The Pro-Rata Rule: What an Old 401(k) Rollover Does to a Backdoor Roth

Rolling an old 401(k) into a traditional IRA drops that balance into the pro-rata pool the IRS measures on December 31 of the conversion year, not the day you converted. A November rollover can make a backdoor Roth you finished in March mostly taxable, on a 2026 contribution capped at $7,500.

Two decisions, not one

Two things are tangled here, and they are worth pulling apart before anything moves.

First, what happens to the old 401(k). That one is about cost, investment lineup, and whether anyone is actually watching the account. Second, whether a backdoor Roth still works for you in the year the rollover happens. That one is about a single line on a single tax form.

Most write-ups on old 401(k)s answer the first question and never mention the second.

The line that does the damage

Form 8606 is where a nondeductible IRA contribution and a Roth conversion meet. Line 6 asks for a balance, and the date attached to that balance is what catches people. From the 2025 Instructions for Form 8606, Nondeductible IRAs, line 6:

“Enter the total value of all your traditional IRAs as of December 31, 2025, plus any outstanding rollovers.”

That is the tax year 2025 revision, the current published one as of September 2026. The tax year 2026 version will not exist until early 2027, and the date printed on the line moves with the tax year.

December 31. Not the conversion date, not the day the money left the plan. The statute underneath it, IRC §408(d)(2), tells you to treat all of your individual retirement plans as one contract, to treat all distributions in a year as one distribution, and to compute the value of that contract as of the close of the calendar year. The pool is measured once, at the end, and anything you put into it before then is in the measurement.

Keep in mind what the fraction does. Your after-tax basis goes over the December 31 balance plus what you converted or distributed during the year. That ratio, not the order of the transactions, decides how much of the conversion comes out tax-free.

A hypothetical, so the arithmetic is visible

Numbers below are assumed for illustration only, not a client and not a projection. Contribute $7,500 nondeductible in March, convert it to Roth the same week, then in November roll an old 401(k) worth $200,000 (a round number picked because it makes the fraction easy to read) into a traditional IRA. On December 31 the pool is $200,000. Basis of $7,500 over $207,500 is about 3.6%, so roughly $271 of the conversion is tax-free and roughly $7,229 is taxable income, on a conversion you had already treated as done in March.

Reverse the order and nothing about the math changes, because the math never looked at the order. It looked at December 31.

What the form actually counts

Employer plan balances are not in the pool. There is no single IRS sentence that says so. It follows from what the statute and the form count: §408(d)(2)(A) aggregates individual retirement plans only, and Form 8606 line 6 asks for the total value of your traditional IRAs. A 401(k) sitting at a former employer is neither.

Sources for the table: IRC §408(d)(2), U.S. Code. 2025 Instructions for Form 8606, line 6 and Definitions, IRS. Publication 590-B (2025), IRS, under “What if You Inherit an IRA?” Dollar figures are tax year 2026, from IRS news release IR-2025-111, November 13, 2025.

Which direction the money moves

The fork is not whether to deal with the old 401(k). It is which way the balance travels.

First, into the pool. Rolling the old 401(k) into a traditional IRA adds that balance to the December 31 measurement. That is not automatically the wrong call, and there are real reasons people do it, which is exactly what the old 401(k) and self-managed IRA review walks through. It is a cost, though, and it belongs in the decision rather than after it.

Second, out of the pool. Moving an existing IRA balance into a current employer’s plan, often called a reverse rollover, takes it out of the line 6 total. The IRS Rollover Chart shows the transaction type is permitted: traditional IRA to a qualified plan such as a 401(k), yes; to a 403(b), yes; to a governmental 457(b), yes, with a note that separate accounts must be maintained.

Third, whether the receiving plan will take it. The chart says the transaction is allowed. It does not say your plan allows it. The IRS FAQ page on retirement plans and IRAs, last reviewed November 16, 2025, states that rolling an IRA into a qualified retirement plan assumes the plan has language allowing it to accept that type of rollover. That FAQ carries its own notice that it may not be relied upon as legal authority, so the plan document and the plan administrator are the thing to check, not the FAQ.

Fourth, basis generally stays behind. Only pre-tax dollars go into an employer plan, which leaves nondeductible basis sitting in the IRA. Worth confirming that split with your CPA, since it changes what is left in the pool.

The consequence runs in both directions. If a reverse rollover completes and is credited before December 31, the pool is smaller on the measuring date. If it lands January 2, the December 31 snapshot still has the money in it, and a conversion you already did gets priced off the larger number anyway.

One more definition that trips people. Line 6 also asks for outstanding rollovers, which per those same instructions means a distribution taken after November 1 and rolled over the following year inside the 60-day window. Those still count in the year-end total even though the money is in transit. If a check is involved rather than a direct trustee-to-trustee transfer, the 60-day window and the 20% withholding are covered on the 401(a) rollover options page.

The 2026 numbers that put you here

Per IRS news release IR-2025-111, issued November 13, 2025, the 2026 Roth IRA income phase-out runs $153,000 to $168,000 for single and head of household filers and $242,000 to $252,000 for married filing jointly. Married filing separately runs $0 to $10,000 and is not adjusted for inflation. Above the top of the range a direct Roth IRA contribution is not available, which is the entire reason a backdoor Roth exists as a topic.

From the same release, for tax year 2026: the IRA contribution limit is $7,500, the age-50 catch-up is $1,100 for a total of $8,600, and the 401(k) elective deferral limit is $24,500. The IRA catch-up moved off $1,000 for the first time under SECURE 2.0 indexing. A lot of 2026 content still prints $1,000, so any figure worth acting on is worth checking against the release itself.

If the plan at your current employer accepts after-tax contributions and in-plan conversions, there may be more Roth capacity available inside the plan than through an IRA at all. That is a separate calculation, and the mega backdoor Roth walk-through covers the mechanics, though it was written against the 2025 limits rather than the 2026 figures above.

What to confirm before December

• Confirm with your CPA the total value of every traditional, SEP and SIMPLE IRA in your name, since that total is what line 6 asks for.
• Evaluate with your CPA or advisor whether a conversion has already happened this year, because the pool is measured at year end regardless of when the conversion occurred.
• Confirm with your plan administrator whether the current employer plan accepts incoming IRA rollovers, and get that in writing from the plan document rather than a phone call.
• Confirm the processing timeline, since December 31 is a measurement date and not a postmark.
• Evaluate the old 401(k) on its own terms too, starting with what it costs you. A 401(k) fee review is the part most people skip, and it is answerable from documents you already have.

Where a review fits

The old 401(k) question and the backdoor Roth question get answered at the same table, or they get answered in the wrong order. A complimentary review of an old 401(k) or self-managed IRA covers what the account is costing you, whether anyone is providing advice on it today, and how it interacts with a conversion. Fee is discussed during that review. You can book the review directly.

Simplest question to answer first: has a Roth conversion already happened for you this year, or not yet?

Questions people ask about this

If I empty the traditional IRA before December 31, does the pro-rata rule still apply?

The measurement is the total value of all your traditional IRAs as of December 31 of the conversion year, plus any outstanding rollovers, per the 2025 Instructions for Form 8606, line 6. How the balance got to zero matters, since a withdrawal is a taxable distribution while a rollover into an employer plan is not. Confirm the sequence with your CPA before initiating it.

Do my spouse's IRAs count in my pro-rata calculation?

Form 8606 is filed per individual and line 6 asks for the total value of all of your traditional IRAs. It is a per-person calculation even on a joint return. Confirm the treatment of both sets of accounts with your CPA, since the joint return combines the results of two separate calculations.

Do Roth IRAs or a Roth 401(k) count in the pool?

No. The Form 8606 definitions state that the term traditional IRA includes traditional SEP IRAs and traditional SIMPLE IRAs. Roth IRAs are not in that definition, and line 6 asks only for traditional IRAs. Employer plan balances are not counted either, since IRC §408(d)(2)(A) aggregates individual retirement plans only.

What about an inherited traditional IRA?

Publication 590-B (2025) explains that basis in an inherited traditional IRA cannot be combined with basis in your own traditional IRAs, and that separate Forms 8606 are required. That is a separate calculation from the one covering your own accounts. Your CPA handles the split.

Can I move a traditional IRA into my current 401(k) to clear the pool?

The IRS Rollover Chart shows a traditional IRA can roll to a qualified plan, a 403(b), or a governmental 457(b), the last one requiring separate accounts. Whether your specific plan accepts it is a plan document question. The IRS FAQ page on retirement plans and IRAs, last reviewed November 16, 2025, notes that this assumes the plan has language allowing it, and that FAQ may not be relied upon as legal authority.

What counts as an outstanding rollover on line 6?

Per the same Form 8606 line 6 instructions, it means a distribution taken after November 1 that is rolled over the following year within the 60-day window. Money in transit across the year end still counts in the year-end total.

Sources: IRC §408(d)(2), U.S. Code. 2025 Instructions for Form 8606, IRS. IRS Rollover Chart. IRS Publication 590-B (2025). IRS Retirement plans FAQs regarding IRAs, page last reviewed November 16, 2025. All tax year 2026 dollar figures from IRS news release IR-2025-111, November 13, 2025. The $7,500 contribution, $200,000 rollover and resulting $271 and $7,229 split are a hypothetical illustration using assumed values, not a projection and not a client result. Form 8606 rounds the line 10 ratio to three decimal places, which moves the tax-free portion by about a dollar against the figure shown here.