It is March, you are doing your taxes, and you realize last year would have been a perfect year for a Roth conversion: low income, bracket headroom to spare. One problem. The answer to "can you do a Roth conversion for the previous tax year" is no. Contributions get the famous April 15 grace period. Conversions do not. A conversion you complete in March 2027 is 2027 income, even if your heart was set on 2026.
This confusion is completely reasonable, because the two actions look alike. You move money into a Roth IRA in the spring, and your custodian lets you tag a contribution as belonging to the prior year, right up until the April filing deadline. A conversion gets no such tag. Your custodian reports it on Form 1099-R for the year the money actually moved, and you report it on Form 8606 with that year's return. The IRS offers no extension for conversions, and a tax return extension does not extend the conversion deadline either.
Why the deadline is December 31, and why it bites
The rule makes the last weeks of December the most dangerous time in Roth planning. The conversion has to be complete by December 31, meaning the funds have actually moved, not that you started the paperwork. Custodians are overloaded at year-end, so a conversion initiated in the final week can easily settle in January, pushing it into the next tax year and blowing up a carefully sized bracket plan.
Worse, December is when your final income picture is murkiest. Mutual fund capital gain distributions, year-end bonuses, and property sales all land late, and any of them can push your income higher than projected. Because conversions have been irrevocable since the 2017 tax law ended recharacterization, there is no undo button. Convert too much, cross an IRMAA tier, and the Medicare surcharge shows up two years later with no recourse.
My practical read: convert in stages. A partial conversion earlier in the year, sized conservatively below whatever threshold you are targeting, followed by a smaller top-up in early December once the income picture is clear. Pre-stage the paperwork in November so the December execution is a button press, not a project.
What you can still do in the spring
The spring is not wasted. You can still make a prior-year IRA contribution by April 15 and convert it later, which is the standard two-step of the backdoor Roth. The contribution carries the prior-year tag; the conversion counts in whatever year you execute it. Just plan both steps together, because if you have other pre-tax IRA balances, the pro-rata rule decides how much of that conversion is taxable.
So mark it once and remember it: contributions get April 15, conversions get December 31. The calendar does not negotiate.
Frequently asked questions
Can you do a Roth conversion for the previous tax year?
No. Conversions are taxable in the calendar year the money moves. A March 2027 conversion is 2027 income, period. Only contributions get the April look-back.
Why can contributions be made for the prior year but conversions can't?
Congress gave contributions a grace period because the year's full income details arrive late. Conversions are reported on Form 1099-R for the year they occur, and the IRS provides no extension.
What is the Roth conversion deadline each year?
December 31, with the funds actually moved, not just initiated. Custodian backlogs mean a late-December initiation can settle in January and land in the wrong year.
Can you undo a Roth conversion if you convert too much?
Not since 2018. The Tax Cuts and Jobs Act ended recharacterization of conversions. Size conservatively and convert in stages instead.
Does this affect the backdoor Roth strategy?
Yes. It splits into two deadlines: the nondeductible contribution can go to the prior year by April 15, but the conversion counts in the year it is executed.
Sources: IRS Instructions for Form 8606; IRS Publication 590-B; Tax Cuts and Jobs Act recharacterization rules. Verified October 2026.