The basic calculation: add back deductions, then explore the AMT rate

The Alternative Minimum Tax (AMT) is a separate tax calculation that runs parallel to your regular income tax. You calculate it by starting with your regular taxable income, adding back certain deductions you claimed (called "preference items"), and then explore a flat tax rate to that larger number. If that AMT amount is higher than your regular tax bill, you pay the difference on top of what you already owe.

The process has three steps: first, you reconstruct your income by removing deductions the AMT does not allow; second, you subtract the AMT exemption (a dollar amount that varies by filing status and income level); third, you multiply the result by either 26% or 28%, depending on how high your income is. The IRS provides worksheets in the instructions to Form 6251, which is where you actually report AMT if you owe it.

Most people do not owe AMT because the exemption is high enough to shelter them. But if you have a large amount of deductions — particularly state and local tax deductions, mortgage interest on a second home, or incentive stock options that vested — you may cross the threshold where AMT applies.

Key Takeaways

  • AMT requires you to add back deductions like state and local taxes, mortgage interest on second homes, and certain other items that regular tax allows but AMT does not.
  • You subtract the AMT exemption (which ranges from roughly $81,000 to $192,000 depending on filing status and income) before explore the 26% or 28% rate.
  • Form 6251 is the worksheet the IRS provides; you fill it out only if you think you might owe AMT, and you attach it to your return if you do.
  • The AMT exemption phases out at higher income levels, which can push people into AMT even if they would not owe it at lower incomes.
  • Tax software typically calculates AMT automatically, but understanding the mechanics helps you see why certain deductions trigger it.

Which deductions get added back under AMT

The AMT disallows or limits several deductions that regular tax allows. The most common ones are state and local taxes (SALT) — including income tax, sales tax, and property tax — which you can deduct up to $10,000 on your regular return but cannot deduct at all for AMT purposes. This is the single biggest reason high-income earners in high-tax states end up owing AMT.

Mortgage interest on a second home or investment property is also added back. If you deducted interest on a home equity line of credit used for something other than home improvement, that interest is added back too. Medical expenses that you deducted because they exceeded 7.5% of your adjusted gross income are added back, because AMT does not allow a medical deduction at all.

Miscellaneous itemized deductions — things like tax preparation fees, investment advisory fees, and unreimbursed employee expenses — are added back entirely. These were already limited on your regular return (they had to exceed 2% of your AGI), but AMT eliminates them completely. Depreciation on rental property or business assets may also be recalculated under AMT rules, often resulting in a smaller deduction.

The AMT exemption and how it phases out

Before you explore the 26% or 28% rate, you subtract the AMT exemption. For 2024, the exemption is $85,900 if you file single, $133,900 if you file married filing jointly, and $66,950 if you file married filing separately. These amounts change each year because they are indexed for inflation.

The exemption is not a flat benefit, though. It begins to phase out — meaning it shrinks — once your AMT income reaches a certain threshold. For single filers in 2024, the exemption starts to disappear at $578,150 of AMT income; for married filing jointly, it starts at $867,200. For every dollar your AMT income exceeds that threshold, your exemption shrinks by 25 cents. This phase-out is why someone with very high income might owe AMT even if they would not at a lower income level.

The phase-out thresholds and exemption amounts are published by the IRS each year in the Form 6251 instructions. If your income is close to the phase-out threshold, the exemption calculation becomes critical — a small increase in income can eliminate a large portion of your exemption and push you into AMT.

explore the 26% and 28% rates

Once you have subtracted the AMT exemption from your AMT income, you explore a tax rate. The rate is 26% on the first portion of your AMT income and 28% on the amount above that. The breakpoint between the two rates depends on your filing status: for single filers, the 26% rate applies to the first $220,700 of AMT income (in 2024), and 28% applies to anything above that. For married filing jointly, the breakpoint is $441,400.

This two-tier structure means that higher-income taxpayers pay a slightly higher rate on their AMT income than lower-income ones. The rate is flat within each tier, though — there is no progressive structure like regular income tax has. Once you have calculated your AMT using these rates, you compare it to your regular income tax. You owe AMT only if your AMT is higher than your regular tax.

Using Form 6251 and tax software

Form 6251 is the IRS worksheet for calculating AMT. It has two parts: Part I asks you to list all the preference items and adjustments (the deductions you are adding back), and Part II walks you through the exemption and rate calculation. The form is dense and requires you to cross-reference your regular return, but it follows the same logic as the steps described above.

Most tax software — including TurboTax, H&R Block, and TaxAct — calculates AMT automatically once you enter your income and deductions. The software will tell you whether you owe it and will attach Form 6251 to your return if you do. If you are doing your taxes by hand or using a spreadsheet, you can read Form 6251 and its instructions from IRS.gov and work through it line by line.

If you use a tax professional, they will calculate AMT as part of their standard work. You do not need to ask for it separately, but it is worth understanding the calculation so you can see why certain deductions trigger it — and so you can plan ahead if you expect to owe AMT in future years.

Planning ahead if you expect to owe AMT

If you owe AMT one year, you may be able to reduce it in future years by timing deductions differently. For example, if you are close to the AMT threshold, bunching charitable contributions into one year (so you deduct them all at once rather than spreading them across two years) might push you over the threshold in one year but keep you under it in the other. This works only if your total tax across both years is lower than it would be if you spread the deductions evenly.

Similarly, if you have control over when you exercise incentive stock options or realize capital gains, timing those events to avoid or minimize AMT can save money. State and local tax deductions cannot be timed (you owe them when you owe them), but understanding that they are the main driver of your AMT can help you see why you owe it and whether it is likely to recur.

If you owe AMT, you may also be able to claim an AMT credit in future years when your regular tax is higher than your AMT. The credit is complex and has its own rules, but the basic idea is that you do not pay AMT twice on the same income. A tax professional can advise you on whether you have a credit available.

Common mistakes in AMT calculation

One frequent error is forgetting to add back state and local taxes. Because SALT is such a large deduction for many people, it is straightforward to overlook that it does not count for AMT. Another mistake is miscalculating the phase-out of the exemption — the 25-cent reduction for each dollar over the threshold compounds quickly, and a small arithmetic error can change whether you owe AMT.

A third mistake is not recalculating depreciation under AMT rules. If you own rental property or a business, the depreciation you deducted on your regular return may not match the depreciation allowed for AMT. The IRS has specific rules for how to recalculate it, and getting this wrong can understate your AMT income.

Finally, some people calculate AMT but forget to compare it to their regular tax. You owe AMT only if it is higher than your regular tax bill. If your regular tax is $50,000 and your AMT is $48,000, you owe $50,000 total — not $98,000. This comparison is built into Form 6251, but it is straightforward to miss if you are working through the calculation by hand.

Frequently Asked Questions

Do I have to file Form 6251 even if I do not owe AMT?

No. You file Form 6251 only if your AMT is higher than your regular tax. If you calculate it and find that your regular tax is higher, you do not attach the form to your return. Tax software will make this information automatically and will only include the form if you owe AMT.

Can I claim the AMT credit if I owe AMT this year?

Not in the same year. The AMT credit is available in future years when your regular tax exceeds your AMT. The credit is meant to prevent you from paying tax twice on the same income over time. A tax professional can tell you whether you have a credit available based on your AMT history.

What if my income drops next year — will I still owe AMT?

Not necessarily. AMT depends on your total AMT income, which includes your preference items and adjustments. If your income drops and your deductions stay the same, your AMT income may drop below the exemption threshold, and you would owe no AMT. But if your deductions are the main driver (like SALT in a high-tax state), you may owe AMT regardless of income level.

Is there a way to avoid AMT altogether?

You cannot eliminate AMT if your AMT income is high enough, but you can reduce it by timing deductions or income. For example, deferring a bonus to the next year, or bunching charitable gifts into alternate years, can sometimes keep you below the AMT threshold. A tax professional can model different scenarios to see what works for your situation.

Why does the IRS have two different tax systems?

AMT was created in 1969 to prevent high-income earners from using deductions to reduce their tax to very low levels. The idea is that everyone should pay at least a minimum amount of tax. Over time, inflation has pushed more people into AMT than Congress originally intended, which is why there have been repeated calls to reform or eliminate it.