IRC 55 Individual Alternative Minimum Tax: Form 6251 and OBBBA Planning Guide for CPAs and EAs

Overview: What Is the Individual Alternative Minimum Tax?

The individual alternative minimum tax (AMT) under IRC 55 is a parallel tax system that runs alongside the regular income tax. Its purpose is to ensure that high-income individuals cannot reduce their regular tax liability to near zero through the accumulation of tax preferences and timing adjustments. Every taxpayer subject to the AMT must compute both their regular income tax and their tentative minimum tax (TMT). If TMT exceeds regular tax, the excess is the AMT owed and is added to the regular tax on Form 1040.

For practitioners advising high-income individuals in 2025 and 2026, the AMT has new urgency. The One Big Beautiful Budget Act (OBBBA) permanently raised the IRC 55(d) exemption amounts above the TCJA levels, eliminating the "AMT cliff" that existed under pre-OBBBA law. At the same time, the OBBBA raised the SALT deduction cap under IRC 164(b)(6) from $10,000 to $40,000, which directly increases the AMT addback under IRC 56(b)(1)(A)(ii) for high-SALT itemizers. The net effect is that some taxpayers who had minimal AMT exposure under the $10,000 SALT cap now face meaningful AMT liability from the $40,000 cap combined with other AMT triggers.

This guide covers the statutory framework, the OBBBA changes, the Form 6251 line-by-line workflow, the ISO AMT trap, passive activity and depreciation adjustments, the IRC 53 minimum tax credit, and pre-year-end planning steps. All threshold amounts, exemption figures, and phase-out ranges cited in this guide should be verified at IRS.gov and in the applicable Revenue Procedure before use in any return or planning engagement.

Statutory Framework

IRC 55: Imposition and the TMT Formula

IRC 55(a) imposes the AMT on every individual (and certain other non-corporate taxpayers) in an amount equal to the excess, if any, of the tentative minimum tax (TMT) over the regular income tax. If TMT does not exceed regular tax, no AMT is owed.

The TMT formula under IRC 55(b) is:

1. Start with regular taxable income.
2. Add or subtract AMT adjustments under IRC 56 (increases and decreases to income that differ between regular tax and AMT).
3. Add AMT preference items under IRC 57.
4. The result is alternative minimum taxable income (AMTI).
5. Subtract the AMT exemption under IRC 55(d) (reduced by the phase-out formula).
6. Apply the AMT rate (26% on the first bracket, 28% above the applicable threshold; verify thresholds at IRS.gov) to the net amount.
7. The result is the TMT. If TMT exceeds regular tax, the difference is the AMT.

IRC 55(b): AMT Rates

The AMT is taxed at two rates: 26% on the portion of AMT base up to the applicable bracket threshold and 28% on the portion above it. These rate thresholds are indexed for inflation annually. Verify the current-year brackets in the applicable Revenue Procedure at IRS.gov before applying rates to a client's AMTI. Qualified dividends and long-term capital gains remain subject to the same preferential rates under both regular tax and AMT (the AMT does not convert LTCG to ordinary income).

IRC 55(d): AMT Exemption and Phase-Out

IRC 55(d) provides AMT exemption amounts for individuals. Under the OBBBA, Congress permanently raised the exemption amounts above the TCJA levels that had been set to expire. This eliminated the "AMT cliff" scenario under pre-OBBBA law in which the TCJA temporary exemptions would have sunset to much lower pre-2018 amounts, exposing millions of additional taxpayers.

Under IRC 55(d)(3) and (4), the exemption phases out at higher AMTI levels, reducing by $0.25 for every $1.00 of AMTI above the applicable phase-out threshold. At sufficiently high AMTI, the exemption is eliminated entirely. Both the exemption amounts and the phase-out thresholds are indexed for inflation annually.

IRC 56: AMT Adjustments

IRC 56 lists items that must be adjusted (added back or modified) when computing AMTI. These are timing and character differences between the regular tax system and the AMT system, not permanent exclusions. The most important adjustments for individual practitioners are:

IRC 56(b)(1): Itemized Deduction Adjustments (Including SALT Addback)

Under IRC 56(b)(1)(A)(ii), no deduction is allowed for AMT purposes for taxes paid or accrued that are deductible under IRC 164 (state and local taxes). This means the entire SALT deduction claimed on Schedule A is added back on Form 6251 when computing AMTI. Under OBBBA, the SALT cap is $40,000 (verify current amount at IRS.gov), so a taxpayer who itemizes the full $40,000 SALT deduction for regular tax must add back the full $40,000 on Form 6251.

Other IRC 56(b)(1) itemized deduction adjustments include: miscellaneous itemized deductions (no deduction allowed for AMT); home equity loan interest that is not for acquisition indebtedness (no AMT deduction); and medical expense floor differences. Charitable deductions are not an AMT adjustment and are allowed under AMT in full.

IRC 56(b)(3): ISO Bargain Element as an AMT Adjustment

Under IRC 56(b)(3), the spread between the exercise price and the fair market value of stock at the time of exercise of an incentive stock option (ISO) is included in AMTI in the exercise year. This adjustment exists because the ISO bargain element is excluded from regular taxable income under IRC 421 and IRC 422 in the year of exercise (when the employee holds the stock). The AMT system treats the bargain element as income for AMT purposes, even though no cash has been received and no regular tax is owed. This is the source of the "ISO AMT trap" described further below.

IRC 56(a): Depreciation Adjustment (MACRS vs. ADS)

For personal property placed in service after 1986, MACRS depreciation is replaced for AMT purposes by the Alternative Depreciation System (ADS) using the straight-line method over the ADS recovery period. The AMT adjustment equals MACRS depreciation claimed for regular tax minus ADS depreciation allowable for AMT. In early years of an asset's life, MACRS produces higher deductions than ADS (positive AMT adjustment, increasing AMTI). In later years, ADS produces higher deductions (negative AMT adjustment, decreasing AMTI). For OBBBA 100% bonus depreciation, the entire cost is deducted in the first year for regular tax, while ADS applies a first-year amount over a longer life for AMT, producing a very large positive AMT adjustment in the year of acquisition.

Other IRC 56 Adjustments

Additional AMT adjustments under IRC 56 include: circulation expenditures (deducted over three years for AMT vs. immediately for regular tax); mining exploration and development costs (deducted over ten years for AMT vs. immediately for regular tax); and long-term contracts completed using the completed contract method for regular tax (must use percentage of completion for AMT). Passive activity losses are also recomputed for AMT purposes using AMT-adjusted income and loss figures, which can produce a different PAL result than the regular tax computation.

IRC 57: AMT Preference Items

IRC 57 lists permanent preference items that increase AMTI regardless of timing. Unlike IRC 56 adjustments, preferences do not reverse in later years. The key preferences for individual taxpayers are:

IRC 57(a)(5): Private Activity Bond (PAB) Interest

Interest from specified private activity bonds is an AMT preference item. While this interest is excluded from regular taxable income, it is added back on Form 6251 when computing AMTI. Not all tax-exempt interest is a preference: interest from governmental bonds and qualifying 501(c)(3) bonds issued before statutory cutoff dates is generally not subject to AMT. See the amber callout below for practitioner guidance on bond classification.

IRC 57(a)(1): Excess Percentage Depletion

The excess of the percentage depletion deduction allowed under IRC 613 or IRC 613A over the adjusted basis of the property at the end of the year is an AMT preference under IRC 57(a)(1). Practitioners with clients who receive percentage depletion from oil, gas, or mineral interests must add the excess amount to AMTI on Form 6251.

The OBBBA Changes: What Practitioners Must Know

Permanent Exemption Increase

Before the OBBBA, the higher TCJA AMT exemption amounts were scheduled to sunset at the end of 2025, which would have reinstated the significantly lower pre-2018 exemption levels and exposed millions of additional taxpayers to AMT. The OBBBA made the higher exemption amounts permanent (and continued inflation indexing), eliminating this "AMT cliff." For the years this guide covers, practitioners should confirm the current exemption amounts in the applicable Revenue Procedure at IRS.gov rather than referencing prior-year figures.

SALT Cap Increase and the New AMT Exposure Vector

The OBBBA raised the SALT deduction cap under IRC 164(b)(6) from $10,000 to $40,000 (verify current amount at IRS.gov). This is a significant AMT planning development for 2025 forward. Under the $10,000 cap, the SALT addback on Form 6251 was capped at $10,000 by the deduction limit itself. Under the $40,000 cap, a taxpayer who deducts $40,000 of SALT on Schedule A adds back $40,000 on Form 6251. This is four times larger than the prior addback.

For taxpayers who also have ISO exercises, bonus depreciation, or PAB interest, the $40,000 SALT addback becomes the variable that tips total AMTI above the point where TMT exceeds regular tax. This is the new AMT planning scenario that practitioners must model before year-end for clients in high-SALT states.

One planning response is the pass-through entity tax (PTET) election: a state PTET election can shift the SALT deduction from Schedule A (where it creates the IRC 56(b)(1) AMT addback) to a business deduction that flows through the entity (where it is not an AMT adjustment on the individual return). For clients with pass-through income in states with conforming PTET regimes, this may reduce individual AMTI. See the IRC 164 guide linked in the Related Guides section below.

Form 6251: Line-by-Line Practitioner Workflow

Form 6251 (Alternative Minimum Tax: Individuals) is the computational form for individual AMT. The following walkthrough covers the principal lines. Because the IRS periodically revises form line numbering, verify all line references against the current Form 6251 and instructions at IRS.gov before use in a client return.

The net AMT from Form 6251 is added to the regular tax on Form 1040 Schedule 2, Line 1. Verify all Form 6251 line references against the current IRS instructions before preparing or reviewing any return.

The ISO AMT Trap: Practitioner Warning

The intersection of incentive stock options and the AMT is one of the most dangerous planning areas in individual taxation. The mechanism is straightforward but the consequences are severe if not modeled in advance.

When an employee exercises an ISO and holds the underlying shares (to qualify for long-term capital gain treatment on a future sale), the bargain element (fair market value at exercise minus exercise price) is excluded from regular taxable income under IRC 421 and IRC 422 in the exercise year. However, under IRC 56(b)(3), that same bargain element is an AMT adjustment included in AMTI. If the bargain element is large enough to push TMT above regular tax, the employee owes AMT on income they have not yet received in cash. The shares may decrease in value after exercise, leaving the taxpayer with a large AMT bill and shares worth less than the tax.

AMT paid on ISO exercises generates an AMT credit carryforward under IRC 53 (Form 8801), but that credit can only be used in years when regular tax exceeds TMT. If the stock price declines after exercise and the employee sells, the regular tax benefit may be in future years that are offset by the carryforward, but the credit provides no immediate cash relief in the year of the AMT liability.

IRC 57: AMT Preferences in Detail

Preference items under IRC 57 differ from IRC 56 adjustments in an important way: they are permanent additions to AMTI, not timing differences that reverse. The two most common individual preference items are PAB interest (covered in the callout above) and excess percentage depletion. If a client receives depletion deductions on oil, gas, or mineral interests, practitioners must compute the excess of percentage depletion over the adjusted basis of the property and include that amount on Form 6251 as a preference.

AMT Treatment of Passive Activity Losses

Passive activity loss rules under IRC 469 apply under both the regular tax and AMT systems, but the PAL computation can differ between the two because the underlying income and loss figures are themselves different for AMT purposes. AMT-adjusted passive activity income or loss is computed using AMT-adjusted income (after applying IRC 56 depreciation adjustments, ISO adjustments, and any other applicable changes). A taxpayer may have a regular-tax PAL that is deductible in the current year under regular tax but not fully deductible for AMT because the AMT-recomputed passive activity income is different. Conversely, some taxpayers find that their regular-tax PAL suspension is partially relieved under AMT because the AMT-recomputed loss is smaller. Practitioners must run separate PAL computations for regular tax and AMT when a client has passive investments with significant depreciation or ISO income.

AMT and Depreciation: Bonus Depreciation Under OBBBA

The OBBBA restored 100% bonus depreciation under IRC 168(k) for qualifying property. For regular tax, the full cost of a qualifying asset is deducted in the year of acquisition. For AMT, bonus depreciation is not allowed; instead, the property is depreciated using the ADS straight-line method over the applicable ADS recovery period. This creates a very large positive AMT adjustment in the acquisition year and negative adjustments (reducing AMTI) in subsequent years as ADS depreciation exceeds the zero MACRS deduction remaining after bonus was taken.

For clients who own pass-through interests (partnerships, S corporations), bonus depreciation flows through on Schedule K-1 and directly increases the partner's or shareholder's Form 6251 Line 2a adjustment. A client with a large partnership K-1 reflecting 100% bonus depreciation may face a substantial AMT adjustment that appears on Form 6251 even though the client took no direct action to claim the depreciation.

The IRC 53 Minimum Tax Credit and Form 8801

When a taxpayer pays AMT attributable to deferral adjustments (ISO exercises, depreciation timing differences, and other items that create AMTI temporarily rather than permanently), that AMT generates a minimum tax credit carryforward under IRC 53. The credit is claimed on Form 8801 (Credit for Prior Year Minimum Tax).

Key features of the IRC 53 credit:

• The credit carryforward has no expiration date. It carries forward indefinitely until used.
• The credit can only be applied in years when the taxpayer's regular tax exceeds their TMT. It cannot reduce AMT liability in a year when AMT applies.
• The credit amount equals the AMT paid attributable to deferral items. AMT attributable to preference items (permanent items such as PAB interest and excess depletion) does not generate a credit.
• In years when regular tax exceeds TMT, the credit reduces the regular tax down to the extent of the excess, with any unused credit continuing to carry forward.

AMT vs. Regular Tax: Comparison Table

The following table compares the treatment of ten common items under the regular tax system and the individual AMT. This comparison is intended as a practitioner reference and should be read alongside the current Form 6251 and its instructions, which are the authoritative source. Verify all items at IRS.gov before use in any return or engagement.

Pre-Year-End AMT Planning Workflow

Effective AMT management is almost entirely a pre-year-end planning exercise. Once December 31 passes, the adjustments and preferences that generate AMT are locked in. The following steps should be completed for every client with AMT exposure risk before year-end:

1. Run a parallel Form 6251 projection. Using current-year estimated income, all anticipated ISO exercises, K-1 depreciation adjustments, SALT deductions at the current cap, and any PAB interest, compute projected AMTI and compare TMT to projected regular tax.
2. Identify the marginal AMT driver. Which single adjustment or preference item, if reduced, would bring TMT below regular tax? Common answers: the ISO bargain element, the SALT addback, or a large bonus depreciation K-1 adjustment.
3. Model ISO exercise timing. If ISO exercises are the marginal AMT driver, model whether deferring the exercise to a year with lower AMTI (lower income, higher exemption absorption) reduces total AMT cost. Account for the holding period clock restart and LTCG implications.
4. Evaluate PTET elections. For clients with pass-through income in states with conforming PTET regimes, a PTET election can shift the SALT deduction from Schedule A (creating an IRC 56(b)(1) AMT addback) to the entity level (no individual AMT addback). Model the net benefit across all affected partners or shareholders.
5. Quantify the IRC 53 credit carryforward. If AMT will be owed regardless of planning, compute the expected credit carryforward and model when it will be usable. Include the present-value discount in any communication about AMT cost.
6. Review Form 8801 for existing carryforwards. If the client has a prior-year AMT credit carryforward from Form 8801, determine whether the current year is a low-AMT year in which the credit can offset regular tax. Apply the credit before considering other withholding adjustments.