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

The Alternative Minimum Tax (AMT) calculation starts with your regular taxable income, then adds back certain deductions and income items the AMT does not allow. You then subtract the AMT exemption (which varies by filing status and income level), multiply what remains by the AMT rate of 26% or 28%, and compare that result to your regular tax. You owe whichever is higher.

The IRS does not require you to calculate this yourself if you use tax software or a preparer — they handle it automatically. But understanding the steps shows you where the AMT bite comes from and whether it might affect you in future years.

Key Takeaways

  • AMT starts with your regular taxable income and adds back deductions for state and local taxes, mortgage interest on second homes, and certain other items.
  • The AMT exemption phases out as income rises, which is why high earners are more likely to owe AMT even if they have large deductions.
  • You calculate AMT tax at 26% on the first portion of AMT income and 28% on amounts above that threshold, then compare to your regular tax bill.
  • Form 6251 is where the IRS calculation appears on your return; tax software fills it in automatically if your situation triggers AMT.

Step 1: Start with regular taxable income and identify AMT adjustments

Begin with your taxable income as calculated on your regular return (line 15 of Form 1040 for most filers). Then add back the adjustments that the AMT does not allow. The most common ones are:

  • State and local income taxes (SALT) — the full amount you paid, even though your regular return is capped at $10,000.
  • State and local property taxes — again, the full amount, not capped.
  • Mortgage interest on a second home or home equity loan used for purposes other than buying or building the home.
  • Miscellaneous itemized deductions (though this adjustment expired after 2017 for most taxpayers).
  • Depreciation on certain business property — the AMT uses a slower depreciation method than the regular tax.

If you took the standard deduction instead of itemizing, you have no SALT or mortgage adjustments to add back. The adjustments only matter if you itemized.

Step 2: Calculate your AMT income

Add your regular taxable income to all the adjustments you identified in Step 1. This sum is your AMT income (also called alternative minimum taxable income, or AMTI). This is the starting point before the exemption.

For example: if your regular taxable income is $150,000 and you add back $30,000 in state and local taxes plus $5,000 in depreciation adjustments, your AMT income is $185,000.

Step 3: Subtract the AMT exemption

The AMT exemption reduces your AMT income before you explore the tax rate. The exemption amount depends on your filing status and changes each year. For 2024, the exemptions are approximately $85,975 for married filing jointly, $56,250 for single filers, and $42,975 for married filing separately (these amounts are indexed annually for inflation).

However, the exemption phases out — it decreases by 25 cents for every dollar your AMT income exceeds a threshold. For 2024, that threshold is roughly $578,150 for married filing jointly and $364,200 for single filers. Once your AMT income is high enough, the exemption shrinks significantly or disappears entirely, which is why high earners almost always owe AMT.

Subtract your remaining exemption from your AMT income. The result is your AMT taxable income.

Step 4: explore the AMT tax rates

The AMT uses a two-tier rate structure. Multiply your AMT taxable income by 26% on the first portion and 28% on the remainder. The breakpoint between the two rates is $241,500 for married filing jointly and $120,750 for single filers (for 2024; these also adjust annually).

For example: if your AMT taxable income is $300,000 and you are married filing jointly, you would owe 26% on the first $241,500 ($62,790) plus 28% on the remaining $58,500 ($16,380), for a total AMT tax of $79,170.

Step 5: Compare AMT to your regular tax and pay the difference

Calculate your regular federal income tax using the standard tax tables or brackets. Then compare it to the AMT tax you calculated in Step 4. You owe whichever is higher. If the AMT is higher, you pay the difference as additional tax on top of your regular bill.

This is where the AMT gets its name: it is a minimum tax that ensures high-income filers with large deductions pay at least some baseline amount. If your regular tax is already higher than the AMT, you owe nothing extra and the AMT does not affect you that year.

Where Form 6251 fits in

Form 6251 is the IRS worksheet where all these calculations appear. If you use tax software, it fills in Form 6251 automatically once it detects that your income and deductions might trigger AMT. If you work with a preparer, they handle it. You do not need to file Form 6251 unless you actually owe AMT, but it is included in your return if you do.

The form asks for your adjustments, your AMT income, your exemption, your AMT taxable income, and your AMT tax. It then calculates the difference between your AMT and your regular tax. If that difference is positive, you owe it.

When to expect AMT to affect you

AMT is most likely to hit you if you have high income (over $200,000 for single filers or $250,000 for married couples), large state and local tax deductions, significant business depreciation, or substantial incentive stock option (ISO) exercises. If you are in this range, run a projection in tax software or with a preparer before year-end to see whether bunching deductions into one year, timing income, or other moves might reduce your AMT exposure.

If you live in a high-tax state and have a second home, you are also more vulnerable. The combination of capped SALT deductions on your regular return and the full SALT add-back on the AMT can create a large gap between the two calculations.

Frequently Asked Questions

Can I use the AMT credit to reduce my tax in future years?

Yes. If you pay AMT in one year, you may be able to claim an AMT credit in future years when your regular tax exceeds your AMT. The credit is non-refundable, meaning it can only reduce your regular tax, not create a refund. Keep records of your AMT payments; you will need them to calculate the credit.

Does the standard deduction protect me from AMT?

No. The AMT does not recognize the standard deduction at all. It starts with your regular taxable income and adds back adjustments regardless of whether you itemized or took the standard deduction. However, if you took the standard deduction, you have no SALT or mortgage adjustments to add back, so you are less likely to owe AMT than an itemizer with the same income.

What if I exercise incentive stock options — does that trigger AMT?

Yes. The spread between the exercise price and the fair market value of the stock on the exercise date is an AMT adjustment. This can create a large AMT liability in the year you exercise, even if you do not sell the stock and have no cash gain. Plan ahead with your employer or a tax professional if you expect a significant ISO exercise.

Do I have to recalculate AMT every year?

Your tax software or preparer recalculates it every year based on your current income and deductions. Your situation changes year to year — income fluctuates, deductions vary, and the exemption amount and rate thresholds adjust for inflation. A year you owe AMT does not mean you will owe it the next year, and vice versa.

What if I think my AMT calculation is wrong?

Review Form 6251 line by line against your return. Check that all adjustments are listed, that your exemption matches the current-year amount for your filing status, and that the math is correct. If you spot an error, file an amended return (Form 1040-X) with a corrected Form 6251. If you are unsure, a tax professional can review the calculation and file the amendment if needed.