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.
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.
Sources
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.