422) requires the holder to have been an employee at all times from the grant date until the day three months before exercise. Miss that, narrow exceptions for disability and death aside, and the option isn't an ISO when exercised, whatever the grant agreement calls it.

This one bites companies that have been generous with exercise windows. If you [extended post-termination exercise to seven or ten years](https://409.ai/articles/extending-post-termination-exercise-window-iso-nso-409a), every option exercised more than three months after the person left is an NSO. It belongs on a W-2, not a Form 3921. Plenty of companies file Form 3921 for those exercises anyway, reporting as an ISO exercise something that stopped being an ISO months earlier.

What the employee does with the form

Employees mostly encounter Form 3921 when their accountant asks for it, and there are two reasons it matters to them.

Alternative minimum tax is the first. [Section 56(b)(3)](https://www.law.cornell.edu/uscode/text/26/56) provides that Section 421 doesn't apply to the transfer of stock acquired through exercise of an incentive stock option, which is the drafting that makes the exercise spread an AMT adjustment in the year of exercise. Box 4 minus box 3, times box 5, is the number that flows into that calculation. An employee holding shares with no way to sell them can owe real cash tax on a paper gain, which is why [the difference between ISO and NSO treatment](https://409.ai/articles/iso-vs-nso-how-stock-options-are-taxed) shapes exercise timing so heavily.

Holding periods are the second. [Section 422(a)(1)](https://www.law.cornell.edu/uscode/text/26/422) requires no disposition within two years of grant and holding the shares for one year after transfer. Box 1 is the grant date and box 2 is the exercise date, so the form carries both clocks. Sell early, in a tender offer or [a secondary sale](https://409.ai/articles/secondary-sale-startup-shares-qsbs-tax-treatment), and the disposition is disqualifying, with the spread becoming ordinary income instead.

The penalties stack

Two separate provisions apply to one exercise. [Section 6721](https://www.irs.gov/payments/information-return-penalties) covers failure to file a correct information return with the IRS. Section 6722 covers failure to furnish a correct payee statement. Missing an exercise entirely trips both.

The IRS publishes the amounts by year. For 2026 returns they run $60 per return if corrected within 30 days, $130 if corrected by August 1, $340 if filed after August 1 or not filed at all, and $680 for intentional disregard. Annual maximums apply and scale with gross receipts, but intentional disregard has no cap.

Run that against 60 ISO exercises you missed and discover in September: 60 exercises times $340 under each provision is $40,800. The underlying tax was correct, nobody underpaid anything, and the exposure comes entirely from paperwork that wasn't filed.

Do this before December instead of in January

Pull the exercise log now, while the year is still open. For each exercise, answer three questions. Was it actually an ISO exercise after the $100,000 split and the three-month rule? What 409A was in effect on the exercise date? Does the valuation date coverage hold up, or did a round, a tender offer or a material change land mid-year without a refresh?

The third question is the only one you can still fix. A [409A valuation](https://409.ai/products/409a) dated before the exercises it needs to support is straightforward. Reconstructing a defensible fair market value for an exercise that happened nine months ago, after a round you didn't get revalued for, is not.

Box 4 isn't a formality. It's the one place your company states, in writing and to the IRS, what a share of common stock was worth on a particular day, and your employee's AMT bill gets computed from it. Get the valuation dates right during the year and January is a data export. Get them wrong and you're mailing corrected forms to people who already filed their returns.