Clear Money Guide
What this guide covers
A quick view of the questions and evidence developed below.
Updated July 28, 2026. Quick answer: A conversion produces taxable income with nothing withheld, so it can create an underpayment penalty even if you pay in full at filing. Reaching a safe harbor — generally 100% of last year’s tax, or 110% above an income threshold — is usually easier than forecasting this year’s.
IRS worksheets read line by line: Form 2210 Schedule AI.
Related reading on this site: 2026 Estimated Tax Worksheet.
Related reading on this site: Estimated tax due dates 2027.
Related reading on this site: Roth conversion deadline 2026.
More IRS forms and schedules, read line by line: Form 2210 instructions.
Owing and being penalised are different
The penalty is not about the balance due at filing; it is about whether you paid enough, early enough, across the year. A large conversion in March with no payment until April of the following year can attract a penalty even if the cheque eventually clears in full.
The prior-year safe harbor is the practical one
Reaching 100% of last year’s total tax — 110% above an income threshold — generally avoids the penalty regardless of how much this year’s conversion adds. You already know last year’s number, which is why this test is easier to guarantee than forecasting a year containing a large discretionary conversion.
Withholding beats estimates on timing. Estimated payments are credited when made, so a Q4 catch-up does not cure an earlier quarter’s shortfall. Withholding is generally treated as paid evenly across the year — which means increasing withholding late in the year can fix a gap that a Q4 estimate cannot.
The timing lever
Converting late in the year leaves less of the year for the payment obligation to have accrued. It is a modest effect and it should not drive the conversion decision, but it is free if the timing is otherwise indifferent.
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IRC §408A (Roth IRAs); IRC §408A(d)(3) (conversions); IRC §1411 (net investment income tax); IRC §86 (taxation of Social Security benefits); IRC §6654 (estimated tax); Tax Cuts and Jobs Act (2017) §13611 (repeal of conversion recharacterisation). Cross-checked July 2026 against professional analyses. Indexed thresholds are described rather than asserted, because they change annually.
This states what the cited authority says. It is not tax advice, and a conversion interacts with the rest of your return in ways one page cannot see.
Related
Nobody withholds for you in retirement. Work out your safe-harbour number — 90% of this year or 100% of last year, whichever is lower — and if the year is already off track, withholding from a December RMD counts as paid evenly across all four quarters, which an estimated payment does not.
Reaching a safe harbor is easier when you know the size of the bill you are covering. The Roth conversion bracket calculator takes your income before any conversion and returns the federal tax on filling your bracket — which is the amount the conversion adds, and the amount that arrives with nothing withheld against it.