Alternative Minimum Tax (AMT): How It Works and Who Pays It

A Parallel Tax System Most People Ignore

Fewer than 200,000 Americans paid AMT in 2023, down from 5.25 million in 2017 before the Tax Cuts and Jobs Act. But for those who do owe it — primarily high-income earners with large incentive stock option exercises, significant itemized deductions, or substantial preference income — the AMT can add tens of thousands of dollars to their tax bill without warning. The IRS does not pre-calculate AMT exposure; it arrives on your return through Form 6251, often as an unpleasant surprise.

The AMT Mechanics

The alternative minimum tax runs parallel to the regular income tax system. You calculate your taxes under both systems and pay whichever is higher. The AMT starts with your regular taxable income, adds back certain deductions and income items called preference items and adjustments, subtracts an exemption amount, and applies AMT rates of 26% or 28% to the result.

AMT income (AMTI) = Regular taxable income + AMT add-backs − AMT exemption

If AMTI × AMT rate > Regular tax, you owe the difference as AMT.

AMT Preference Items and Adjustments

The items that get added back are where most taxpayers run into trouble. The most significant:

• Incentive stock option (ISO) spread: When you exercise ISOs, the spread between grant price and fair market value is not regular income — but it is 100% an AMT preference item in the year of exercise. This is the most common AMT trigger for tech employees.
• State and local tax (SALT) deduction: Deductible under regular tax (up to $10,000 post-TCJA), but entirely disallowed for AMT purposes. Pre-TCJA, high-SALT states like California and New York drove mass AMT exposure.
• Standard deduction: Entirely disallowed for AMT. The AMT has its own exemption instead.
• Accelerated depreciation: The difference between regular MACRS depreciation and the slower AMT depreciation method is an adjustment item for businesses and landlords using accelerated schedules.
• Percentage depletion: The excess of percentage depletion over cost depletion on natural resources is a preference item.
• Tax-exempt interest from certain private activity bonds: Even though exempt from regular tax, is an AMT preference item.

AMT Exemption and Phaseout (2024)

The exemption phaseout reduces by $1 for every $4 of AMTI above the threshold. A married couple with $1,418,700 in AMTI has their exemption reduced by $50,000 ([$1,418,700 − $1,218,700] ÷ 4), leaving $83,300 of effective exemption. Above the phaseout completion threshold, the exemption is fully eliminated.

The TCJA dramatically increased these exemptions for 2018–2025. Pre-TCJA exemptions were $54,300 (single) and $84,500 (MFJ) with much lower phaseout thresholds — which is why AMT hit millions of upper-middle-income taxpayers before 2018. Without congressional action, AMT parameters revert to pre-TCJA levels in 2026.

AMT Rates

Two rates apply. That's it.

• 26% on AMTI up to $232,600 ($116,300 for married filing separately)
• 28% on AMTI above those thresholds

Note that long-term capital gains and qualified dividends receive the same preferential rates under the AMT as under regular tax — a critical point sometimes misunderstood. The AMT can still affect high-capital-gain taxpayers by eliminating the regular deductions that offset other income, but the gains themselves are taxed at 0/15/20%, not 26/28%.

The ISO Stock Option Trap

Incentive stock options generate no regular income at exercise (only at sale), but the ISO spread — the difference between the strike price and fair market value on the exercise date — is a 100% AMT preference item in the exercise year. This creates a brutal scenario for employees at pre-IPO companies:

Example: Engineer exercises 100,000 ISOs at $1 strike price when fair market value is $15 per share. Spread = $1,400,000. This $1.4 million appears in AMTI. At a 28% AMT rate, the tentative minimum tax from this one item could be $280,000 above any regular income tax. The cash to pay it must come from somewhere — but the shares cannot be sold without triggering regular income tax and losing ISO treatment on a disqualifying disposition.

• The AMT paid on ISO exercise generates an AMT credit (Form 8801) usable in future years when regular tax exceeds AMT
• If the company goes public and stock value holds or rises, the credit eventually offsets future tax liability
• If the stock collapses after exercise (a common IPO lockup scenario), you may owe AMT on income that no longer exists in marketable form
• Tax planning for ISOs includes spreading exercises over multiple years to stay below the AMT breakeven point

Form 6251: Your AMT Calculation

AMT liability is computed on Form 6251 (Individuals) or Form 4626 (Corporations). The form walks through: line-by-line adjustments and preferences added to taxable income, the exemption calculation, the tentative minimum tax, comparison to regular tax, and the resulting AMT (or zero). You can calculate estimated AMT exposure mid-year using estimated income and option exercise plans — avoiding the year-end surprise is infinitely preferable to managing an unexpected $50,000 liability in April.

This article is for informational purposes only and does not constitute financial or tax advice.