Roth Conversion + Capital Gains Calculator (2026)

A Roth conversion raises the ordinary income floor under your long-term capital gains. If it pushes enough of your stack above the 0% LTCG ceiling, gains that would have been tax-free suddenly cost you 15%. This calculator shows the exact interaction — and the maximum conversion you can do before that shift happens.

How LTCG stacking works

The IRS calculates your tax in two layers. First, ordinary income (wages, IRA withdrawals, Roth conversions, pension, most interest) fills the bottom of the stack. Long-term capital gains and qualified dividends sit on top. Each dollar of capital gain is taxed at the rate that applies to wherever it falls in the combined stack.

The 0% LTCG rate applies to gains that fall below the threshold — $98,900 of taxable income for MFJ filers, $49,450 for single filers in 2026. Once the ordinary income floor rises above that ceiling (because of a Roth conversion), the capital gains sitting on top are now in the 15% zone.

2026 long-term capital gains brackets

Source: IRS Rev. Proc. 2025-32; Kiplinger 2026 capital gains tax brackets. Thresholds apply to taxable income (gross income minus deductions), not gross income.

The 2026 standard deduction

The standard deduction reduces ordinary income before the LTCG stack is calculated, which gives capital gains more room in the 0% zone. For 2026:

• MFJ: $32,200 base, plus $1,650 for each spouse age 65 or older
• Single: $16,100 base, plus $1,650 if age 65 or older

A couple where both spouses are 65+ uses a $35,500 standard deduction — giving $35,500 more space before LTCG starts rising in the stack.¹

Three ways to coordinate conversions and capital gains in the same year

1. Convert before you harvest gains

Run this calculator before realizing any capital gains. If your planned conversion would push LTCG into the 15% bracket, you can decide in advance: reduce the conversion, delay the harvest, or plan the harvest for a different year when you're doing fewer conversions.

2. Separate the years

For a retiree in the golden window (roughly 60–73), there's often flexibility on timing. Convert heavily in years when capital gains are small or zero. Harvest gains in years when conversions are smaller. The two-year IRMAA lookback means you already need a multi-year conversion calendar — folding in capital gains timing is a natural extension.

3. Intentionally cross the 0% threshold — with eyes open

Sometimes it's worth it. If you have a large unrealized gain you want to take now (selling a business, settling an estate, rebalancing a concentrated position), the 15% rate on capital gains may be worth paying alongside the conversion. The question is whether the lifetime Roth conversion savings justify the one-year capital gains cost. This calculator shows you the interaction cost; your advisor models whether the lifetime math works.