7 Mistakes That Turn an ISO Exercise Into an AMT Bill You Did Not Plan For

7 Mistakes That Turn an ISO Exercise Into an AMT Bill You Did Not Plan For

An ISO exercise AMT bill is the only tax bill people get for a transaction where no money changed hands and no income showed up on a W-2.

That is the whole problem in one sentence. When you exercise an incentive stock option, the spread between your strike price and the fair market value of the shares is not wages and is not regular taxable income. It is an adjustment for alternative minimum tax purposes. Regular tax does not see it. AMT sees all of it.

So the exercise feels free in October and produces a wire request in April. Here are seven ways that happens, and 2026 makes several of them worse.

1. Not Knowing That 2026 Changed the AMT Math

This is the one to read even if you skip the rest.

The One Big Beautiful Bill Act kept the higher AMT exemption amounts permanent, which sounds like good news, and then changed two things that matter more. Starting in 2026, the income level where the exemption begins to phase out resets to $500,000 for single filers and $1,000,000 for joint filers, indexed going forward. And the rate at which the exemption phases out doubles, from 25 percent to 50 percent.

The 2026 exemption amounts themselves are $90,100 for single filers and $140,200 for married filing jointly.

Put those together and the practical effect is that the exemption disappears faster and starts disappearing at a lower income than it did in 2025. An exercise that generated a tolerable AMT number last year can generate a materially larger one this year with no change to the option, the strike, or the share price.

If your plan for exercising was built on a 2024 or 2025 model, the model is stale. That is not a reason to do anything in particular. It is a reason to rerun the number before you act.

2. Treating the ISO Exercise Bargain Element as Theoretical

The AMT adjustment is strike price subtracted from fair market value at exercise, multiplied by shares. For a private company, fair market value generally comes from the most recent 409A valuation.

People discount this because the shares are illiquid and the valuation feels like a paper number. The IRS does not discount it. The adjustment is computed on that number whether or not there is any market to sell into.

Ten thousand options at a $1 strike with a $9 valuation is an $80,000 AMT adjustment on a transaction that cost you $10,000 in cash and produced zero dollars of proceeds.

3. Exercising in December Instead of January

Timing inside the calendar year changes your options later, and December is usually the worst month for it.

Here is the mechanism. If you exercise and then sell the shares in the same calendar year, the transaction generally becomes a disqualifying disposition. You lose the favorable ISO treatment and the spread becomes ordinary income, but the AMT adjustment goes away because there is no year-end AMT basis difference to carry. For someone facing an AMT bill they cannot pay, that is an escape hatch.

Exercise in December and the window to use that hatch is a few weeks. Exercise in January and you have most of a year to watch what happens and decide.

The tradeoff is real and cuts both ways: an early-in-year exercise starts the holding-period clock sooner, but it also means holding an illiquid position through more of the year with the AMT exposure live. Neither timing is correct in the abstract. What is incorrect is exercising in the last week of December without knowing you just shortened your own decision window.

4. Ignoring Form 3921 and the Second Basis

After an ISO exercise, your shares have two cost bases. Regular tax basis is what you paid. AMT basis is what you paid plus the adjustment you already got taxed on.

Your employer files Form 3921 and sends you a copy. It has the exercise date, the strike, and the fair market value. It is the input for both bases.

The failure mode is mundane and expensive: the 3921 gets filed in a drawer, the shares get sold three years later, and the return reports one basis instead of two. You pay AMT on the spread at exercise, then pay again on the same spread at sale because nobody adjusted the AMT gain. Recovering that means amending, if it is even caught.

Keep the 3921 with your tax records permanently, not for three years. The document matters at sale, which can be a decade after the exercise.

5. Assuming the ISO Exercise AMT Credit Comes Back Quickly

When AMT applies because of an ISO exercise, you generally generate a minimum tax credit that carries forward and can offset regular tax in later years to the extent your regular tax exceeds your tentative minimum tax.

That is a real asset. It is not a refund.

Whether you use it, and how fast, depends on your income in future years. Someone whose income stays high and whose regular tax comfortably exceeds their AMT can recover the credit over a few years. Someone whose income drops, or who leaves the company, or whose situation keeps them near the AMT line, can carry the credit for a long time. It also has no value at all if you never have enough regular tax to absorb it.

Planning that treats the credit as "I get it back next year" is planning on an assumption about your own future income. Say the assumption out loud so you can check it.

6. Exercising Shares You Cannot Sell to Pay the Tax

This is the mistake that actually hurts people, and it is not really a tax mistake.

An early exercise on a private company creates a cash obligation, potentially in two parts: the strike price now and the AMT in April. The asset backing both is stock you cannot sell. If the company does well, this looks like brilliant planning. If the company flatlines, you paid real cash and real tax for an illiquid position that may end up worth less than the tax you paid on it.

The risk is not hypothetical. Companies that were priced at a high 409A valuation have subsequently repriced downward, and there is no mechanism that refunds AMT you already paid because the valuation later fell. A worthless-stock loss and a large prior-year AMT payment are two separate events, and the loss does not undo the payment.

This is the section where an actual conversation with a qualified professional is worth more than any article. Cash you need for something else should not be the source of an exercise, and no amount of tax efficiency makes a concentrated illiquid position appropriate for everyone.

7. Doing It Without Coordinating the Return and the Books

The exercise happens in an equity system. The AMT shows up on a personal return. The company's cap table, its 3921 filings, and its stock compensation expense sit in a third place. When those three do not reconcile, the person holding the options is the one who finds out.

The SaaS Bookkeeper is based in Austin and was founded in 2017. The practice works with small business owners, venture-backed startups, high net worth individuals, and specifically on equity compensation, which is the intersection that matters here: the company-side accounting for stock comp and the individual-side reporting of an exercise are usually handled by people who never speak to each other.

The limitation is worth stating. A firm that handles the accounting side is not the same as a licensed investment adviser, and the question of whether to exercise at all is an investment question about concentration, liquidity, and your own balance sheet. Many people in this situation need both a preparer who understands the mechanics and an adviser who will tell them no. If you are comparing on the advisory side, the Sam's List financial advisor directory lists vetted advisors with verified client reviews.

The One AMT Number to Run Before an ISO Exercise

Compute your tentative minimum tax with the exercise and without it. The difference is the cost of the exercise. Then ask where that cash comes from.

If the answer is "I would have to sell the shares," you have discovered the constraint, and you found it in October instead of April.

You can compare accountants and read verified client reviews in the Sam's List accountant directory. Every reviewer authenticates through LinkedIn, Google, or Twitter before submitting, and firms cannot delete negative reviews. What a directory cannot do is model your exercise. That takes a professional looking at your actual return.

Frequently Asked Questions

Do I owe tax when I exercise an ISO? Not for regular tax purposes. Exercising an incentive stock option and holding the shares generally produces no regular taxable income, which is why nothing appears on your W-2. The spread between strike and fair market value is an alternative minimum tax adjustment, so you can owe AMT for the year of exercise even though your regular tax return shows no income from the transaction.

How much AMT will I owe on an ISO exercise? There is no percentage that answers this, because AMT is computed on your whole return rather than on the exercise alone. The adjustment is the spread times the shares, but whether it produces tax depends on your other income, your exemption after phaseout, and your regular tax for the year. For 2026 the exemption is $90,100 single and $140,200 joint, phasing out starting at $500,000 and $1,000,000 respectively at a 50 percent rate.

What is a disqualifying disposition and why would I want one? It is a sale that breaks the ISO holding-period requirements, generally selling within two years of grant or one year of exercise. It converts the spread to ordinary income and forfeits long-term capital gain treatment, which is usually worse. The reason someone might accept it is that selling in the same calendar year as the exercise generally eliminates the AMT adjustment, which can be the better of two bad outcomes if the tax is otherwise unpayable.

Does the AMT credit expire? The minimum tax credit generally carries forward indefinitely rather than expiring on a schedule, but it can only be used in years where your regular tax exceeds your tentative minimum tax. So the practical risk is not expiration, it is never having a year where the credit is usable. That makes the credit worth including in a multi-year projection rather than counting on as a near-term recovery.

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About the author: Kimberly Green is the cofounder of Sam's List, where business owners and high earners find vetted CPAs, financial advisors, and fractional CFOs. She's met one-on-one with 400+ financial professionals and writes from the real data behind thousands of client-advisor matches. Ask her anything about finding an accountant - she's heard it all, including the questions people are afraid to ask.