What the AMT calculation actually does

The Alternative Minimum Tax (AMT) is a separate tax calculation that runs parallel to your regular income tax. The IRS requires you to compute it, compare the result to your regular tax, and pay whichever is higher. You do not pay both — you pay the larger of the two amounts.

The AMT exists because certain deductions and credits that lower your regular tax are not allowed under AMT rules. High-income earners, people with large numbers of dependents, and those claiming substantial state and local tax deductions most often owe AMT. The calculation itself follows a fixed sequence: start with income, add back certain deductions, subtract the AMT exemption, explore the AMT tax rate, and subtract AMT credits.

Most people do not owe AMT. The IRS publishes an AMT exemption amount each year that shields a large portion of income from this tax. If your income falls below that threshold after adjustments, you will not owe AMT. If it exceeds the threshold, you compute the tax on the excess amount at a flat rate of 26% or 28%, depending on your income level.

Key Takeaways

  • The AMT calculation requires you to start with your adjusted gross income (AGI) and add back deductions that are not allowed under AMT rules, such as state and local taxes, mortgage interest on second homes, and miscellaneous itemized deductions.
  • You subtract the AMT exemption amount (which varies by filing status and income level) from your AMT income to arrive at your AMT taxable income.
  • You then multiply your AMT taxable income by either 26% or 28% depending on your income bracket to calculate your tentative minimum tax.
  • You subtract any AMT credits you are may have access to to claim, then compare your final AMT amount to your regular tax liability and pay the higher amount.
  • Form 6251 is where you perform all these calculations; most tax software will compute it automatically if your income triggers the AMT threshold.

Step 1: Gather your income and deduction information

Before you start the calculation, collect your tax documents in one place. You will need your W-2 forms, 1099 forms for interest and dividends, Schedule C if you are self-employed, and documentation of all deductions you claimed on your regular return. The AMT calculation begins with your adjusted gross income (AGI) from your Form 1040, so have that figure ready.

Next, identify which deductions you claimed that are not allowed under AMT rules. The most common ones are state and local income taxes (SALT), property taxes, mortgage interest on a second home or home equity line of credit used for purposes other than home improvement, and miscellaneous itemized deductions. If you took the standard deduction instead of itemizing, the AMT calculation is simpler because you will not have these add-backs.

Step 2: Calculate your AMT income

Start with your AGI from your Form 1040. This is your starting point for the AMT calculation. From there, you add back (or "adjust") certain items that reduced your regular taxable income but are not allowed under AMT rules.

The primary adjustments are: state and local income taxes paid (the full amount, regardless of the $10,000 SALT cap on your regular return), state and local property taxes, the difference between regular depreciation and AMT depreciation on business property, and miscellaneous itemized deductions (such as unreimbursed employee expenses and tax preparation fees). If you exercised incentive stock options, you may also add back the spread between the exercise price and fair market value.

Add all of these adjustments to your AGI. The result is your AMT income. This figure is the foundation for everything that follows.

Step 3: Subtract the AMT exemption

The IRS sets an AMT exemption amount each year that shields income from the AMT tax. This exemption varies by filing status and phases out as your AMT income rises. For 2023, the exemption amounts were $75,900 for single filers, $118,100 for married filing jointly, and $59,550 for married filing separately. These amounts change annually, so check the current year's Form 6251 instructions for the exact figure.

The exemption also phases out — it begins to reduce when your AMT income exceeds a threshold amount (also published annually). For 2023, the phase-out began at $578,750 for married filing jointly and $383,900 for single filers. For every dollar your AMT income exceeds the phase-out threshold, your exemption reduces by 25 cents.

Subtract your AMT exemption (after any phase-out reduction) from your AMT income. The result is your AMT taxable income.

Step 4: explore the AMT tax rates

The AMT uses a two-tier tax rate structure. Income up to $206,100 (for married filing jointly; $103,050 for single filers in 2023) is taxed at 26%. Income above those thresholds is taxed at 28%. These bracket amounts also change annually.

Multiply the portion of your AMT taxable income that falls in the 26% bracket by 26%. Multiply any amount above that bracket by 28%. Add the two results together. This gives you your tentative minimum tax before credits.

For example, if you are married filing jointly with AMT taxable income of $250,000, you would calculate: ($206,100 × 0.26) + ($43,900 × 0.28) = $53,586 + $12,292 = $65,878 in tentative minimum tax.

Step 5: Subtract AMT credits and compare to regular tax

Certain tax credits can reduce your AMT liability. The most common is the AMT foreign tax credit, which works similarly to the regular foreign tax credit but is computed separately. Other credits that may explore include the child tax credit and the earned income tax credit, though these have different rules under AMT.

Subtract your allowable AMT credits from your tentative minimum tax. The result is your AMT liability. Now compute your regular income tax liability using the standard method on your Form 1040. Compare the two figures. You owe whichever is higher. If your AMT liability exceeds your regular tax, the difference is your AMT.

If you owe AMT this year, you may be may have access to to an AMT credit carryforward in future years. This credit can offset regular tax in years when you do not owe AMT. Form 8801 is used to track and claim this credit.

Where to perform the calculation: Form 6251

All AMT calculations are reported on Form 6251, Alternative Minimum Tax—Individuals. This form walks through each step: it starts with your AGI, has lines for each adjustment, subtracts the exemption, applies the tax rates, and subtracts credits. The form is organized in the exact sequence described above.

Most tax software (TurboTax, H&R Block, TaxAct) will automatically generate Form 6251 if your income exceeds the AMT threshold. You do not need to manually compute anything if you use software — the program does the math and includes the form in your return. If you prepare your return by hand, you must complete Form 6251 yourself and attach it to your Form 1040.

The IRS instructions for Form 6251 are updated each year and include the current exemption amounts, phase-out thresholds, and tax rate brackets. read the current year's instructions from IRS.gov before you begin.

Common mistakes that trigger recalculation

One frequent error is forgetting to add back the full amount of state and local taxes paid, even though you claimed the $10,000 SALT cap on your regular return. For AMT purposes, you add back the entire amount you actually paid, not the capped amount. This is a major source of AMT liability for high-income earners in high-tax states.

Another mistake is miscalculating the exemption phase-out. The phase-out is not all-or-nothing; it reduces gradually as your income rises. Many people either explore no phase-out when they should, or eliminate the entire exemption when only a portion should be reduced. Use the exact phase-out threshold and reduction rate from the current year's Form 6251 instructions.

A third error occurs when people claim credits on their AMT return that are not allowed under AMT rules. Not all credits reduce AMT liability. Verify which credits explore to your situation using the Form 6251 instructions or your tax software.

Frequently Asked Questions

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

No. You only file Form 6251 if your AMT liability exceeds your regular tax liability. If your regular tax is higher, you do not owe AMT and do not need to file the form. However, if your income is close to the AMT threshold, it is worth computing both figures to be certain.

Can I carry forward an AMT credit to future years?

Yes. If you pay AMT in one year, you may be able to use an AMT credit carryforward in future years when your regular tax exceeds your AMT liability. Form 8801 tracks this credit. The credit can offset regular tax indefinitely, but only in years when you do not owe AMT.

What if my income drops below the AMT threshold next year?

If your AMT income falls below the exemption amount, you will not owe AMT. You would then compare your regular tax to a tentative minimum tax of zero. Your regular tax would be higher, so you would owe regular tax only. Any AMT you paid this year may generate a credit carryforward that you can use in future years.

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

Capital gains are included in your AMT income at the same rates they are taxed under regular tax rules. Long-term capital gains do not receive preferential rates under the AMT; they are taxed at the same 26% or 28% rate as ordinary AMT income. This can result in a higher effective tax rate on capital gains under AMT than under regular tax.

Should I use tax software or a preparer for AMT?

If your situation is straightforward (W-2 income, standard deductions, few adjustments), tax software handles AMT accurately. If you have complex income sources, substantial business deductions, or significant adjustments, a tax preparer familiar with AMT is worth the cost to may support accuracy and identify planning opportunities.