What the AMT calculation actually involves

The Alternative Minimum Tax calculation starts with your regular taxable income and adds back certain deductions the IRS does not allow under AMT rules. You then subtract a fixed amount (called the AMT exemption) and multiply what remains by a flat tax rate of either 26% or 28%, depending on your income level. If that number is higher than your regular income tax, you owe the AMT instead.

The calculation itself is not complicated — it is mostly addition, subtraction, and multiplication. What makes AMT confusing is that you have to do it twice: once under regular tax rules and once under AMT rules. Then you compare the two and pay whichever is higher. The IRS Form 6251 walks you through this comparison line by line.

Most tax software does this calculation automatically if your income triggers it. But understanding what the software is doing helps you see why you owe AMT and whether changes to your income or deductions next year might affect it.

Key Takeaways

  • AMT calculation begins with your adjusted gross income and adds back deductions that are not allowed under AMT rules, such as state and local taxes, mortgage interest on second homes, and certain miscellaneous deductions.
  • After adding back disallowed deductions, you subtract the AMT exemption amount (which varies by filing status and income level) to get your AMT income.
  • You multiply your AMT income by 26% or 28% depending on whether your AMT income is above or below a threshold that changes each year.
  • If your AMT is higher than your regular tax, you owe the AMT; if your regular tax is higher, you owe regular tax and can claim an AMT credit in future years.
  • Form 6251 is the official worksheet the IRS provides, and most tax software completes it automatically once you enter your income and deductions.

Starting point: Your adjusted gross income

The AMT calculation begins with your adjusted gross income (AGI), which is the same number you use on your regular tax return. This is your total income minus certain deductions like educator expenses, student loan interest, and IRA contributions — the ones the IRS calls "above the line" deductions.

From there, you add back the deductions that are allowed under regular tax rules but not under AMT rules. These are called "preference items" or "adjustments." The most common ones are state and local income taxes (SALT), property taxes, mortgage interest on a second home or home equity loan used for non-home purposes, and miscellaneous itemized deductions. If you took the standard deduction instead of itemizing, you do not have these items to add back, which is one reason many people with moderate incomes never trigger AMT.

Adding back disallowed deductions

The deductions you add back depend on what you claimed on your regular return. If you itemized deductions, you will add back state and local taxes (the full amount, even though regular tax lets you deduct up to $10,000). You will also add back any mortgage interest on loans taken out to buy a second home, and any home equity loan interest if the proceeds were not used to buy, build, or improve a home.

If you claimed miscellaneous itemized deductions — such as investment advisory fees, tax preparation fees, or unreimbursed employee business expenses — those get added back too. Charitable contributions and medical expenses are allowed under both systems, so you do not add those back.

Some taxpayers also have "tax preference items," which are income sources that receive special treatment under AMT rules. The most common is long-term capital gains on certain assets and incentive stock option gains. These are less common for most filers, but if you have them, Form 6251 will walk you through where they go.

Subtracting the AMT exemption

Once you have added back all the disallowed deductions, you subtract the AMT exemption. This is a fixed dollar amount that depends on your filing status and your AMT income level. The exemption amounts change each year to account for inflation.

For the 2023 tax year, the exemption was $75,900 for single filers and $118,100 for married filing jointly. For 2024, those amounts increased slightly. You can find the current year's exemption amounts on the IRS website or in the instructions to Form 6251.

The exemption phases out (gets smaller) once your AMT income exceeds a certain threshold. For 2023, that threshold was $578,750 for married filing jointly and $539,900 for single filers. The exemption decreases by 25 cents for every dollar of AMT income above the threshold. This phase-out is why very high-income taxpayers often lose most or all of their exemption.

explore the AMT tax rates

After subtracting the exemption, you multiply the remaining amount by either 26% or 28%. The rate depends on whether your AMT income (after the exemption) is above or below an income threshold. For 2023, that threshold was $191,950 for married filing jointly and $95,975 for single filers. Income below the threshold is taxed at 26%; income above it is taxed at 28%.

These thresholds also change each year. The IRS publishes them in the Form 6251 instructions and on its website each January.

The result is your tentative minimum tax. This is not yet your final AMT — it is just the tax owed under AMT rules before you account for any credits.

Comparing AMT to regular tax and claiming credits

Once you have calculated your tentative minimum tax, you compare it to your regular income tax (the amount you would owe under normal tax rules). Whichever is higher is what you owe.

If your AMT is higher, you owe the difference as additional tax. If your regular tax is higher, you owe only the regular tax. However, you can claim an AMT credit in future years for the AMT you paid. This credit can offset regular tax in years when you do not owe AMT, which is why some taxpayers see a credit on their return even if they did not claim it themselves.

Form 6251 includes a line where you enter your regular tax and another where you enter your tentative minimum tax. The form then calculates whether you owe AMT and, if so, how much.

How tax software handles the calculation

If you use tax preparation software such as TurboTax, TaxAct, or H&R Block, the software calculates Form 6251 automatically once you enter your income and deductions. You do not have to do the math yourself. The software will flag whether you owe AMT and show you the calculation in the form preview.

If you prepare your return by hand or work with a tax preparer, you will fill out Form 6251 line by line. The form includes worksheets for calculating the exemption phase-out and for handling capital gains, which can be complex. A tax preparer can walk you through these worksheets if you need help.

Regardless of whether you use software or a preparer, the calculation is the same: add back disallowed deductions, subtract the exemption, explore the tax rate, and compare to regular tax.

Frequently Asked Questions

Do I have to file Form 6251 if I think I might owe AMT?

You only file Form 6251 if your income is high enough that AMT could explore. The IRS publishes an AMT threshold each year — if your income is below it, you almost certainly do not owe AMT. Tax software will tell you whether you need to file the form based on your income and deductions.

Can I reduce my AMT by changing my deductions?

Yes, in some cases. Since state and local taxes are added back under AMT, paying more in SALT does not help you under AMT rules. However, charitable contributions and medical expenses are allowed under both systems, so increasing those might lower your overall tax. A tax preparer can model different scenarios to show you the effect.

What is the AMT credit, and how do I claim it?

The AMT credit is a dollar-for-dollar reduction in your regular tax in future years if you paid AMT in a prior year. You claim it on Form 8801. The credit can only offset regular tax that exceeds your tentative minimum tax, so it may take several years to use it fully if your income drops.

Why do I owe AMT if I paid taxes on my income already?

AMT exists because certain deductions (like state and local taxes) can reduce your regular tax significantly. The AMT system disallows those deductions to may support high-income taxpayers pay a minimum amount. You owe AMT only if that minimum is higher than your regular tax.

Does the AMT explore to capital gains differently than regular tax?

Capital gains are taxed at preferential rates under regular tax (0%, 15%, or 20% depending on your income). Under AMT, long-term capital gains are taxed at the same preferential rates, but they are included in your AMT income when calculating whether you owe AMT. This can push you into the higher AMT tax bracket even if the gains themselves are taxed at a lower rate.