The Pro-Rata Rule — How It Affects Roth Conversions

The pro-rata rule means the IRS treats all your Traditional, SEP, and SIMPLE IRAs as one combined pool when calculating the taxable portion of a Roth conversion. You cannot convert "just the nondeductible portion" and avoid taxes on the rest. Form 8606 generally divides your basis by an annual denominator that includes the December 31 IRA value plus relevant same-year distributions and net conversions. If you have $100,000 of pretax IRA money, add $10,000 of nondeductible basis and convert $10,000, $9,090.91 of that conversion is taxable under these simplified facts.

The Aggregation Rule: How the IRS Pools Your IRAs

When you perform a Roth conversion, the IRS doesn't look at individual IRA accounts. Instead, it adds up all your Traditional IRA, SEP IRA, and SIMPLE IRA balances—regardless of where they're held or what they contain—into a single aggregate pool. This is called the aggregation rule, and it's codified in IRC §408(d)(2).

Here's what's critical: the aggregation applies only to certain IRA types. Your Roth IRAs are completely separate and don't count. Inherited Traditional IRAs (from someone other than your spouse) are treated separately. And if you have a 401(k), 403(b), or other employer plan, those are not included in the aggregation.

The rule is mechanical and has no exceptions. If you have three different Traditional IRAs at three different banks—one with $50,000, one with $30,000, and one with $20,000—for conversion purposes, the IRS treats you as if you have a single $100,000 pre-tax IRA pool. When you convert any amount from any of those accounts, the conversion is calculated against the full $100,000 aggregate balance.

The Pro-Rata Formula: How Your Tax Bill Is Calculated

The formula itself is straightforward, but the implications are profound. Here's how it works:

Suppose you have $93,000 of pretax IRA money, add $7,500 of nondeductible basis and convert $7,500. If $93,000 remains in the counted IRA pool on December 31 and there are no other relevant transactions, the annual denominator is $93,000 + $7,500 = $100,500.

The calculation: ($7,500 ÷ $100,500) × $7,500 = $559.70 nontaxable. The other $6,940.30 is taxable. These examples use the full-precision ratio and round the result to the nearest cent; a filed Form 8606 follows its line-specific decimal and whole-dollar rounding instructions.

The key point is that a taxpayer cannot designate the conversion as coming only from the recent nondeductible contribution while leaving pretax IRA money outside the calculation. Form 8606 allocates basis across the owner’s aggregate Traditional, SEP, and SIMPLE IRA pool under the annual ratio.

The December 31 Rule: Why Timing Matters

One of the most overlooked aspects of the pro-rata rule is the timing: the conversion-date balance does not freeze the result. Form 8606 generally builds one annual denominator from the December 31 Traditional/SEP/SIMPLE value plus relevant same-year distributions and net conversions.

Example: You have $100,000 in a Traditional IRA in November. You make a $7,500 nondeductible IRA contribution on December 15, then convert $7,500 on December 20.

It does not work as a basis-only conversion. After the conversion, $100,000 remains in the counted IRA pool on December 31; Form 8606 then adds the $7,500 conversion, producing a $107,500 denominator. The full-precision ratio makes $523.26 nontaxable and $6,976.74 taxable—6.9767% basis and 93.0233% pretax.

An inbound rollover can change an earlier conversion too. If you convert in January and later roll $100,000 from a workplace plan into a Traditional IRA that remains there on December 31, that year-end balance enters the annual denominator. Conversely, eligible pretax IRA money accepted by an employer plan can leave the IRA pool by December 31 even if the Roth conversion happened earlier. Cleanup need not precede the conversion; completion by the conversion-year deadline is what matters for line 6.

Form 8606: Tracking Your Non-Deductible Basis

For backdoor-Roth purposes, Form 8606 is generally required for a year in which you make a nondeductible traditional IRA contribution, convert traditional IRA money to Roth, or receive a traditional IRA distribution while you have basis. The instructions list additional events, including certain Roth IRA distributions. Basis that merely carries through an otherwise inactive year does not by itself require a new form.

Form 8606 has three parts. Part I reports nondeductible traditional IRA contributions, tracks traditional-IRA basis, and calculates the nontaxable share of distributions and conversions under the pro-rata rule. Part II reports traditional-IRA-to-Roth conversions and their taxable amount. Part III is for Roth IRA distributions.

For a conversion involving basis, Part I uses your prior basis, current nondeductible contribution, aggregate December 31 value of all your traditional IRAs, other distributions, and conversion amount. Part II then reports the conversion and subtracts the nontaxable amount calculated in Part I.

File Form 8606 whenever its instructions require it, even when a conversion's resulting taxable amount is zero. Keep each filed form and its supporting records so line 14 basis can be carried to line 2 the next time Part I applies.

Which IRAs Count in the Aggregation?

IRAs that ARE included: Traditional IRA, SEP IRA (Simplified Employee Pension), SIMPLE IRA. This includes accounts you think you've "abandoned" or that have been inactive for years. Even if you opened an SEP IRA for a side business five years ago and haven't contributed since, it still counts in the aggregation.

IRAs that are NOT included: Roth IRA (conversions to Roth are what we're calculating; the destination account doesn't affect the calculation), Roth conversions you made in prior years (only the current year's conversion is aggregated), and inherited Traditional IRAs from someone other than your spouse. Inherited IRAs are subject to separate rules and don't aggregate with your own IRAs.

Employer plans that are NOT included: 401(k), 403(b), SIMPLE 401(k), Solo 401(k), Profit-Sharing Plan, Defined Benefit Plan, or any other qualified employer plan. If your employer 401(k) holds $500,000, that balance does not enter the IRA pro-rata calculation. Moving IRA money into a plan is separate: the plan must accept the exact IRA source, and only eligible pretax money can move.

Why This Matters for Backdoor Roth Conversions

After compensation, shared IRA room and direct Roth eligibility are checked, the pro-rata rule determines how much of a Traditional-IRA-to-Roth conversion is allocated to basis under these simplified facts.

For example, a saver whose Roth MAGI is above the direct-contribution band might make an eligible $7,500 nondeductible Traditional IRA contribution and convert $7,500. If substantiated basis equals the annual denominator, the conversion has no taxable portion. Other distributions, conversions, gains, losses or counted IRA balances can change the result.

But if $100,000 of pretax IRA money remains on December 31, the same $7,500 conversion is different: $523.26 is nontaxable and $6,976.74 is taxable. At a 22% marginal federal rate, the illustrative federal tax attributable to that taxable amount is $1,534.88, before state tax and other return effects.

This is why the pro-rata rule deserves a full owner-wide inventory. An old rollover, SEP, or SIMPLE IRA that remains in the counted pool on December 31 can make an earlier conversion partly taxable; the conversion date alone does not settle the result.

The December 31 Trap: A Real-World Example

Here's a scenario that catches even sophisticated planners off guard:

The Setup: It's mid-November. Jennifer has $100,000 in a Traditional IRA. She wants to do a backdoor Roth for $7,500. She plans to contribute the $7,500 non-deductible on December 15, then convert it on December 20.

What she thinks: "I'll add $7,500 to my IRA, making it $107,500. Then I'll convert $7,500, leaving $100,000. The conversion will be fully non-taxable because I'm converting only the non-deductible portion I just added."

What Form 8606 does: On December 31, Jennifer has $100,000 remaining in her Traditional IRA after the conversion. Part I then adds that year-end value to her $7,500 conversion when building the denominator: $100,000 + $7,500 = $107,500.

Her full-precision basis percentage is $7,500 ÷ $107,500 = 6.9767%. Applied to the $7,500 conversion, $523.26 is nontaxable and $6,976.74 is taxable when rounded to cents. The denominator happens to equal her pre-conversion balance in this simple example, but it comes from the annual Form 8606 lines—not a snapshot taken on December 20.

The distinction matters when the balance changes later. If Jennifer converted in July and then finished the year with $110,000 in counted IRAs, Form 8606 would generally combine that $110,000 December 31 value with the $7,500 conversion and any other relevant distributions. It would not reuse the July balance or calculate each conversion from a separate transaction-date ratio.

Before You Do a Backdoor Roth: Your Checklist

If you're planning a backdoor Roth conversion, use this checklist to avoid the pro-rata trap:

The Conditional Cleanup: Moving Eligible Pretax IRA Money Into an Employer Plan

One possible way to reduce the year-end IRA pool is a direct roll-in of eligible pretax IRA money to the same owner's employer plan. It is not automatic and is not available under every plan.

How it works: A qualified employer plan may accept the otherwise taxable portion of an eligible IRA rollover. IRA basis cannot be rolled into the plan. When properly completed, the accepted pretax amount leaves the Traditional/SEP/SIMPLE pool used by Form 8606.

The catch: The receiving plan is not required to accept IRA-origin money. Confirm that it accepts the exact source, not merely plan-to-plan rollovers. Traditional SIMPLE IRA money can also face its initial two-year transfer restriction.

Timing: The qualifying roll-in must be completed by December 31 of the conversion year; it need not occur before the conversion itself. A January 2 completion does not change the prior conversion year's December 31 value.

Future Roth conversion: Employer-plan money does not have to return to a Traditional IRA first. Depending on plan terms and distribution eligibility, pretax plan money may be converted through an in-plan Roth rollover or moved directly to a Roth IRA; either route generally creates taxable income. The IRA pro-rata rule becomes relevant again only if pretax money first enters a Traditional/SEP/SIMPLE IRA and remains in that owner's annual IRA calculation.

How to File Form 8606 for Your Conversion

Form 8606 is required for every traditional-IRA-to-Roth conversion. It is also generally required when you make a nondeductible traditional IRA contribution, take a traditional IRA distribution while you have basis, or receive certain Roth IRA distributions. Carrying basis through an otherwise inactive year does not itself require a new form.

What you'll need: Your available nondeductible basis from prior Forms 8606 and the current year, the December 31 value of all same-owner Traditional, SEP, and SIMPLE IRAs, and every relevant same-year traditional-IRA distribution and net conversion used in the annual denominator.

The calculation on the form: Part I line 1 reports current-year nondeductible contributions and line 2 carries prior basis. Line 6 reports the December 31 aggregate traditional-IRA value plus outstanding rollovers; line 7 reports other traditional IRA distributions and line 8 reports conversions. Lines 10–14 calculate the nontaxable amounts and remaining basis. Part II lines 16–18 report the conversion, its nontaxable share, and the taxable amount.

Filing: Attach Form 8606 to Form 1040 when the instructions require it. The taxable conversion amount is included in income for that year. Keep the prior basis forms, transaction confirmations and year-end statements together so the annual calculation can be supported.

Inherited IRAs Are Not Aggregated: The Single Most Valuable Exclusion

IRC §408(d)(2)(B) applies to "all individual retirement plans of the taxpayer," but Treas. Reg. §1.408-8, Q&A-5 and the instructions to Form 8606 both treat inherited IRAs as separate from the taxpayer's own IRAs for pro-rata purposes. If you inherited a $500,000 Traditional IRA from a parent, that balance is entirely excluded from your backdoor Roth pro-rata computation. You can still execute a clean $7,500 backdoor even while holding a massive inherited pre-tax balance.

The inherited IRA is its own legal beast: it has its own RMD schedule (generally the 10-year rule post-SECURE for non-spouse beneficiaries), its own 1099-R each year, and its own basis tracking. Crucially, if the decedent had filed Form 8606s tracking their own non-deductible basis, that basis carries over to the inherited IRA and the beneficiary uses a separate Form 8606 (not commingled with the beneficiary's own) to recover it. This is why heirs should always request copies of the decedent's prior-year Form 8606 filings as part of estate administration—without them, the beneficiary often loses basis that would reduce their 10-year-rule tax bill.

A spouse who inherits and rolls into their own IRA loses this protection. A surviving spouse who elects to treat an inherited Roth or Traditional IRA as their own (the "spousal rollover" under §408(d)(3)(C)) then loses the inherited-IRA pro-rata exclusion. For surviving spouses who plan to continue backdoor Roths, leaving the account in inherited-IRA form (rather than rolling over) preserves the aggregation separation—at the cost of potentially less favorable RMD rules. This is a trade-off worth modeling.

Roth IRAs Have Their Own Aggregation Rule—Different from Traditional Pro-Rata

It's a common misconception that "IRAs are aggregated." In reality, Traditional-side aggregation (for conversion pro-rata) operates under IRC §408(d)(2), while Roth-side aggregation (for withdrawal ordering) operates under IRC §408A(d)(4). They are separate statutory rules, and balances from one side don't cross over.

Your existing Roth IRA balance does not enter the Traditional-side pro-rata ratio. For example, a $500,000 Roth IRA would not change the ratio for a separate $7,500 Traditional IRA contribution and conversion. Under simplified facts with $7,500 of substantiated basis, no other same-year distributions, and a $0 conversion-year December 31 balance across the owner’s Traditional, SEP, and SIMPLE IRAs, Form 8606 would allocate the $7,500 conversion to basis. Investment gains or another pretax IRA balance would change that result.

On the Roth side, however, all Roth IRAs are aggregated for the three-tier ordering (contributions, conversions, earnings) and for the 5-year clocks. That's a separate topic; see our 5-Year Rule guide for the Roth-specific rules.

Notice 2014-54: Splitting a Plan Distribution by Tax Character

IRS Notice 2014-54 explains how one employer-plan distribution sent to multiple destinations is allocated. When the requirements are met, the participant can direct the pretax portion to a Traditional IRA and the after-tax contribution portion to a Roth IRA. The notice applies to qualified-plan distributions, not to distributions from an IRA.

Simplified example: A 401(k) holds $200,000 of pretax money and $50,000 of after-tax contributions, with no earnings included in the after-tax subaccount for this illustration. The participant could direct $200,000 to a Traditional IRA and $50,000 to a Roth IRA. The pretax rollover is not currently taxable; the after-tax contribution amount is not taxed again. Real plan records may also show earnings on after-tax contributions, and those earnings are pretax. Confirm the plan's source accounting before choosing destinations.

The Cream-in-the-Coffee Problem: You Cannot Earmark an IRA Conversion

A common metaphor for IRA pro-rata is coffee and cream: a conversion cannot select only the basis after pretax and after-tax amounts share the owner's IRA pool. With $100,000 pretax and $7,500 basis under the simplified facts above, the annual ratio is 93.0233% pretax and 6.9767% basis. That year's ratio applies across the distributions and conversions covered by Form 8606; it is recalculated in a later year from that year's basis, transactions and December 31 value.

The metaphor has an important limit: eligible pretax IRA money may be moved to an accepting employer plan under IRC §408(d)(3)(A)(ii), while IRA basis stays behind. That conditional roll-in can change the conversion-year denominator if completed by December 31. Otherwise, converting the pretax portion generally produces taxable income.

Each Spouse Has a Separate IRA Pool

The pro-rata rule aggregates each spouse's IRAs separately. A married couple filing jointly does not combine IRA balances for this calculation—each spouse's Form 8606 uses only that spouse's Traditional, SEP, and SIMPLE IRAs.

Separate-owner example: Wife has a $300,000 pretax Traditional IRA from a 2005 workplace-plan rollover. Husband has no Traditional, SEP, or SIMPLE IRA balance. Under otherwise-clean facts, the husband's separate conversion can avoid pro-rata tax even though the wife's IRA remains. Her own conversion would use her separate pool unless eligible pretax money left it through a qualifying employer-plan roll-in by her conversion-year December 31.

One spouse's pretax IRA therefore does not enter the other spouse's conversion ratio. Each spouse still needs separate records, and any employer-plan roll-in depends on that spouse's plan accepting the exact eligible pretax source.

When Prior-Year Basis Records Are Missing

A missing Form 8606 does not prove that basis is zero. If an earlier nondeductible Traditional IRA contribution may not have been reported, reconstruct the history before relying on the current-year ratio. Useful records include prior federal returns, Forms 8606, Forms 5498, account statements, and documents showing any intervening distributions, conversions, rollovers, or transfers.

The current Form 8606 instructions tell a taxpayer with basis from prior years to enter the total basis carried into the year. They do not create a shortcut for an unsupported estimate. A tax professional can help determine whether a prior-year return or Form 8606 should be corrected and how much basis can be substantiated. Until the history is resolved, treat the conversion result as undetermined rather than assuming either zero basis or a tax-free conversion.