Backdoor Roth IRA Limits: The Pro-Rata Rule Most People Miss

The backdoor Roth IRA limit is the ordinary IRA limit: $7,500 for 2026, or $8,600 if you turn 50 or older during the year. The backdoor is a route, not a bigger container. It gets money past the Roth income cap without raising how much money is allowed through.

That surprises people who go looking for backdoor Roth IRA limits and expect a separate, higher number. There isn't one. And there is a second rule, buried in the tax code at section 408(d)(2), that decides whether the route costs you nothing or costs you a four-figure tax bill on money you already paid tax on once.

What the backdoor actually is

Two ordinary transactions, done back to back.

First, a nondeductible contribution to a traditional IRA. Nondeductible means you claim no deduction for it, so the dollars going in are after-tax dollars. That creates basis, which is the tax term for money in a retirement account that has already been taxed and will not be taxed again on the way out.

Second, a conversion of that traditional IRA to a Roth IRA. A conversion is a taxable event in principle. You pay income tax on whatever pre-tax money moves across.

The trick lives in an asymmetry the IRS states plainly: "Regardless of the amount of your adjusted gross income, you may be able to convert amounts from a traditional IRA into a Roth IRA." Conversions have no income limit. Direct Roth contributions do.

The income wall that makes the detour necessary

For 2026 the Roth IRA contribution phase-out runs on modified adjusted gross income:

Above the top of the range, a direct Roth contribution is zero. Meanwhile the traditional IRA deduction phases out much earlier for anyone covered by a workplace plan: $81,000 to $91,000 for a single filer in 2026. So a high earner with a 401(k) sits in a specific spot. Too much income to fund a Roth directly, too much income to deduct a traditional contribution, but still allowed to make the contribution as a nondeductible one under section 408(o). The conversion door is open at any income. That gap is the entire mechanism.

The limit that binds is still the contribution limit

Nothing about the backdoor increases the $7,500. The ceiling attaches to the person rather than to the account, and it covers traditional and Roth IRAs together, so opening a second or third Roth changes nothing about the number. That is the same reason holding several Roth IRAs at once does not multiply what you can put in them.

A married couple filing jointly gets two limits because each spouse has their own, and the pro-rata calculation below is also run separately for each spouse. One partner's legacy IRA balance does not contaminate the other's conversion.

The conversion itself has no annual cap. You could convert $400,000 of an old rollover IRA in one year if you were willing to pay the tax. What is capped is the $7,500 of new after-tax money you can feed into the pipe each year.

The pro-rata rule: every IRA you own gets a vote

Here is the part that catches people.

Section 408(d)(2) says that for purposes of figuring the taxable portion of a distribution, "all individual retirement plans shall be treated as 1 contract," all distributions during the year are treated as one distribution, and the values are computed as of the close of the calendar year.

Read that again with a conversion in mind. You cannot point at the $7,500 of clean after-tax money and convert only that. The IRS looks through your account labels and sees one pool made of every traditional, SEP and SIMPLE IRA you own. The conversion takes a proportional bite of that pool, whatever account it physically came from.

The nontaxable fraction is your basis divided by the whole pool:

Nontaxable share = total basis ÷ (year-end value of all traditional, SEP and SIMPLE IRAs + amounts converted or distributed during the year)

Roth IRAs are not in the denominator. Neither are 401(k), 403(b) or 457(b) balances. Inherited IRAs you hold as a beneficiary are computed separately. What is in the denominator, and what does the damage, is usually a rollover IRA created when someone moved an old 401(k) out of a former employer's plan.

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The curve is a hyperbola, so almost all of the damage happens in the first $50,000. At $200,000 of pre-tax IRA money, $271 of the conversion escapes tax. The taxable share is 96.4%.

The worked example

Take someone with $60,000 in a rollover IRA from a job they left in 2019, all of it pre-tax. In 2026 they contribute $7,500 nondeductible to a traditional IRA and immediately convert $7,500 to a Roth.

They wrote a check for $7,500 of already-taxed money and got a $1,600 tax bill for the privilege of moving it.

The $6,667 of unused basis is not destroyed. It stays with them as basis in the traditional IRA and reduces tax on some future distribution, possibly decades away. But the promise of the backdoor was a free move, and this is not one.

Form 8606 is where the basis lives

The IRS does not track your basis for you. Form 8606, Nondeductible IRAs, is the only place it exists.

Part I of the form runs the pro-rata calculation directly. Line 1 is the year's nondeductible contribution, line 2 is basis carried in from the prior year's line 14, line 6 is the year-end value of all traditional, SEP and SIMPLE IRAs, and line 8 is the amount converted. Line 6 is where the next section does its work, because what sits in those accounts on December 31 is what sets the ratio.

Filing is required in the year of the nondeductible contribution and again in the year of the conversion. The instructions attach a $50 penalty for failing to file when required, absent reasonable cause, and $100 for overstating nondeductible contributions.

The real cost of a missing form is larger than $50. Basis you cannot document is basis you effectively lose, and the same money gets taxed twice at withdrawal. The chain of line-14 figures across years is the record.

Emptying the denominator

Because line 6 excludes amounts rolled into an employer plan, moving pre-tax IRA money into a 401(k), 403(b) or a governmental 457(b), the only version of that plan that can accept it, takes it out of the calculation entirely. This is the standard way the obstacle gets cleared.

The direction matters. Section 408(d)(3)(A)(ii) allows a rollover from an IRA into an eligible retirement plan but caps it at "the portion of the amount received which is includible in gross income." Only pre-tax dollars can go. After-tax basis is not allowed into the plan, which is convenient here: the basis stays behind in the IRA, exactly where it needs to be.

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Three conditions have to hold for this to be available at all.

• The employer plan has to accept incoming rollovers from IRAs. Many do, some do not, and the plan document is what settles it.
• SEP and SIMPLE IRAs sit in the denominator too, so a self-employed side business creates the same problem when it saves through a SEP IRA rather than a solo 401(k). SIMPLE IRAs also carry a waiting period after the first contribution before the balance can be rolled elsewhere.
• The move has to be finished by December 31 of the conversion year.

There is a trade-off on the other side of it. Money inside a 401(k) is restricted to the plan's investment menu, and a plan lineup can carry expense ratios well above what the same exposure costs in an IRA. It also loses the flexibility of an IRA's open architecture. What it gains is the creditor protection that attaches to ERISA plan assets and, for someone still working at the age when distributions would otherwise start, the ability to defer required minimum distributions on that balance. Most people running this play weigh the annual tax on the conversion against those differences rather than treating the rollover as automatic.

The mega backdoor is a different machine

The names are similar and the mechanics are not. The mega backdoor happens inside a 401(k), uses a different set of limits, and is unaffected by anything sitting in your IRAs.

The $72,000 figure is the section 415(c) annual additions limit for 2026, and it covers everything credited to the account in a year: your $24,500 of elective deferrals, the employer match, profit sharing, and after-tax contributions. The after-tax headroom is whatever is left over, which is why the mega backdoor is large for people with small matches and modest for people with generous ones.

One shared piece of plumbing is worth naming. When after-tax money leaves a 401(k), the plan's own pro-rata rule applies to that distribution: a withdrawal carries a proportional share of pre-tax and after-tax amounts. The 2014 IRS guidance is what lets a single distribution be split, sending the pre-tax portion to a traditional IRA and the after-tax portion to a Roth IRA. Without that split rule, earnings on the after-tax contributions would drag taxable dollars into the Roth conversion.

What actually limits you

Three numbers, in order of how often they bind.

The $7,500 is the hard ceiling on new money through the IRA door, and no amount of account-opening changes it. The pro-rata fraction is what determines the price of walking through, and it is set by balances you may have created years ago for unrelated reasons. Form 8606 is what makes the basis real to the IRS rather than real only to you.

The uncomfortable implication is that the backdoor is cheapest for people who have never used a traditional IRA and most expensive for people who diligently rolled every old 401(k) into one. That is not a rule anybody designed. It falls out of treating every IRA as a single contract.