A Roth conversion sounds like a transfer. It is really a tax event dressed up as one.
When you convert, you move money out of a traditional IRA or a pre-tax 401(k) and into a Roth account. Nothing leaves your control, no check gets written to you, and your net worth on paper barely moves. But the IRS treats every dollar you convert as ordinary income in the year you do it, the same as if your employer had handed it to you in a paycheck. You pay tax on it now so that it grows and comes out tax free later.
That is the whole idea. The complications live in the details, and there are more of them than most explanations let on.
Why anyone would volunteer to pay tax early
The case for converting is a bet about tax rates: that the rate you pay today is lower than the rate you or your heirs would pay when the money eventually comes out.
The 2026 brackets give that bet some shape. The 12% bracket runs up to $50,400 of taxable income for a single filer and $100,800 for a married couple filing jointly. Above that, 22% takes over until $105,700 and $211,400 respectively. Layer in the 2026 standard deduction of $16,100 for singles and $32,200 for joint filers, and a retired couple with modest income can have a surprising amount of unused room in the low brackets.
The classic candidate is someone in the gap years, retired but not yet claiming Social Security and not yet taking required minimum distributions. Their income dips. Their traditional IRA keeps growing. Converting a slice each year, sized to fill up the 12% or 22% bracket and stop, spreads the tax over several returns instead of getting hit with a large forced distribution later.
The other reason people convert has nothing to do with their own retirement. Under current rules, most non spouse beneficiaries have to empty an inherited IRA within ten years. If your kids will be in their peak earning years when that clock runs, the tax on an inherited traditional IRA can be brutal. Inherited Roth money still has to come out in ten years, but it comes out tax free.
The pro-rata rule, which surprises almost everyone
Here is the rule people learn about after the fact.
If you have made nondeductible contributions to a traditional IRA, you have basis, meaning money you already paid tax on. It would be reasonable to assume you could convert just that portion and owe nothing. You cannot.
The IRS aggregates every traditional, SEP, and SIMPLE IRA you own and treats them as a single pool. Any conversion is deemed to come out proportionally, part pre-tax and part after-tax, based on the ratio across all of them measured as of December 31 of the conversion year.
An example makes it concrete. Say you have $14,000 of nondeductible basis sitting in a small IRA and $126,000 of pre-tax money in a rollover IRA from an old job. That is $140,000 total, of which 10% is basis. Convert $14,000 and you do not get a tax free conversion. You get a conversion that is 10% basis and 90% taxable, so $12,600 lands on your return as income. The rest of your basis stays behind, pro-rated forever.
Two things soften this. Employer plan balances do not count in the calculation, so money still sitting in a 401(k) or 403(b) is invisible to the pro-rata math. That is why people who want a clean backdoor Roth sometimes roll their IRA balances into a current employer’s plan first, if the plan accepts incoming rollovers. And the December 31 measurement date means a conversion in March can still be spoiled by a rollover you do in November.
Every conversion involving basis gets reported on Form 8606, and that form is how the IRS tracks what you have already paid tax on. Losing track of it is a good way to pay tax twice on the same dollars.
Two five-year clocks, not one
The phrase “the five-year rule” is doing a lot of work in most articles, because there are actually two separate rules and they answer different questions.
The first clock is about the account. Earnings inside a Roth IRA come out tax free only if the distribution is qualified, which requires that you be 59 and a half or older (or meet a narrow exception) and that five tax years have passed since you first funded any Roth IRA. That clock starts once and never restarts. If you opened your first Roth in 2019, you cleared it in 2024, regardless of what you have done since.
The second clock is about each conversion. If you are under 59 and a half and you withdraw converted principal within five years of converting it, you owe a 10% penalty on it, even though you already paid income tax on that money going in. Each conversion carries its own clock. Charles Schwab has a readable walkthrough of how the two Roth five-year rules interact, which is worth reading before you rely on converted money for near term spending.
The clocks start on January 1 of the year of the conversion, not on the day you did it. A conversion executed on December 28, 2026 gets credited back to January 1, 2026. That quirk is one of the few places the tax code hands you free time, and it is a reason year end conversions are common.
You cannot take it back
Before 2018, a conversion could be undone. If the market dropped after you converted and you were suddenly paying tax on a value that no longer existed, you could recharacterize and pretend it never happened.
That door closed. Conversions made after December 31, 2017 are irrevocable. The IRS confirms this in its own guidance on Roth IRA conversions and recharacterizations. You convert, the income is on your return, and the only way out is a time machine.
That permanence changes how you should size a conversion. Converting in one large chunk and hoping the numbers work is a different risk than converting a measured amount you have already run through a tax projection.
The costs that do not show up on the conversion form
The income tax is the obvious cost. Several others hide behind it.
Medicare uses a two year lookback for its income related surcharge, so your 2026 income determines what you pay for Part B and Part D in 2028. For reference, the 2026 surcharge starts at $109,000 of modified AGI for single filers and $218,000 for joint filers, on top of a standard Part B premium of $202.90 a month. A conversion that pushes you one dollar over a threshold moves you into the next tier for a full year. There is no partial credit.
If you buy coverage through the marketplace, conversion income can shrink or eliminate a premium tax credit. If you are already collecting Social Security, added income can increase the share of those benefits that is taxable. And because long term capital gains stack on top of ordinary income, a conversion can push gains that would have been taxed at 0% into the 15% band.
Then there is the question of where the tax payment comes from. Paying the bill out of the converted money itself shrinks the amount that ends up in the Roth and, if you are under 59 and a half, the withheld portion is treated as a distribution subject to the 10% penalty. A conversion generally only makes sense if you can pay the tax from money outside the retirement account.
The date that is not April 15
Roth contributions can be made up until the tax filing deadline for the prior year. Conversions cannot. A conversion counts for the calendar year in which it is actually completed, so anything meant to land on your 2026 return has to be done by December 31, 2026.
Custodians get busy in late December and processing is not always same day, so the practical deadline is earlier than the legal one. Mid December is a safer target.
None of this makes conversions a good idea by default. They tend to help people with low income years, a long runway before withdrawals, cash on hand to cover the tax, and a reasonable belief that rates will not be lower later. They tend to hurt people who are already in a high bracket, who would have to raid the account to pay the tax, or who are close enough to a Medicare or subsidy threshold that the second order costs swamp the benefit.
Run the numbers before December, not after.