Imagine a taxpayer who earns a substantial income and claims several deductions and tax preferences. Under the regular federal income tax system, these deductions may significantly reduce taxable income.

But the U.S. tax system asks another question: after applying a separate set of rules, should this taxpayer still pay at least a minimum level of federal income tax? That question is answered by the Alternative Minimum Tax (AMT).

Quick take: The AMT is a parallel tax calculation. Taxpayers compare their regular tax liability with a “tentative minimum tax” — whichever is higher becomes what they actually owe.

What Exactly is AMT?

The Alternative Minimum Tax is a parallel tax system that runs alongside the regular U.S. federal income tax framework. Certain deductions, exclusions, and preference items that receive favorable treatment under the regular tax system may be adjusted or added back for AMT purposes.

The taxpayer then calculates Alternative Minimum Taxable Income (AMTI) and applies AMT rules to arrive at a tentative minimum tax. The IRS uses Form 6251, Alternative Minimum Tax—Individuals, for this calculation.

Regular Tax vs. AMT — Two Parallel Paths

Step	Regular Tax	AMT
Rules applied	Regular tax rules	Alternative tax rules
Deductions	Regular deductions	Certain deductions adjusted
Income base	Regular taxable income	AMTI
Rates	Regular tax rates	26% / 28% AMT rates
Result	Regular liability	Tentative minimum tax
Final step	Compare both	Higher amount determines additional AMT owed

A Worked Example

Suppose a taxpayer’s regular tax = $48,000, but their tentative minimum tax = $55,094. Because the tentative minimum tax is higher, the taxpayer owes the difference as additional AMT liability:

$55,094 − $48,000 = $7,094 additional AMT

Figure 1. How the tentative minimum tax splits into regular tax and additional AMT in this example.

Why Was AMT Introduced?

The AMT is designed to prevent taxpayers with relatively high economic income from reducing their federal tax liability too substantially through deductions, exclusions, and other tax preferences.

In other words: AMT does not necessarily tax additional income — it often changes the way certain deductions and tax benefits are treated for a subset of higher-income filers.

AMT for Tax Year 2025 — Important Numbers

For Tax Year 2025, the IRS provides the following AMT exemption amounts and phaseout thresholds by filing status:

Filing Status	AMT Exemption	Phaseout Begins	Complete Phaseout
Single / Head of Household	$88,100	$626,350	$978,750
Married Filing Jointly (MFJ)	$137,000	$1,252,700	$1,800,700
Married Filing Separately (MFS)	$68,500	$626,350	$900,350

Figure 2. 2025 AMT exemption amounts by filing status.

Figure 3. AMT exemption phaseout thresholds by filing status.

How the Phaseout Works — John’s Example

For a single taxpayer in 2025: the AMT exemption is $88,100, the phaseout begins once AMTI exceeds $626,350, the exemption is reduced by 25% of the AMTI above that threshold, and it is completely phased out at $978,750.

Suppose John has Alternative Minimum Taxable Income (AMTI) of $700,000.

Particulars	Amount
AMTI	$700,000
Less: Phaseout threshold	$626,350
Excess AMTI	$73,650
Phaseout reduction (25% of excess)	$18,412.50
Original AMT exemption	$88,100
Reduced AMT exemption	$69,687.50

So, John does not get the full $88,100 exemption — his reduced exemption is $69,687.50.

John’s Full AMT Calculation

Particulars	Amount
AMTI	$700,000
Normal 2025 AMT exemption	$88,100
Excess over phaseout threshold	$73,650
25% phaseout reduction	($18,412.50)
Actual AMT exemption available	$69,687.50
AMT taxable amount	$630,312.50

Instead of $700,000 − $88,100 = $611,900, John ends up with $700,000 − $69,687.50 = $630,312.50 subject to AMT — meaning $18,412.50 more of his AMTI becomes taxable because his exemption was phased down.

AMT Tax Rates for 2025

For most individual taxpayers, the AMT applies two flat rates to the AMT taxable excess:

2025 AMT Rate	Applies to AMT Taxable Excess
26%	Up to $239,100
28%	Above $239,100

Note: for Married Filing Separately (MFS), the 26%/28% breakpoint is lower, at $119,550.

2025 AMT — Quick Revision Table

A fast reference for exam prep or client conversations:

Question	2025 Answer
What is AMT?	Alternative Minimum Tax
Individual AMT form	Form 6251
Single exemption	$88,100
MFJ exemption	$137,000
MFS exemption	$68,500
Single phaseout starts	$626,350
MFJ phaseout starts	$1,252,700
Single complete phaseout	$978,750
MFJ complete phaseout	$1,800,700
First AMT rate	26%
Second AMT rate	28%
26%/28% breakpoint	$239,100
MFS breakpoint	$119,550

Bottom line: AMT rarely changes what income you report — it changes which deductions and preferences you get to use. Running both the regular tax and AMT calculations each year is the only way to know which one applies to you.