Roth Conversions and Capital Gains: The Hidden Interaction

Most pre-retirees know that a Roth conversion is taxed as ordinary income. What many miss is that the conversion also affects the tax rate on long-term capital gains they've already realized — or plan to. Because of how federal income tax is calculated, a large enough conversion can shift tens of thousands of dollars in LTCG from the 0% bracket straight into the 15% bracket. Understanding this interaction can save $5,000–$15,000 in a single year.

How the tax stack works

The U.S. federal income tax is layered: ordinary income (wages, IRA distributions, Roth conversion amounts, Social Security, interest) fills the bottom of the stack. Long-term capital gains and qualified dividends sit on top.

That means the long-term capital gains rate that applies to a given dollar of gain depends on where that dollar falls in the combined income stack — not on the gain itself in isolation. Specifically:

• Long-term capital gains taxed at 0% — the portion of your stack that falls below the 0% threshold
• Long-term capital gains taxed at 15% — the portion that falls between the 0% and 20% thresholds
• Long-term capital gains taxed at 20% — the portion above the 20% threshold (applies to very high incomes)

When you add Roth conversion income, it raises the floor under your capital gains — pushing gains higher in the stack and into higher-rate territory.

2026 LTCG thresholds

Source: IRS Rev. Proc. 2025-32. Thresholds apply to taxable income (gross income minus deductions), not gross income.

For reference, the 2026 standard deduction for MFJ is $32,200, plus $1,650 for each spouse age 65 or older — so a couple where both are 65+ uses a $35,500 standard deduction.¹ That deduction applies to ordinary income, not to LTCG directly, but it lowers the ordinary taxable income floor, which in turn gives LTCG more room in the 0% zone.

The stacking math: a worked example

Mark and Patrice are both 68, recently retired, and have no pension. They receive $52,000/year in combined Social Security — at their income level, 85% of SS is taxable, so $44,200 flows into gross income. Their only other income is $8,000 in qualified dividends from their taxable portfolio.

This year they also want to rebalance a long-held stock position, generating $70,000 in long-term capital gains. And they're considering a $90,000 Roth conversion from their $1.8M traditional IRA.

Scenario A: LTCG harvest only — no conversion

• Ordinary income: $44,200 (SS) + $8,000 (dividends) = $52,200
• Standard deduction: $35,500 (MFJ, both 65+)
• Ordinary taxable income: $52,200 − $35,500 = $16,700
• LTCG stack: sits from $16,700 up to $86,700
• 0% LTCG ceiling (MFJ 2026): $98,900 → $86,700 is well below
• All $70,000 LTCG: 0% tax
• Capital gains tax: $0

Scenario B: LTCG harvest + $90,000 Roth conversion

• Ordinary income: $44,200 + $8,000 + $90,000 (conversion) = $142,200
• Standard deduction: $35,500
• Ordinary taxable income: $142,200 − $35,500 = $106,700
• LTCG stack: now sits from $106,700 up to $176,700
• 0% LTCG ceiling: $98,900 → completely below the stack
• All $70,000 LTCG at 15%
• Capital gains tax: $70,000 × 15% = $10,500

Why this happens (and why it's easy to miss)

The conversion amount is taxed at ordinary income rates — that part most people understand. But because long-term capital gains sit on top of ordinary income in the tax calculation, raising ordinary income also raises the floor that LTCG starts from. Every dollar of conversion income that pushes ordinary taxable income past $98,900 (MFJ) also shifts a dollar of long-term capital gain from 0% to 15%.

In this example, the conversion of $90,000 pushed the ordinary taxable income from $16,700 to $106,700 — clearing the $98,900 LTCG ceiling entirely. The full $70,000 of capital gains shifted from 0% to 15%.

The true cost of the conversion is therefore not just the 12%/22% bracket rate on the conversion amount, but also the additional 15% on the gains that got bumped out of the 0% zone.

Three coordination strategies

1. Separate gains and conversions into different years

The cleanest solution: harvest capital gains in years when you're not converting, and convert in years when you're not harvesting gains. In the golden window (roughly ages 60–72 for most), you may have 10+ years to alternate between the two. Gain years and conversion years don't have to coincide.

Using Mark and Patrice's example: if they harvest the $70,000 in gains in Year 1 at $0 tax, then do the $90,000 conversion in Year 2, their total capital gains bill over the two years is still $0. The conversion tax is the same either way — approximately $8,700 in ordinary income tax on the $90,000 converted. No interaction penalty.

2. Size the conversion to preserve 0% LTCG room

If you know you're harvesting gains this year, limit conversions to the amount that keeps your ordinary taxable income below the 0% LTCG ceiling minus the gains you plan to realize.

In the Mark and Patrice example:

• 0% LTCG ceiling: $98,900 MFJ
• If they harvest $70,000 in gains, ordinary taxable income must stay ≤ $98,900 − $70,000 = $28,900
• Their ordinary taxable base (no conversion): $16,700
• Maximum conversion without disrupting 0% LTCG: $28,900 − $16,700 = $12,200

They could do a $12,200 partial conversion this year and convert the remaining $77,800 in a future year — preserving all $70,000 of gains at 0%.

3. Accelerate gain harvesting before conversions use up the window

Some pre-retirees prioritize a few years of heavy gain harvesting early in the golden window — while the 0% LTCG room is plentiful — before shifting to conversion-focused years. If your taxable portfolio has substantial embedded gains that will eventually be realized (rebalancing, estate planning, gifting), harvesting them in the lowest-income years first, before conversions crowd out the 0% space, can save significant tax over the full window.

The NIIT layer

There is a third interaction worth knowing: the Net Investment Income Tax (NIIT). Under IRC § 1411, a 3.8% surtax applies to net investment income (which includes long-term capital gains) when MAGI exceeds $250,000 for MFJ filers or $200,000 for single filers.² These thresholds are not indexed for inflation and have not changed since the tax was enacted in 2013.

Roth conversion amounts are included in MAGI even though they are not themselves "net investment income." So a large conversion can push MAGI over the NIIT threshold, causing investment income you already have to become subject to the 3.8% surtax.

For Mark and Patrice, their total MAGI in Scenario B is $44,200 + $8,000 + $90,000 + $70,000 = $212,200 — below the $250,000 threshold, so no NIIT. But a couple converting $180,000 instead of $90,000 would have MAGI of $302,200: $52,200 above the threshold. The NIIT would apply to the lesser of their NII ($78,000) or the excess ($52,200): $52,200 × 3.8% = $1,984 in additional tax.

At typical golden-window conversion amounts, the NIIT is a secondary concern — but it becomes material when aggressive conversions push MAGI well above $250,000.

The 0% gain-harvesting window is a once-in-a-lifetime opportunity

The golden window between retirement and RMDs is not only a Roth conversion opportunity — it is often the only extended period in a person's life when their ordinary income is low enough to make large capital gains realizations truly tax-free. During their working years, their salary filled the stack past the 0% LTCG ceiling. During RMD years (73+), mandatory distributions do the same. Only in the retirement window does ordinary income fall low enough to create 0% LTCG room.

If you have a taxable portfolio with embedded gains — appreciated stocks, mutual funds, real estate held through a 1031 exchange, or inherited assets with low basis — the conversion planning process should incorporate capital gain harvesting as a parallel objective, not an afterthought. The multi-year conversion schedule and the tax bracket calculator can both help model how much room exists in a given year for gains vs. conversions vs. both.

When the interaction matters most

• Taxable portfolio with embedded gains ($100K+). The more gains you need to realize over the golden window, the more valuable it is to coordinate the timing of harvests and conversions.
• Modest ordinary income base. Pre-retirees with little or no pension, no part-time work, and SS deferred have the most 0% LTCG room — and therefore the most to lose from unplanned conversions crowding it out.
• Rebalancing or estate planning triggering gains. A single large rebalancing event, a sale of an appreciated stock, or a property sale can create a one-time LTCG spike. If a large conversion is also planned that year, the interaction cost can be substantial.
• Years where MAGI is approaching $250,000 MFJ / $200,000 single. Adding both a conversion and a large gain realization in the same year can trigger NIIT on the gains.