Chapter 7: ISOs and the AMT
Jordan is about to exercise 20,000 stock options, receive zero dollars of cash, and owe the IRS $25,360. Nothing in that sentence is a mistake, and nothing in it is avoidable by filing cleverly in April. The bill is set the day he exercises. This chapter exists so that you see that bill before you click the button, the way Jordan does, instead of eleven months later, the way thousands of engineers did in the year 2000.
Incentive stock options (ISOs) are the tax-favored option breed, the one startups hand to employees. Chapter 6 covered their plainer cousin, the NSO, whose spread becomes wages the day you exercise. ISOs make a better offer with a hidden toll, and the toll has a name: the alternative minimum tax.
The promise
Exercise an ISO and, under the regular tax system, nothing happens. No W-2 income, no withholding, no tax. If you then hold the shares at least two years from the grant date and one year from the exercise date, the entire gain from your strike price to your final sale price is long-term capital gain. That is the qualifying disposition, and it is a genuinely great deal. On a big win, the difference between ordinary rates and long-term rates can be 15 to 20 cents on every dollar.
Sell before either clock runs out and you have a disqualifying disposition: the spread gets reclassified as ordinary wage income, roughly matching the NSO treatment. Hold that thought, because later in this chapter the disqualifying disposition turns out to be a useful escape hatch rather than a failure.
The parallel tax
The alternative minimum tax (AMT) is a second federal tax calculation that runs alongside the regular one. It starts from your regular income, adds back certain items the regular system ignores, subtracts a large exemption, and applies its own rates: 26%, and 28% above a threshold ($244,500 of AMT base in 2026). You compute both systems and pay whichever is higher. Most people never notice the AMT because the exemption ($90,100 for singles in 2026, $140,200 for joint filers) keeps the parallel bill below the regular one.
ISOs are the classic way ordinary employees crash into it. The bargain element, the gap between the share's fair market value and your strike price on exercise day, is invisible to the regular tax but counts as income in the AMT calculation. It is called a preference item. Exercise enough ISOs and the parallel bill climbs past the regular one, and the difference is the AMT you owe, in real dollars, on shares you have not sold.
Jordan's exercise, traced to the dollar
Jordan holds 40,000 ISOs at a $2.40 strike. The current 409A value of the stock is $9.00 (chapter 9 explains what a 409A is; for now, it is the official price of his private shares). He exercises 20,000 of them in 2026. His salary and other income total $160,000, and he files single. The bargain element is $9.00 minus $2.40, or $6.60 per share, times 20,000 shares: $132,000.
First, his regular 2026 tax, which ignores the exercise entirely:
| Regular tax, 2026, single | Amount |
|---|---|
| Income | $160,000 |
| Standard deduction | −$16,100 |
| Taxable income | $143,900 |
| 10% on the first $12,400 | $1,240 |
| 12% on the next $38,000 | $4,560 |
| 22% on the next $55,300 | $12,166 |
| 24% on the last $38,200 | $9,168 |
| Regular tax | $27,134 |
Now the parallel calculation. The AMT starts from taxable income, adds back the standard deduction, and adds the bargain element:
| AMT calculation, same year | Amount |
|---|---|
| Taxable income | $143,900 |
| Add back the standard deduction | +$16,100 |
| Add the ISO bargain element | +$132,000 |
| Alternative minimum taxable income (AMTI) | $292,000 |
| AMT exemption (single, 2026; no phaseout below $500,000) | −$90,100 |
| AMT base | $201,900 |
| Tentative minimum tax at 26% (base is under the 28% threshold) | $52,494 |
| Regular tax | −$27,134 |
| AMT owed on top of regular tax | $25,360 |
Jordan pays the higher of the two systems: $52,494 in total federal income tax, which is his $27,134 regular bill plus $25,360 of AMT. He also wired the company $48,000 to cover the strike price (20,000 times $2.40). Count the cash flows for the year and the picture is stark.
$48,000 of strike cost plus $25,360 of AMT, all paid in real dollars.
Jordan owns 20,000 private shares he cannot sell. The $132,000 gain is paper.
The AMT bill works out to about 19% of the paper gain. That is the toll for starting the long-term capital gains clock while holding the shares. The calculator below runs this same math on your own numbers, and Form 3921, which your company must send after any ISO exercise, supplies the inputs.
Your AMT room
The exemption creates a useful concept: AMT room, the amount of bargain element you can absorb in a year before the parallel bill overtakes the regular one. At Jordan's income, the crossover sits near a $34,000 bargain element, about 5,200 of his shares. He could exercise that many every year and owe no AMT at all.
Two more 2026 rules shape the big-exercise end of that chart. First, the exemption phases out at 50 cents per dollar of AMTI above $500,000 for singles ($1,000,000 joint), a tighter squeeze than in earlier years, and it is gone entirely at $680,200. A very large exercise loses the shield exactly when it is needed most. Second, the $100,000 limit: ISOs first exercisable in any single year are capped at $100,000 of strike value, and the excess is treated as NSOs from the start. Jordan's whole grant carries $96,000 of strike value (40,000 times $2.40), so his options all stay ISOs; a 50,000-share grant vesting in one year would have split.
The escape hatch
Suppose Jordan exercises in March and by November the company has stumbled, or the AMT bill simply looks unbearable next to his savings. Selling the shares in the same calendar year as the exercise is a disqualifying disposition, and it removes the bargain element from the AMT calculation entirely. He pays ordinary income tax on his actual profit, the real one, measured by what the shares actually fetched. Ordinary rates on a real gain beat AMT rates on a vanished one every time. The catch for private-company holders is brutal in its simplicity: the escape requires a buyer, and chapter 9 explains why there often is none.
Paying tax on money that no longer exists
In March 2000 the Nasdaq peaked, and that spring thousands of employees exercised ISOs at the top and held for the one-year clock. The AMT did exactly what the tables above show: it taxed the bargain element as of exercise day. Then the shares fell 70, 80, 90 percent. The tax bills did not fall with them.
The bill is frozen on exercise day. The stock is not.
After the 2000 crash, people owed six-figure AMT bills on bargain elements that had evaporated, sometimes more than the shares were worth by the time the return was due. Some sold homes to pay tax on gains they never touched. The same movie ran again in 2021 and 2022, when employees exercised at peak 409A valuations and watched the next valuation come in 60% lower. The lesson is not "never exercise." The lesson is that exercising and holding fixes your tax bill at today's price while leaving you fully exposed to tomorrow's, so never exercise more than your AMT math, and your savings, can survive.
The credit comes back, slowly
AMT paid because of an ISO exercise is not pure loss. It generates a minimum tax credit (claimed on Form 8801) that you can use in later years, in any year when your regular tax exceeds your tentative minimum tax. In effect, the AMT is a forced prepayment that dribbles back. Jordan's $25,360 will return over future filings, a few thousand dollars a year at his income, and faster in the year he finally sells the shares. Track it every year; the credit is famously forgotten, and tax software only carries it forward if last year's return came along for the ride.
Jordan runs the numbers twice and goes ahead with the 20,000-share exercise, with three things in writing first. The $25,360 sits in a savings account labeled AMT before he clicks exercise, on top of the $48,000 strike cost. A calendar entry for December 1 says "check the 409A; if the company has cracked, sell before December 31 and disqualify." And his spreadsheet notes the minimum tax credit so Form 8801 gets filed every year until the $25,360 comes home. He leaves the other 20,000 options unexercised, which chapter 9 turns into a much harder question the day a recruiter calls.
Exercise ISOs when the bargain element is small, the AMT room exists, and you would buy the stock with cash anyway. Before any exercise that exceeds your room, compute the AMT to the dollar, hold that cash in savings, and write down your same-year escape plan. Never exercise an amount whose tax bill assumes the stock keeps its price.
Where people go wrong
- Exercising without running the parallel tax. The regular system shows $0 due and people stop reading. The AMT calculation is the whole game for ISOs.
- Confusing paper gains with the ability to pay. The $132,000 is in shares; the $25,360 is due in cash. Match the bill to a funding source before exercising.
- Missing the December 31 escape. The same-year sale that erases the AMT preference dies at midnight on New Year's Eve. The decision belongs in December, not at filing time.
- Forgetting the credit. AMT from an ISO exercise is recoverable. People abandon five-figure credits by switching tax software and losing the carryforward.
- Treating the $100,000 limit as a footnote. Big grants with fast vesting quietly split into ISOs and NSOs, with different tax treatment per slice. Read the grant notice; your company's Form 3921 covers only the ISO part.
Key takeaways
- ISOs charge no regular tax at exercise, but the bargain element counts as income in the parallel AMT system, at 26% to 28% above a $90,100 exemption (single, 2026).
- Jordan's 20,000-share exercise at a $2.40 strike and $9.00 value creates a $132,000 bargain element and an AMT bill of $25,360 on top of his $27,134 regular tax, with $0 of cash received.
- At his income, about $34,000 of bargain element per year fits inside the AMT room at zero cost. Multi-year slices beat one giant exercise, especially with the exemption now phasing out above $500,000 of AMTI.
- Selling in the same calendar year erases the AMT preference; AMT actually paid comes back later as a credit via Form 8801.
- March 2000 is the permanent warning: the tax is frozen at exercise-day prices and the stock is not.
Sources: IRS: Topic 427, Stock Options · IRS: About Form 6251, Alternative Minimum Tax · IRS: About Form 3921, Exercise of an Incentive Stock Option · Finvest Tax Playbook