A Roth conversion is taxable as ordinary income in the year of conversion (per IRC §408A(d)(3)). The federal tax bill on the converted amount is determined by your marginal tax bracket, plus any state tax, IRMAA Medicare surcharges, ACA premium tax credit clawback, and Net Investment Income Tax effects. Pay the tax from money OUTSIDE the IRA — using the converted balance to pay tax destroys most of the conversion's economic benefit. The three mechanisms: custodian withholding at the time of conversion, quarterly estimated tax payments, or increased W-2 withholding from your job.

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Quick Facts

  • warningPay tax from outside funds, not from the conversion. Withholding from the conversion creates a deemed distribution of the withheld amount and triggers the 10% penalty if you're under 59½.
  • infoThree payment options: custodian withholding, quarterly estimated tax (Form 1040-ES), or increased W-2 withholding (Form W-4 update).
  • infoDefault custodian withholding is 10% on IRA distributions per IRC §3405(b). Often less than your actual liability — the rest comes due at filing.
  • check_circleSafe harbor avoids underpayment penalty. Per IRC §6654, no penalty if withholding+estimated tax is at least 100% of last year's tax (110% if AGI > $150K).
  • warningState tax also applies in most states. California, Oregon, Hawaii, etc. tax conversions as ordinary income. Some states (FL, TX, WA, etc.) have no income tax.

Why You Should Pay From Outside Funds

The economic case for a Roth conversion depends on shifting dollars from a tax-deferred account to a tax-free one. If you use part of the converted amount to pay the tax, two things happen: (1) only the after-tax balance ends up in the Roth IRA, and (2) if you're under 59½, the IRS treats the withheld amount as a regular distribution from the traditional IRA — which gets a 10% early-withdrawal penalty under IRC §72(t).

Worked example: $100,000 conversion at a 22% federal bracket (assuming no state tax). If you withhold $22,000 from the conversion to pay the tax, only $78,000 reaches the Roth — and $22,000 is treated as a normal early distribution, adding $2,200 to the penalty if you're under 59½. Total cost: $24,200 lost from the retirement system.

If instead you pay the $22,000 tax bill from a taxable brokerage account, all $100,000 reaches the Roth, and there's no §72(t) penalty exposure. That's a $24,200 swing on a single transaction.

Three Payment Mechanisms Compared

1. Custodian withholding. When you initiate the conversion, you elect the withholding percentage. Default is 10% federal (state withholding varies). Pros: simple, automatic. Cons: only useful if you have outside funds to "replace" the withheld amount in the Roth before the 60-day rollover window — otherwise the withheld portion fails to reach the Roth.

2. Quarterly estimated tax (Form 1040-ES). Pay the conversion's tax liability directly to the IRS in the quarter the conversion occurred. Pros: clean, doesn't reduce the converted balance. Cons: you have to remember the quarterly deadlines (April 15, June 15, September 15, January 15) and calculate the right amount. Underpayment in the right quarter triggers a small underpayment penalty per IRC §6654.

3. Increased W-2 withholding. If you have a regular paycheck, file an updated Form W-4 with your employer to bump withholding for the rest of the year. Pros: automated, no quarterly deadlines, IRS treats W-2 withholding as paid evenly throughout the year (avoids quarter-specific underpayment exposure). Cons: requires knowing the conversion is coming early enough to adjust.

The Safe-Harbor Rule for Avoiding Underpayment Penalty

IRC §6654 imposes a small underpayment penalty if you don't pre-pay enough tax during the year. The penalty is calculated at the IRS short-term rate plus 3% (currently around 8%) on the underpaid amount for the period it was underpaid.

You avoid the penalty entirely if your total federal withholding plus estimated tax payments equals or exceeds whichever is smaller:

  • 90% of the current year's actual tax, OR
  • 100% of last year's total tax (110% if last year's AGI was over $150,000 single / $75,000 MFS)

The 100%-of-last-year approach is the simplest safe harbor. If your tax bill last year was $20,000, ensure your 2026 withholding + estimated payments hit $20,000 before December 31. Any additional tax owed in April carries no penalty.

For high-income readers (over $150K AGI), the 110% rule means if last year's tax was $20,000, you need $22,000 prepaid this year to qualify for the safe harbor.