Form 3921 is the IRS information return your company must file whenever an employee exercises an incentive stock option (ISO). If any of your team members exercised ISOs last year, you have a filing obligation — and a set of deadlines that don't care whether this is your first time dealing with it. Miss them and the penalties start at $60 per form.
This guide covers what Form 3921 is, who has to file it, exactly what data goes on it, when it's due, and what it means for your employees on the other side.
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Generating Form 3921 manually means pulling grant dates, exercise dates, strike prices, and 409A FMV values from separate records, then formatting them into IRS-compliant output for each exercise. For a company with ten or twenty exercises in a year, that's a meaningful admin task with real error risk.
Cake generates IRS-ready Form 3921 reports directly from your cap table data. The platform auto-pulls strike prices, 409A fair market values matched to each exercise date, grant records, and exercise details — producing all three copies in one action. Copy B is automatically distributed to employees' Documents area in MyCake, and built-in deadline tracking keeps the February and March dates visible without a separate reminder system.
See how Form 3921 works in Cake
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What is Form 3921?
Form 3921 is an IRS information return filed by corporations when an employee exercises an incentive stock option (ISO). Its official name: "Exercise of an Incentive Stock Option Under Section 422(b)."
Think of it like a 1099. The company doesn't pay tax on it, and neither does the employee at exercise time in most cases. But the IRS wants a record. The form tells the agency that a specific person received compensation in the form of an ISO exercise — giving it the data needed to verify what shows up (or should show up) on that person's tax return.
One form is required for each individual exercise, not each employee. If an employee exercised three separate grants in the same year, that's three forms.
"Made it easy to get our startup's shares issued and compliant." — Eric Shuss, Director at Armanino LLP
Who has to file Form 3921?
Any US corporation that transferred stock to an employee via ISO exercise during the calendar year must file Form 3921. The obligation falls on the company, not the employee.
A few things founders often miss:
Former employees count. If someone left your company but still had options and exercised within their post-termination exercise period, you're still required to file for them.
One form per exercise, not per person. If the same employee exercised options on multiple dates, each exercise is a separate form.
Nonresident aliens are excluded. If you have an employee who is a nonresident alien and not required to receive a W-2, Form 3921 doesn't apply to that exercise.
ISOs only, not NSOs. This form applies specifically to incentive stock options, not NSOs. Non-qualified stock options have different reporting requirements, typically handled through W-2 Box 12.
What information goes on Form 3921?
Form 3921 has five required data fields. They are straightforward once you know where each number comes from.
| Box | What it asks for | Where the data comes from |
|---|---|---|
| Box 1 | Date the option was granted | Grant date on the equity plan record |
| Box 2 | Date the option was exercised | Exercise date confirmed by the company |
| Box 3 | Exercise price per share | The strike price set at grant |
| Box 4 | Fair market value per share on the exercise date | Your most recent 409A valuation |
| Box 5 | Number of shares transferred | Total shares acquired in this exercise |
The number that matters most is the spread: Box 4 minus Box 3. If an employee exercised 10,000 options at a $1.00 strike price when the FMV was $5.00, the spread is $4.00 per share, or $40,000 total. That spread drives AMT exposure — more on that in the employee section below.
Form 3921 deadlines
Form 3921 has three separate deadlines, each covering a different obligation.
February 2 — Provide Copy B to every employee (or former employee) who exercised ISOs in the prior calendar year.
March 2 — File Copy A with the IRS if filing on paper.
March 31 — File Copy A with the IRS if filing electronically.
E-file threshold. If your company files 10 or more information returns of any type in total during the year (Forms 1099, W-2, 3921, 3922, and others combined), you're required to e-file. Most startups with multiple ISO exercisers hit this threshold. The IRS FIRE system or an authorized e-file provider handles submission.
Three copies exist for a reason:
- Copy A: Filed with the IRS
- Copy B: Provided to the employee (used when filing their tax return)
- Copy C: Retained in company records
Penalties for missing the deadline
The IRS structures Form 3921 penalties per form, based on how late the filing is.
| When you file | Penalty per form | Annual max (small businesses) |
|---|---|---|
| Within 30 days of deadline | $60 | $630,500 |
| 31 days late through August 1 | $120 | $1,981,500 |
| After August 1 | $310 | $3,783,000 |
A startup with 20 ISO exercises that files two months late faces around $2,400 in penalties. Avoidable with a little calendar management. Intentional disregard removes the annual cap entirely, so penalties scale without limit in that scenario.
Penalty amounts are indexed to inflation and increase annually.
Form 3921 vs Form 3922
Form 3921 and Form 3922 are often mentioned together because they both fall under IRC Section 6039 reporting. They cover different situations.
| Form 3921 | Form 3922 | |
|---|---|---|
| Trigger | Employee exercises an ISO | First transfer of stock acquired through an ESPP |
| Who files | The company | The company |
| Key data reported | Grant date, exercise date, strike price, FMV at exercise, shares | Grant date, purchase date, FMV at both dates, purchase price, transfer date, shares |
| Employee copies due | February 2 | February 2 |
If your company doesn't have an Employee Stock Purchase Plan, Form 3922 doesn't apply. Most early-stage startups only deal with Form 3921.
What Form 3921 means for employees
Form 3921 isn't just a company compliance form — employees need it to file their own taxes correctly.
The spread between the strike price and the FMV at exercise (Box 4 minus Box 3) is an AMT preference item in the year of exercise. It gets added to the employee's alternative minimum taxable income and can trigger an AMT liability even if no shares were sold. This is one of the most common surprises for employees who exercise a large position in a strong year.
"Cake Equity is solving the complexity and inefficiency often associated with managing company equity... it simplifies issuing, tracking, and reporting on share options for both founders and team members." — G2 Reviewer (Verified)
Two things employees commonly get wrong:
The AMT basis adjustment. When an employee exercises ISOs and later sells those shares, they have two different cost bases: a regular tax basis (the strike price) and an AMT basis (the strike price plus the spread recognized at exercise). If they use only the regular basis when calculating capital gains, they effectively double-count the bargain element and overpay taxes. Form 3921 is what makes the AMT basis traceable years later.
Exercise date vs sale date. The ISO holding period clock starts at exercise, not at sale. Employees need both dates to confirm whether a disposition is qualifying (long-term capital gains treatment) or disqualifying (ordinary income). Box 2 of Form 3921 — the exercise date — is the anchor for that calculation.
Employees should give Form 3921 to their CPA before filing. For more on the mechanics of exercising ISOs and what to expect at tax time, the exercising stock options guide covers it in full.
Your Form 3921 questions, answered
Does every ISO exercise require a separate Form 3921?
Yes. One form is required for each ISO exercise. If an employee exercised three separate grants in the same year, that's three forms — even when all three forms go to the same employee.
What if an employee exercised ISOs but no longer works at the company?
The company is still required to file. The obligation follows the exercise event, not current employment status. Provide Copy B to the former employee's last known address.
What FMV value goes on Form 3921 — is it the 409A valuation in effect at the time of exercise?
Box 4 should reflect FMV on the date of exercise. In practice, companies use the most recent 409A valuation as of that exercise date. If your 409A was completed before the exercise, that value applies. Consult your accountant if there's any ambiguity around timing.
Do S-Corps need to file Form 3921?
Yes. The form applies to corporations, including S-Corps that have issued ISOs. Most LLCs don't issue ISOs (which require a corporate structure), so Form 3921 typically doesn't arise for them. If you're unsure whether your grants qualify as ISOs, your legal counsel can confirm.
What if I discover an error on a submitted Form 3921?
File a corrected Form 3921 as soon as possible. Check the "Corrected" box on the new form and send it to both the IRS and the affected employee. Correcting errors promptly generally reduces or eliminates associated penalties.
Form 3921 is a manageable filing once you understand the structure: one form per exercise, five data fields, three copies, three deadlines. The complexity for most startups isn't the form itself — it's assembling the right data. Matching grant records to exercise records to 409A valuations for every individual exercise takes time when that information lives in different places.
Getting employees their Copy B by February 2 matters more than founders usually expect. That form is what employees need to file AMT correctly, establish their cost basis, and confirm holding periods. A late or missing Copy B creates real confusion.
For founders also thinking about broader compliance — stock-based compensation reporting under ASC 718, 409A valuations, Rule 701 — Form 3921 fits into a picture that gets easier when your equity data lives in one place.
This guide is for general information only and is not legal, financial, or accounting advice. Tax rules can change, and your situation may have specific nuances that affect your obligations. Consult a qualified CPA, tax advisor, or legal counsel before making decisions about your equity plan or filing obligations.
This article is designed and intended to provide general information in summary form on general topics. The material may not apply to all jurisdictions. The contents do not constitute legal, financial or tax advice. The contents is not intended to be a substitute for such advice and should not be relied upon as such. If you would like to chat with a lawyer, please get in touch and we can introduce you to one of our very friendly legal partners.
