Five AMT mistakes you can still fix before December 31

You hold incentive stock options (ISOs). The strike is low, the company is up, and exercising sounds straightforward. Then a parallel federal tax called Alternative Minimum Tax (AMT) shows up and changes the math. Five common traps decide whether your exercise is a free lever or a six-figure surprise. Two of them are pure calendar timing.

AMT is a calendar-year trap. By the time April arrives, the decisions that moved your bill are already made.

The five most expensive AMT mistakes

If you read nothing else, read this. These are the traps that cost real money, ordered roughly by how often they're missed.

1. Lump-sum exercise in your bonus year. December stock bonus, January RSU vest, big ISO exercise (all in the same tax year) is the classic AMT trap. The bonus and vest push your regular tax up, the AMT preference from the exercise pushes AMTI up further, and the AMT bill arrives in April for a number that could have been much smaller if the exercise had landed in a different year. Always check what other equity events fall in the same tax year before scheduling a large exercise.
2. Wrong calendar year by a few days. An exercise on December 30 counts against this year's crossover. An exercise on January 2 counts against next year's. Same ISOs, same bargain element, two different tax years, and potentially two very different AMT bills. If your crossover is tight in the current year, deferring an exercise into early January can dramatically lower the total AMT.
3. Ignoring state AMT. A single filer in California exercising a $50,000 bargain element pays roughly $13K of federal AMT plus $3.5K of California AMT: a 33% effective rate. Federal-only modeling underestimates the cost meaningfully in CA and MN.
4. Exercising without the cash to pay AMT. AMT is owed in April, regardless of whether you sold any shares. If you exercise without selling, you owe AMT in cash on top of the strike price you already paid. Make sure the liquidity is there, or plan a same-year disqualifying disposition to fund the bill.
5. Triggering the exemption phaseout unintentionally. For 2025, the AMT exemption shrinks by 25 cents for every dollar your AMTI exceeds $626,350 single / $1,252,700 MFJ. Starting in 2026, the OBBBA tightens this: thresholds drop to $500K single / $1M MFJ and the phaseout rate doubles to 50 cents per dollar. A large exercise can push you past the threshold and erode $20K-$40K of exemption that was protecting you. Spreading the exercise across years keeps the phaseout intact.

Each trap is worth a closer look once you have the underlying mechanics down. The rest of this article walks through how AMT actually charges you, how to find your own crossover, and how the multi-year credit eventually pays back most of what you owed.

What AMT actually charges you for

When you exercise an ISO, you pay the strike price and receive shares worth the current share price. The difference between those two is your bargain element, and it is your unrealized gain at exercise.

Regular tax does not care about the bargain element until you sell the shares. AMT does. The full bargain element gets added to your Alternative Minimum Taxable Income (AMTI) in the year you exercise, even if you never sell a share.

The AMT exemption and rates

AMT has its own version of the standard deduction. For tax year 2025, the AMT exemption is $88,100 for single filers and $137,000 for married filing jointly (per IRS Form 6251 Instructions). AMTI below your exemption is not taxed. The One Big Beautiful Bill Act made these higher exemption levels permanent; the exemption itself adjusts upward for inflation in 2026 and beyond.

Above the exemption, AMT charges 26% on the first $239,100 of taxable AMTI (2025) and 28% on the rest. The result is your tentative AMT. Subtract your regular tax liability from your tentative AMT; whatever is left over is the AMT you write a check for in April.

There is also a phaseout. For 2025, the exemption shrinks by 25 cents for every dollar your AMTI exceeds $626,350 (single) or $1,252,700 (MFJ). Starting in 2026, the OBBBA tightens this materially (Section 70107): the phaseout starts at $500,000 single / $1,000,000 MFJ, and the phaseout rate doubles to 50 cents per dollar over the threshold. For equity-rich tech employees, the phaseout matters more in 2026 than in 2025: a large exercise can push you past the new lower thresholds and erode the exemption at twice the previous rate.

Your AMT crossover

Your AMT crossover is the largest exercise you can do in a single year before AMT starts adding tax on top of your regular bill. Below the crossover, AMT is zero (or small enough that your regular tax already covers it). Above the crossover, every additional ISO you exercise costs you 26-28% of the bargain element in extra federal tax, plus state AMT in two states.

Why this is the number that matters: ISOs are valuable specifically because their bargain element does not trigger ordinary income at exercise. Exercising up to your crossover preserves that benefit. Exercising above it gives part of the benefit back as AMT.

Your specific crossover depends on:

• Your salary and any other regular income
• Your filing status (single, married filing jointly)
• Other deductions and credits
• The strike price vs. the current share price
• Any state AMT you owe on top of federal

Why timing matters: the multi-year strategy

Here is the move that turns the crossover from a hard ceiling into a planning lever: it resets every January.

If exercising your whole stack in one year would push you well past the crossover, split the exercises across multiple tax years. Each year, exercise up to your crossover, pay zero (or minimal) AMT, and start the long-term capital-gains clock on those shares. The total bargain element does not change. But you stay in regular-tax territory for as much of it as possible.

The AMT credit (and how you get it back)

AMT is technically a prepayment, not a permanent tax. The portion of your AMT caused by deferral items (like an ISO exercise where you do not sell the shares that year) generates an AMT credit you carry forward to future years on Form 8801.

In any future year where your regular tax exceeds your tentative AMT, the credit pays you back. Selling some of those ISO shares as a qualifying disposition (held 2 years from grant and 1 year from exercise per IRS Topic 427, taxed at long-term capital gains) spikes your regular tax past tentative AMT, which is what triggers credit recovery.

Practically: a multi-year exercise plus hold plus sell plan can recover most of the AMT you paid, but only if the regular-tax-vs-AMT crossover flips in your favor in those later years. If your future regular tax stays low (extended career break, no qualifying dispositions), the credit sits unused on your return year after year.

State AMT can change the math

Two states currently have their own AMT that applies on top of federal AMT:

• California (7%): applies to most federal AMT preference items, including the ISO bargain element (FTB Schedule P)
• Minnesota (5.8%): applies to the federal AMTI base with state adjustments (Schedule M1MT)

If you live in CA or MN, your effective AMT rate on the bargain element runs roughly 33-35% (federal 26-28% plus state). That is enough to change which exercise size is worth doing in a given year. Most other states have no separate AMT; they had one historically and either repealed it (New York in 2014 per TSB-M-15(1)I, Iowa later) or never adopted one.

Where to go from here

For your specific numbers, the AMT + ISO Exercise Calculator computes your crossover, builds a multi-year exercise schedule, and tracks AMT credit recovery year by year. It also handles the 90-day post-termination exercise window (per IRC §422(a)(2)) with departure-date-aware planning.

If you also have NSOs, RSUs, or a concentrated single-stock position to think about, the NSO sell-vs-hold, RSU withholding gap, and concentration risk articles cover the adjacent decisions.

The calculators here handle one decision in isolation. OptionsAhoy plans the full picture jointly: every ISO, NSO, RSU vest, concentrated position, and hedge, across bullish, neutral, and bearish scenarios. The output is a year-by-year Plan optimized for total after-tax wealth. Free during beta.