Alternative Minimum Tax Calculator

Background.

The alternative minimum tax is a second, flatter version of the federal income tax that runs alongside the ordinary one. You work out your tax under both sets of rules and pay whichever is higher; the AMT that appears on the return is only the difference. It exists to stop people with large deductions and tax preferences from cutting their bill to almost nothing, and since the 2017 tax law raised the exemption sharply it reaches far fewer households than it used to.

Those it still catches usually have one of a few things in common. Exercising incentive stock options and holding the shares is the classic trigger: the bargain element, market value minus strike price, is not income for regular tax but is for the AMT. A large itemized deduction for state and local taxes, which the AMT adds back, and interest from private activity bonds can tip people over as well.

The arithmetic here starts once you know your alternative minimum taxable income (AMTI). It subtracts the exemption you enter, taxes the first slice of what remains at 26% and anything above that slice at 28%, and sets that tentative minimum tax against your regular tax. If the tentative figure is larger, the gap is your AMT.

Three things stay in your hands. The exemption shrinks at higher incomes and no phase-out is applied here, so enter the reduced amount if yours is cut. The exemption and the 26% band change every year and depend on filing status, so take both from the current Form 6251 instructions. And long-term capital gains and qualified dividends, which keep their lower rates under the AMT, are taxed here at 26% or 28%, so the estimate runs high for anyone with substantial investment income. It is a planning figure, not tax advice.

What is alternative minimum tax calculator?

Alternative minimum taxable income is your taxable income rebuilt under AMT rules: some deductions are disallowed and some items the regular system ignores, such as the spread on exercised incentive stock options, are added in. Subtracting the exemption gives the amount subject to AMT. The tentative minimum tax is that amount taxed at 26% up to a yearly threshold and 28% above it. The alternative minimum tax is only the excess of the tentative minimum tax over your regular tax, and it is zero when regular tax is the larger of the two.

How to use this calculator.

1. 01Enter your alternative minimum taxable income, the AMTI total at the end of Part I of Form 6251. It is your taxable income with the AMT adjustments and preference items added back, so work through Part I first.
2. 02Enter the AMT exemption for your filing status and tax year from the Form 6251 instructions. If your AMTI is high enough for the exemption to be phased out, use the reduced figure from the instructions' worksheet rather than the full amount.
3. 03Enter the amount taxed at 26%, the threshold printed on Form 6251 for that year. Married people filing separately use half the figure that applies to everyone else.
4. 04Enter your regular income tax as Form 6251 defines it: essentially the tax on your taxable income before credits, not the balance due after withholding.
5. 05Read the tentative minimum tax beside the AMT owed. When the AMT is zero but the tentative figure sits just below your regular tax, a modest amount of extra AMT-only income could push you over.

The formula.

First the exemption comes off: base = max(0, AMTI − exemption). Then the tentative minimum tax is TMT = 26% × min(base, band) + 28% × max(0, base − band), where the band is the amount you entered as taxed at 26%. Finally AMT = max(0, TMT − regular tax). Form 6251 writes the same step differently: above the threshold it multiplies the whole base by 28% and subtracts a fixed amount equal to 2% of the band, which is $4,652 when the band is $232,600, and the two routes give identical answers. Because the rates are flat, every extra dollar of AMTI adds 26 or 28 cents to the tentative tax, and every dollar of exemption lost to the phase-out does the same. Income that counts only for the AMT, such as an incentive stock option spread, therefore feeds almost straight into the amount owed once you are paying AMT, since it leaves regular tax untouched.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Internal Revenue Service. Form 6251 instructions current 2026. Alternative minimum tax computation. Retrieved 2026-08-06.
2. [2]U.S. Internal Revenue Service. Tax guidance current 2026. Official forms, instructions and tax topics. Retrieved 2026-08-06.
3. [3]OpenStax, Rice University. Principles of Finance, 2022. Chapters 6–10. Retrieved 2026-08-06.