What Is Bracket-Filling?
Instead of converting a large lump sum (which pushes you into higher brackets), bracket-filling means converting just enough each year to fill your current tax bracket — or the next bracket up — before the rate jumps.
For example, if you're MFJ with $190,000 in taxable income, you're in the 24% bracket (which ends at $394,600 for 2025). You have $204,600 of room before hitting 32%. Converting $204,600 from traditional to Roth costs 24 cents on every dollar. Converting $250,000 means the last $45,400 costs 32 cents.
The Math: When Bracket-Filling Beats Lump-Sum
2025 Federal Tax Brackets (MFJ)
| Bracket | Taxable Income Range |
|---|---|
| 10% | $0 – $23,850 |
| 12% | $23,851 – $96,950 |
| 22% | $96,951 – $206,700 |
| 24% | $206,701 – $394,600 |
| 32% | $394,601 – $501,050 |
| 35% | $501,051 – $751,600 |
| 37% | Over $751,600 |
Key insight: The jump from 24% to 32% is 8 percentage points — the largest single bracket jump in the code. This is the most common bracket-filling target.
Watch for NIIT
Roth conversions increase MAGI. If the conversion pushes MAGI above $250,000 (MFJ) or $200,000 (single), you may trigger the 3.8% Net Investment Income Tax (NIIT) under IRC §1411 on your investment income.
The conversion amount itself isn't subject to NIIT, but the higher MAGI can cause existing investment income to become subject to it. Factor this into your bracket-filling calculation.
Multi-Year Spreading
If you have a large traditional IRA/401(k) balance, spreading conversions over multiple years keeps each year's conversion within lower brackets.
Example: $500,000 traditional IRA, currently in the 22% bracket with $80,000 of room before 24%.
- Lump sum: $500K conversion → taxed across 22%, 24%, 32%, and 35% brackets. Blended rate ≈ 28%.
- Spread over 5 years at $100K/year: Each year stays in 22-24% bracket. Blended rate ≈ 23%.
- Tax savings from spreading: ~$25,000 over 5 years
Frequently Asked Questions
What is Roth conversion bracket-filling?
Bracket-filling is a Roth conversion strategy where you convert just enough traditional IRA/401(k) each year to 'fill' your current tax bracket before the rate increases. This minimizes the blended tax rate on conversions compared to a large lump-sum conversion.
How much room do I have in my current tax bracket for a Roth conversion?
Subtract your current taxable income from the top of your bracket. For example, MFJ with $200,000 taxable income is in the 24% bracket (ends at $394,600). Room = $394,600 - $200,000 = $194,600 that can be converted at 24%.
Does a Roth conversion trigger NIIT?
The conversion itself isn't subject to the 3.8% NIIT (IRC §1411), but it increases your MAGI. If your MAGI exceeds $250,000 (MFJ) or $200,000 (single), existing investment income may become subject to NIIT. Include this in your conversion cost calculation.
Should I convert my entire IRA to Roth at once?
Usually not. A large lump-sum conversion pushes income through multiple brackets, increasing the blended tax rate. Spreading conversions over 3-7 years to stay within lower brackets typically saves 3-8% in total tax cost. The optimal strategy depends on your age, expected future tax rates, and current bracket room.