⬤ ISO Reporting · Tax Compliance
IRS Form 3921, Decoded for Founders and Finance teams
When an employee exercises an incentive stock option, the clock starts on a federal information return most companies underestimate. Here is exactly what Form 3921 reports, who files it, when it is due, and how a defensible fair market value keeps the whole filing clean.
ISO Exercise Record
AMT Spread · (Box 4 − Box 3) × Box 5
$53,000
Form 3921 is one of those filings that feels minor until it is missed. It carries its own deadlines, its own penalty schedule, and a single field – the fair market value on the exercise date – that ripples into every employee’s alternative minimum tax. This guide walks through all of it in plain language, then shows where the numbers actually come from.
What Form 3921 actually is
Form 3921, formally titled Exercise of an Incentive Stock Option Under Section 422(b), is an information return a corporation files with the IRS each time an employee exercises an incentive stock option (ISO). It is not a tax payment and it does not, by itself, create a tax bill. Its job is to put four facts on the record the grant, the exercise, the price paid, and the value received so the IRS and the employee can later calculate any tax the exercise triggers.
A separate Form 3921 is filed for every ISO exercise by every employee. Ten employees who each exercise once means ten forms. One employee who exercises three separate grants in the same year may generate three.
Each form comes in three copies, and each copy has a different destination:
A
Goes to the IRS
The official copy transmitted to the IRS, either electronically or on the IRS’s scannable red-ink paper. This is the copy with the hard deadline and the penalties attached.
B
Goes to the IRS
The official copy transmitted to the IRS, either electronically or on the IRS’s scannable red-ink paper. This is the copy with the hard deadline and the penalties attached.
C
Goes to the IRS
The official copy transmitted to the IRS, either electronically or on the IRS’s scannable red-ink paper. This is the copy with the hard deadline and the penalties attached.
- If you paper-file Copy A, you cannot print it from a PDF on plain paper. The IRS processes Copy A through optical scanners that only read its official red-ink stock, which you order from the IRS. A plain-paper printout can be rejected outright.
Who has to file it
The filing obligation sits with the company that issued the stock, not the employee. If your corporation granted ISOs and an employee exercised any of them during the calendar year, you file Form 3921 the following year, before the deadlines below.
A quick illustration. Say you granted ISOs to an engineer in 2022. In 2025, she exercises a portion of them. That exercise is a reportable event for the 2025 tax year, so your company files her Form 3921 in early 2026 and gives her Copy B by the January deadline. The form tells the IRS how many shares transferred and at what value – nothing about whether she has sold them yet.
- If you paper-file Copy A, you cannot print it from a PDF on plain paper. The IRS processes Copy A through optical scanners that only read its official red-ink stock, which you order from the IRS. A plain-paper printout can be rejected outright.
Why it exists: the AMT link
To see why this form matters, follow the money at exercise. An employee pays the strike price and receives shares. Because ISOs are usually granted years before exercise, the company’s value has often climbed in the meantime, so the shares received are worth more than the price paid. That gap – the difference between fair market value on the exercise date and the strike price – is called the bargain element, or spread.
Here is the wrinkle that surprises people: for regular income tax, exercising an ISO and holding the shares creates no taxable income at all. But for the alternative minimum tax (AMT), that same spread is counted as a preference item in the year of exercise. An employee can owe AMT on a paper gain from shares they have not sold and cannot necessarily sell.
Form 3921 is what makes that calculation possible. It hands the employee the exact figures – strike price, FMV, and share count – needed to compute the spread and feed it into their AMT worksheet. Without the form, neither the employee nor the IRS has a clean record of the number.
A box-by-box Anatomy
Most of Form 3921 is information you already have on hand. One box is the exception and it is the one that drives the tax math.
Transferor details
Name, address, and employer identification number of the corporation transferring the stock.
Employee details
Name, address, and taxpayer identification number of the person who exercised the option.
Date option granted
The date the ISO was originally awarded to the employee.
Date option exercised
The date the employee actually exercised — the moment the reportable event occurred.
Exercise (strike) price per share
What the employee paid per share, fixed back at the grant date.
Fair market value per share on the exercise date
The single number you usually do not already know. For a private company it comes from a current 409A valuation, and it sets the size of the AMT spread.
Number of shares transferred
How many shares moved to the employee through the exercise.
Account number
Optional. Used only when a company maintains multiple accounts for one employee requiring more than one form.
- Boxes 2, 3, and 5 are facts the employee usually knows. Box 4 is the variable - and getting it right is a valuation question, not a clerical one.
How to file: E-file vs. Paper
There are two routes to the IRS, and recent rule changes have made one of them the default for almost everyone.
Filing electronically
Under current IRS rules, businesses filing 10 or more information returns of any type in aggregate during the year must e-file a sharp drop from the old 250-return threshold. In practice, most companies issuing ISOs now fall on the electronic side of the line. Electronic filing runs through the IRS’s information-return systems, and first-time filers need a Transmitter Control Code (TCC) before they can submit, which takes time to obtain. Start early.
To file electronically you will generally need a list of employees who exercised, their taxpayer identification numbers, your TCC, and an account set up to transmit returns.
Filing on paper
If you fall under the e-file threshold, you may paper-file Copy A — but only on the official IRS scannable form ordered directly from the IRS. The downloadable PDF is for reference; it is not acceptable for submission. Copies B and C, by contrast, can be printed normally for the employee and your records.
- Thresholds and procedures shift from year to year. Confirm the current e-file requirement and form-ordering process on IRS.gov before you file, and build in lead time for a TCC if this is your first electronic filing.
Deadlines that matter
Form 3921 has not one deadline but three, staggered across the early months of the year after the exercise.
By January 31
Furnish Copy B to employees
Every employee who exercised an ISO last year must receive their recipient statement. They need it to prepare their own return, so this is the deadline that affects real people most directly.
By End of February
Paper-file Copy A with the IRS
If you are eligible to paper-file, Copy A must reach the IRS on the official scannable form by the late-February deadline.
By End of March
E-file Copy A with the IRS
Electronic filers get an extra month. Since most companies now sit above the e-file threshold, this is the operative deadline for the majority.
Penalties for late filing
The penalty for a late or incorrect Form 3921 scales with how late you are – and it applies per form, meaning per employee exercise. The figures below are the structure the IRS uses; the exact dollar amounts and annual caps are indexed for inflation and change each year, so treat these as the framework rather than the current rate.
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When you correct the filing
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Penalty tier (per form)
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What it means
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|---|---|---|---|---|
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Within 30 days of the deadline
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Lowest tier
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The smallest per-form amount, with an annual cap that is reduced for small businesses.
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||
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After 30 days, but by August 1
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A higher per-form amount and a higher annual cap.
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|||
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After August 1, or never filed
|
||||
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Intentional disregard
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No cap
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|||
- The penalties stack. Failing to file with the IRS and failing to furnish the employee copy are treated as two separate violations - so ignoring both effectively doubles your exposure. Filing on time is far cheaper than any tier above.
Form 3921 vs. Form 3922
These two forms are siblings and are easy to mix up. They cover different equity events under different parts of the tax code.
Form 3921
Exercise of an ISO
- Filed when an employee exercises an incentive stock option
- Governed by Internal Revenue Code Section 422(b)
- Its figures feed directly into the employee’s AMT calculation
- Box 4 (FMV at exercise) is the key driver
Form 3922
Transfer under an ESPP
- Filed when stock is first transferred under an employee stock purchase plan
- Governed by Section 423(c)
- Informational only - not entered on the employee’s return at transfer
- Its figures matter later, when the employee eventually sells
Where Box 4 comes from
Everything on Form 3921 is straightforward except the fair market value on the exercise date and that one field carries the most weight. For a publicly traded company, FMV is simply the market price. For a private company, there is no market price to read off a screen, so FMV has to be established by an independent appraisal: a 409A valuation.
That valuation is what populates Box 4. If the figure is too low, the IRS can challenge it and reassess the employee’s AMT exposure. If it is stale based on a snapshot from many months earlier it may not defensibly represent value on the actual exercise date. A current, properly supported 409A valuation is what keeps the whole filing on solid ground, and it is grounded in the same fair-market-value principles the IRS has applied for decades under Revenue Ruling 59-60.
This is precisely where Countsure works. We prepare independent, audit-ready 409A valuations that give you a defensible Box 4 figure, alongside the broader business-valuation work that surrounds equity compensation. If your team is approaching a wave of ISO exercises, getting the valuation right before the forms go out saves a great deal of trouble later.
Here is the wrinkle that surprises people: for regular income tax, exercising an ISO and holding the shares creates no taxable income at all. But for the alternative minimum tax (AMT), that same spread is counted as a preference item in the year of exercise. An employee can owe AMT on a paper gain from shares they have not sold and cannot necessarily sell.
Form 3921 is what makes that calculation possible. It hands the employee the exact figures – strike price, FMV, and share count – needed to compute the spread and feed it into their AMT worksheet. Without the form, neither the employee nor the IRS has a clean record of the number.
Frequently asked questions
Not from the form itself. Form 3921 is purely an information return. Exercising and holding an ISO creates no regular income tax, though the spread may count toward the employee’s AMT for that year. The form simply supplies the figures needed to work that out.
Per exercise. A single employee who exercises two separate ISO grants in the same year can generate two forms. The count of forms you file equals the count of reportable exercise events.
Penalties apply per form and increase the longer the error stands, with a much steeper, uncapped penalty for intentional disregard. Correcting quickly keeps you in the lowest tier, so fix mistakes as soon as you spot them rather than waiting.
No, not for the IRS copy if you are paper-filing. Copy A must be the official scannable form ordered from the IRS. The PDF version is informational. Copies B and C, however, can be printed normally.
For a private company, from a current 409A valuation performed by an independent appraiser. That valuation establishes the per-share FMV on the exercise date, which is the figure that sets the size of the AMT spread.
Get Started
Filing Form 3921 soon? Start with a defensible Box 4.
Countsure delivers independent, audit-ready 409A valuations that give you a fair market value you can stand behind – before the forms go out and the AMT figures reach your employees.
This guide is general information, not legal, tax, or accounting advice. IRS thresholds, deadlines, and penalty amounts change periodically always confirm the current rules on IRS.gov or with a qualified professional for your specific situation.
