Exercising incentive stock options without selling the shares can generate a tax bill that has nothing to do with cash in your bank account. The alternative minimum tax treats the spread between your exercise price and the stock's fair market value as income, even though you have not sold anything. Employees at fast-growing companies routinely discover five- and six-figure AMT bills they did not expect, cannot easily pay, and in some cases owe on gains that evaporate before tax day.
Exercising ISOs and holding the shares can generate a five- or six-figure tax bill on paper gains you have not realized and may not be able to sell. The AMT does not care whether the stock is liquid or whether the price has dropped since you exercised. Run the AMT calculation before you exercise -- not in March of the following year when your tax preparer delivers the news.
Under the regular tax system, exercising incentive stock options produces no taxable event. You pay the exercise price, you receive the shares, and nothing appears on your W-2. The IRS does not tax the transaction until you sell. IRC Section 421(a) excludes the exercise of statutory stock options from gross income for regular tax purposes.
The alternative minimum tax has a different view. The spread at exercise -- the fair market value of the shares on the exercise date minus the price you paid -- is an AMT "preference item" that gets added back to your income when computing alternative minimum taxable income (AMTI). Technical detail
This adjustment only applies when you exercise and hold. If you exercise ISOs and sell the shares in the same calendar year (a disqualifying disposition), the preference item is zero and the AMT does not apply to that exercise. The trap is specifically for employees who exercise and hold through the end of the tax year, either to meet the ISO qualifying disposition holding periods or because they cannot sell (lockup periods, trading windows, or illiquid private stock).
How the AMT Calculation Works
The AMT runs as a parallel tax system. You compute your regular tax liability and your tentative minimum tax. If the tentative minimum tax is higher, you pay the difference as AMT on top of your regular tax. Technical detail
Start with your regular taxable income. Add back AMT preference items and adjustments. For stock option purposes, the big item is the ISO exercise spread. The result is your alternative minimum taxable income (AMTI).
Subtract the AMT exemption. For 2025, the exemption is $88,100 for single filers and $137,000 for married filing jointly. For 2026, those amounts rise to $90,100 and $140,200 respectively. Rev. Proc. 2024-40 (2025 amounts); OBBBA-adjusted projections for 2026. The exemption phases out at 25 cents per dollar of AMTI above $626,350 (single) or $1,252,700 (married filing jointly) for 2025.
Apply the AMT tax rates. The first $239,100 of AMT income above the exemption (2025, all filing statuses; $119,550 for married filing separately) is taxed at 26%. Everything above that threshold is taxed at 28%. IRC Section 55(b)(1)(A). The 28% rate breakpoint is adjusted annually for inflation.
Compare to your regular tax. If the resulting tentative minimum tax exceeds your regular tax, the excess is your AMT liability.
A Real-Numbers Example
Meet David. He is single, earns a $50,000 salary at a startup, and exercises 10,000 ISOs with an exercise price of $2 per share. On the exercise date, the company's 409A valuation puts the fair market value at $22 per share. The spread is $20 per share, or $200,000 total.
Regular tax picture: David's taxable income is approximately $50,000 (salary minus the standard deduction). His regular federal tax is roughly $5,150. The ISO exercise does not appear anywhere on his regular return.
AMT picture: David's AMTI is approximately $50,000 + $200,000 = $250,000. After subtracting the $88,100 AMT exemption (2025), his AMT base is $161,900. At the 26% rate, his tentative minimum tax is approximately $42,094.
The damage: David's AMT is $42,094 minus his regular tax of $5,150 = approximately $36,944 in additional tax. On a $50,000 salary, that is roughly 74% of his gross annual pay -- owed on stock he has not sold and, if the company is private, may not be able to sell.
David now has a $36,944 tax bill, paper gains he cannot monetize, and four months until April 15.
- $200,000 AMT adjustment added to income
- Tentative minimum tax of ~$42,094
- AMT bill of ~$36,944 on top of regular tax
- Equals ~74% of gross annual salary owed in tax on unsold stock
- Year 1: Exercise 3,000 shares ($60K adjustment, minimal AMT)
- Year 2: Exercise 3,500 shares (adjusted for income changes)
- Year 3: Exercise 3,500 shares
- Each year stays below or near the AMT breakeven point
The AMT on ISO exercises is calculated based on the stock's fair market value on the exercise date. If the stock price drops after you exercise but before you sell, you still owe the AMT based on the higher valuation.
This is exactly what happened to thousands of employees during the dot-com collapse. People exercised ISOs when their company's stock was trading at peak valuations. By the time tax returns were due the following April, the stock had lost 80% or 90% of its value -- but the AMT bill was computed on the peak-date spread. Some employees owed more in taxes than their shares were currently worth.
The fix, if you recognize the risk in time, is to sell shares before December 31 of the exercise year. Selling converts the exercise from an AMT preference item to a disqualifying disposition. You will owe ordinary income tax on the spread, but you will have cash from the sale to pay it, and you avoid the AMT calculation entirely.
The AMT Credit: Getting Your Money Back (Eventually)
AMT paid on ISO exercises is not permanently lost. It generates a minimum tax credit that you can carry forward indefinitely and use in future years when your regular tax exceeds your tentative minimum tax. Technical detail
The credit works like this: in any future year where your regular tax is higher than your tentative minimum tax, you can apply the stored credit to reduce your regular tax down to (but not below) the tentative minimum tax amount. The remaining unused credit carries forward again.
In practice, recovering the full AMT credit can take years or even decades, depending on your income trajectory and whether you have ongoing AMT adjustments. If David from our example sells his shares in a future year and generates a large capital gain, the resulting regular tax may exceed his tentative minimum tax by enough to absorb some or all of the credit. But if he continues to have AMT adjustments from new ISO exercises, the credit may sit unused for a long time.
The credit is real -- it is not a deduction or a vague promise -- but it is cold comfort when you need to write a check for $37,000 today.
1. Exercise in Stages Across Multiple Tax Years
Rather than exercising all your ISOs in a single year, spread the exercises across two, three, or more tax years. Each year, exercise only enough shares to keep the AMT adjustment below the level that triggers significant additional tax. A CPA can calculate the precise number of shares you can exercise each year before the AMT exceeds your regular tax.
2. Exercise When the Spread Is Small
The AMT adjustment equals the spread at exercise. If you exercise early in the company's growth -- shortly after joining, or right after a low 409A valuation -- the spread is minimal and the AMT impact is negligible. Exercising 10,000 shares with a $2 spread generates a $20,000 AMT adjustment. Exercising those same shares two years later with a $20 spread generates a $200,000 adjustment.
This is why early exercise (where available) paired with an 83(b) election is one of the most effective tax strategies for startup employees. If you exercise when the FMV equals or nearly equals the exercise price, the AMT adjustment is close to zero.
3. Exercise and Sell in the Same Calendar Year
Selling ISO shares in the same tax year you exercise them eliminates the AMT preference item entirely. The sale is a disqualifying disposition, which means the spread is taxed as ordinary income rather than capital gains. But you have cash in hand to pay the tax, and you avoid the AMT calculation and its cascading consequences.
This is the right choice when the AMT cost of holding would exceed the tax benefit of qualifying disposition treatment. For employees with large grants or high spreads, the numbers often favor the disqualifying disposition.
4. Monitor the Stock Price Through Year-End
If you exercised ISOs earlier in the year and the stock price has dropped significantly by November or December, consider selling before December 31. Selling before year-end converts the exercise to a disqualifying disposition, which eliminates the AMT adjustment. You may realize a loss or a reduced gain, but you avoid paying AMT on a spread that no longer exists.
This is the emergency valve that protects you from the dot-com-era nightmare scenario.
5. Run the AMT Calculation Before You Exercise
Before committing to an ISO exercise, model the AMT impact with your actual income numbers. You need your current year's projected taxable income, the number of shares you plan to exercise, the exercise price, and the current FMV. A CPA or tax advisor can run this calculation and tell you exactly how many shares you can exercise before triggering AMT, or how much AMT a given exercise will cost.
The worst time to learn about your AMT exposure is in March of the following year when your tax preparer delivers the news.
The Form You Need to Know
Form 6251 (Alternative Minimum Tax -- Individuals) is where the AMT calculation lives. If you exercise ISOs and hold the shares through year-end, you must file Form 6251. The ISO adjustment goes on line 2i. Your employer provides Form 3921 (Exercise of an Incentive Stock Option Under Section 422(b)) with the exercise price, FMV, and number of shares -- the inputs you need for line 2i.
If you owe AMT in any year, you should also be aware of Form 8801 (Credit for Prior Year Minimum Tax), which is how you claim the AMT credit carryforward in subsequent years. Many taxpayers who paid AMT on ISO exercises forget to file Form 8801 in later years, leaving money on the table.
The AMT credit carries forward indefinitely -- it does not expire. But you can only use it in years when your regular tax exceeds your tentative minimum tax. If you paid AMT in any prior year, file Form 8801 every year until the credit is fully recovered. Many taxpayers leave thousands of dollars on the table by forgetting this form.