- Incentive stock options: How ISOs work and tax treatment
- What are incentive stock options?
- How do incentive stock options work?
- How incentive stock options are taxed
- The alternative minimum tax and ISOs
- Qualifying disposition
- Disqualifying disposition
- Reporting your ISOs
- When can you exercise ISOs?
- When do incentive stock options expire?
- When can you sell your ISOs?
- Incentive stock options vs. non-qualified stock options
- ISOs vs. NSOs
- ISOs vs. equity awards
- Getting the most from your incentive stock options
- Frequently asked questions about incentive stock options
What are incentive stock options?
Incentive stock options (ISO) are a type of equity that give employees the right to purchase company stock at a predetermined price, known as a strike price or exercise price. ISOs are required to be granted with an exercise price at or above the fair market value (FMV) of the company's common stock on the grant date, making them a popular form of equity compensation at startups and other fast-growing companies. ISOs are a type of stock option, and are not actual shares of stock. You must exercise your options to become a shareholder.
For employees, ISOs offer the opportunity to buy shares at a set price and benefit as the stock appreciates. Unlike other option types, ISOs also offer significant tax advantages when certain holding conditions are met.
For companies, ISOs are a cost-effective way to attract, motivate, and retain talent with competitive compensation packages outside of base pay and bonuses. ISOs are also structured with vesting schedules, which creates a long-term incentive for employees to stay and realize the full value of their grants.
Important terms to know:
Grant date: The date an employee or service provider is granted options
Strike price: The fixed price at which an employee can purchase shares
Vesting schedule: The timeline defining when ISOs can be exercised
Holding period: The time required for ISOs to qualify for favorable tax treatment
Exercise date: The date on which an employee purchases shares by exercising ISOs
ISOs are sometimes called statutory stock options, which is the IRS classification for this type of option. Non-qualified stock options (NSO), by contrast, are called nonstatutory stock options.
How do incentive stock options work?
Most incentive stock options move through five stages: grant, vesting, exercise, hold, and sell.
Your company gives you the right to buy a set number of shares at your strike price. Vesting spreads that right out over time, usually on a fixed schedule. A common schedule vests your options over four years with a one-year cliff, so you earn nothing until your first anniversary, then earn the rest gradually.
Once options vest, you can exercise them. Exercising means buying the shares at your strike price, even if the company is worth more by then. If you do not have the cash to buy and hold, a cashless exercise lets you sell some shares to cover the cost of the rest. The difference between your strike price and the FMV at the time you exercise is called the spread. The spread matters because it drives how your ISO shares get taxed.
Your strike price is not chosen at random. For a private company, it comes from a 409A valuation, which is an independent appraisal of the company's common stock. The 409A sets the FMV, and your company uses that number to price your options at or above it.
How incentive stock options are taxed
The main advantage of ISOs is how they're taxed. There are two points in time when an ISO can trigger a tax: when you exercise the options, Nx when you go to sell the resulting share. At exercise, there is the potential to trigger the alternative minimum tax (AMT), while at sale, you are typically looking at capital gains tax.
When you exercise incentive stock options, you owe no ordinary income tax on the spread at that moment. Ordinary income tax is the rate applied to wages and salary. If you then meet the holding rules, your eventual gain is taxed at the lower long-term capital gains rate. Capital gains tax applies to profit from selling an investment you have held.
The alternative minimum tax and ISOs
The alternative minimum tax (AMT) is a separate way of calculating tax liability to make sure taxpayers pay an appropriate amount of federal income taxes. If you exercise ISOs but don't sell the shares in the same year, the difference between the strike price and the FMV at exercise, known as the bargain element, is treated as taxable income when calculating AMT. However, if you exercise and sell the shares within the same year, this spread is not included in AMT income but is instead treated as regular income. Talk to your tax advisor to see if AMT might affect you. If the FMV has grown significantly before you exercise your options, this increases the chance that you will trigger AMT from exercising your ISOs.
Qualifying disposition
When you exercise ISOs, you don't have to sell the resulting shares right away. If you exercise ISOs and hold your stock for over one year after purchase and over two years after grant, your stock should be eligible for favorable tax treatment when you sell and you will pay the lower long-term capital gains tax rate on the increase in value. However, you may be subject to the alternative minimum tax (AMT) when you exercise.
The 83(b) election and ISOs
An 83(b) election is a notice you file with the IRS within 30 days of receiving unvested stock. It only applies to ISOs when your company allows early exercise and you buy shares before they've vested. For a standard exercise of already-vested options, it doesn't apply.
The main reason to file is AMT. Without an 83(b) election, the spread between your strike price and the FMV at each future vesting date counts as AMT income in the year those shares vest. Filing locks in that calculation at the date of early exercise, when your strike price and FMV are often close and the spread is small. The 30-day deadline is strict and the IRS does not grant extensions, so talk to a tax advisor before you early exercise to decide whether filing makes sense for your situation.
The $100K ISO limit
The $100K ISO limit restricts employees from receiving favorable tax treatment on more than $100,000 worth of incentive stock options that become exercisable for the first time within a single year.
The limit applies per calendar year and is based on the fair market value of your shares at grant. Say $150,000 worth of your ISOs become exercisable for the first time in one year. The first $100,000 keeps its ISO tax treatment, and the remaining $50,000 is treated as non-qualified stock options. Only the portion that first becomes exercisable in a given year counts toward the limit, so grants that vest across several years are measured year by year, depending on your vesting schedule.
Disqualifying disposition
If you exercise and sell your ISOs right away, the shares you sell won't qualify for favorable tax treatment or advantages. Instead, you'll pay ordinary income tax on the spread between your strike price and the FMV at the time of sale. Any gain above that FMV is taxed as short-term capital gains if you've held the shares less than a year, or long-term capital gains if you've held them longer.
Reporting your ISOs
When you exercise ISOs, your company files Form 3921 with the IRS and sends you a copy early in the following year. This form records the details of your exercise, including your strike price, the fair market value on the exercise date, and the number of shares you bought. Keep it with your tax records, because you'll need those figures if you sell the shares and for calculating AMT.
If exercising your ISOs triggers the AMT, you may also need to file Form 6251 to calculate what you owe. Not everyone who exercises ISOs will owe AMT, so whether you need this form depends on your situation. Confirm the details with your tax advisor before you file.
When can you exercise ISOs?
Usually, you can't buy all of your shares right away and have to work for the company over time to be able to purchase your shares. This process is called vesting. You can exercise your ISOs as soon as your options have vested, but it's not required.
In some cases, you might be able to exercise your ISOs before they vest. You can check your option grant or ask your company to see if it allows early exercising. Note that this may result in a taxable event, so also consult with your tax advisor.
→ Learn more about exercising stock options.
When do incentive stock options expire?
ISOs expire 10 years from your grant date. However, your company might enforce a post-termination exercise period (PTEP) that gives you a shorter amount of time to exercise options after you leave the company. If you don't exercise them before that period ends or before they expire, you may lose the opportunity to purchase them.
Even if your company gives you a long time to exercise ISOs after you leave, if you don't exercise them within three months of leaving, they'll lose their ISO tax treatment and will be taxed like NSOs if you ultimately exercise them.
When can you sell your ISOs?
You have to exercise ISOs and purchase shares before you can sell them. If you choose to exercise your ISOs, you usually have two options: pay for the total in cash or do a same-day sale (sell a portion of your shares to cover the cost of exercise).
Selling to cover exercise costs is called a cashless exercise. It's less risky because you haven't invested your own money. However, selling shares right after exercising prevents you from taking advantage of ISOs' favorable tax structure. Not all companies allow cashless exercises, so check to see if yours does before exercising.
Incentive stock options vs. non-qualified stock options
Non-qualified stock options (NSO) are another type of stock options U.S. companies may offer to employees. With NSOs, you pay ordinary income tax at exercise and capital gains tax when you sell. This usually means you pay more taxes with NSOs than with ISOs.
ISOs vs. NSOs
ISO | NSO | |
Exercise | May be subject to alternative minimum tax | May be subject to ordinary income tax |
Sell | Ordinary income or capital gains | Capital gains |
ISOs vs. equity awards
Instead of stock options, some startups use alternative equity, such as restricted stock awards (RSA) or restricted stock units (RSU), depending on the company's stage. RSAs are typically used for very early-stage companies, while RSUs are common for more mature ones. Both RSAs and RSUs are grants of stock, not options of stock, so you typically don't need to exercise them.
→ Learn more about the differences between RSUs and stock options
Getting the most from your incentive stock options
Whether you issue equity or hold it, a few decisions make the biggest difference. Start with the fundamentals, then plan the moments that carry tax consequences:
Understand your grant terms: Know your strike price, your vesting schedule, your expiration date, and your post-termination window before you need them.
Model the cost before you exercise: Add up the cash to buy your shares and the potential AMT impact so the bill does not surprise you.
Track your holding periods: Watch the one-year and two-year marks so you can time a sale for a qualifying disposition.
Companies use Carta's equity management software to issue and track options, perform 409A valuations that set the strike price, and manage an accurate cap table as grants and limits change. New founders can start with free cap table software, and if you manage a team, Carta’s Equity Advisory services can answer common questions for your employees.
Managing equity doesn’t have to be complicated. If you are issuing options and want a system that tracks grants, vesting, and the $100,000 limit in one place, request a demo to see how Carta manages incentive stock options end-to-end.
Frequently asked questions about incentive stock options
What is the difference between an RSU and an incentive stock option?
A restricted stock unit (RSU) is a promise of shares granted at no cost that is taxed as ordinary income when it vests or settles, while an ISO is a right to buy shares at a set price with the potential for capital gains tax implications.
How do you report incentive stock options on your taxes?
Your company issues Form 3921 when you exercise ISOs, as the IRS's Form 3921 instructions confirm, and you report a qualifying sale on your capital gains schedule. If you hold the exercised shares into the next tax year, you may need to file Form 6251 for an AMT adjustment.
Can incentive stock options be granted to contractors or advisors?
No. ISOs can be granted only to employees, so contractors and advisors receive non-qualified stock options instead.
When should you exercise your incentive stock options?
It depends on your situation, but many people exercise once their options have vested, after weighing the cost, the potential AMT impact, and how long they can hold the shares.
What happens to your ISOs if your company goes public?
An initial public offering (IPO) doesn't automatically convert your ISOs into shares. You still need to exercise them, and the same tax rules apply. Most companies impose a lock-up period of 90 to 180 days after the IPO, during which employees can't sell shares. Once the lock-up expires, you have a liquid market, which makes exercising and selling more straightforward than at a private company. Keep in mind that if the stock price is well above your strike price by the time you exercise, the spread could generate a significant AMT bill, so talk to a tax advisor before you act.
DISCLOSURE: This communication is on behalf of eShares, Inc. dba Carta, Inc. ("Carta"). This communication is for informational purposes only, and contains general information only. Carta is not, by means of this communication, rendering accounting, business, financial, investment, legal, tax, or other professional advice or services. This publication is not a substitute for such professional advice or services nor should it be used as a basis for any decision or action that may affect your business or interests. Before making any decision or taking any action that may affect your business or interests, you should consult a qualified professional advisor. This communication is not intended as a recommendation, offer or solicitation for the purchase or sale of any security. Carta does not assume any liability for reliance on the information provided herein. © 2026 Carta. All rights reserved. Reproduction prohibited.
