AMT and Long-Term Capital Gains: Do Preferential Rates Still Apply?

Long-term capital gains retain their preferential tax rates even under alternative minimum tax (AMT), but gains that exceed the AMT exemption trigger a phaseout that substantially raises effective tax liability on those gains. The interplay between preferential rates and exemption erosion confuses many investors, yet understanding it is essential to forecasting your actual AMT bill.

How preferential rates survive AMT

The IRS does not force you to abandon the 0%, 15%, or 20% brackets for long-term capital gains when computing AMT. Instead, the AMT system preserves those rates within the AMT calculation. This is not a favor; it is simply how the code is drafted.

When you report long-term capital gains on your AMT calculation, you enter them at the preferential rate just as you would under the regular tax. If you owe AMT, you pay whichever is higher: regular tax or AMT, computed using those same preferential rates.

The catch is not the rate itself—it is the exemption.

The exemption phaseout trap

The real leverage of AMT on capital gains comes through the exemption phase-out. The AMT exemption is a fixed dollar amount:

• Single filers: approximately $85,950 (2024)
• Married filing jointly: approximately $133,200 (2024)

This exemption applies to all AMT income, including long-term capital gains.

Once your total AMT income exceeds the exemption, the exemption begins to phase out at a rate of 25 cents per dollar. This means every $4 of additional income erodes the exemption by $1.

Example: A single filer with an exemption of $85,950 realizes $100,000 in long-term capital gains (the only income item). AMT income is $100,000. The exemption phases out by 0.25 × ($100,000 − $85,950) = $3,512.50. The remaining exemption is $82,437.50. The first $82,437.50 is taxed at 0%. The remaining $17,562.50 is taxed at 15%. That is 15% × $17,562.50 = $2,634 in AMT tax on the gains—an effective rate of 2.63% on the $100,000, not the 0–15% advertised bracket.

Why the effective rate climbs

The phaseout mechanism introduces a hidden marginal rate. As you add AMT income, you lose exemption at 25 cents per dollar. For a taxpayer in the 15% long-term gain bracket, the marginal cost of that additional income is 15% (the stated rate) plus the value of the lost exemption.

If you are in the 15% preferential bracket and the exemption is phasing out, the true marginal cost is:

• 15% on the gain itself
• Plus an implicit tax on the lost exemption

This effective rate can easily exceed 20%, approaching 40% in some configurations, because you are simultaneously paying the preferential rate and accelerating into a higher AMT band.

Stacking gains and the exemption collapse

Realize $200,000 in long-term gains, and the exemption collapses entirely if you are a single filer:

$200,000 − $85,950 = $114,050 over the threshold × 0.25 = $28,512.50 eroded away. But the exemption only has $85,950 to lose, so the full exemption is consumed and then some.

Once the exemption is gone, every dollar of additional gain is taxed at the preferential rate with no shelter. That is still 15% or 20%, not 26% or 28%, but the exemption has already cost you the bulk of your planning room.

Interaction with other income

Your ordinary income, dividends, and gains all count toward AMT income and all participate in the exemption phaseout. A large long-term gain realized in a year when you also have substantial wages or business income triggers a faster phaseout and can push you into a higher bracket on the gain than you might anticipate.

Conversely, a year in which your ordinary income is low might allow room for gains to be taken at a lower effective rate before the exemption fully phases out.

Calculating and planning

To estimate your AMT exposure, compute your tentative minimum tax (TMT) first:

1. Take your regular taxable income and add back preference items and tax adjustments.
2. Subtract the exemption (subject to phaseout for your income level).
3. Multiply by the 26% or 28% marginal rate.
4. Compare this to your regular tax.

If AMT is higher, you owe the difference. The phaseout of the exemption directly raises the base on which the 26% or 28% rate applies, inflating your AMT bill.

Investors with large capital gains often find that realizing gains across two years, or timing ordinary income down in a high-gain year, smooths the exemption phaseout and reduces total AMT exposure. This requires forecasting, but the numbers often justify the effort.

See also