The pro-rata rule: the tax trap inside the backdoor Roth

Every backdoor Roth tutorial tells the same tidy story: contribute $7,500 to a traditional IRA without deducting it, convert it to Roth a few days later, and owe the IRS nothing, because every dollar already paid its tax. The story is true only when your traditional IRA balance is zero before you start. The moment an old rollover IRA from a previous job’s 401(k) sits anywhere in your name, a provision called the pro-rata rule rewrites the ending and attaches a very real bill to the “tax-free” conversion.

The maneuver exists because the front door is closed to high earners. For 2026, IRS Notice 2025-67 phases direct Roth IRA contributions out between $153,000 and $168,000 of modified adjusted gross income (MAGI) for single filers, and between $242,000 and $252,000 for married couples filing jointly. Anyone above those lines who still wants Roth dollars goes in through the step-by-step backdoor Roth process: a nondeductible traditional IRA contribution — up to $7,500 in 2026, or $8,600 with the age-50 catch-up — followed by a conversion. The mechanics are legal and settled. What ambushes people is not the law but the arithmetic, and the arithmetic lives on a form most filers have never read: Form 8606.

The pro-rata rule means you cannot convert only your after-tax contribution. Under Section 408(d)(2) of the tax code, the IRS aggregates every traditional, SEP and SIMPLE IRA you own into one pot, and each converted dollar carries the pot’s overall blend of pre-tax money and after-tax basis. Form 8606 sets the tax-free fraction: nondeductible basis divided by the December 31 value of all those IRAs plus the year’s conversions. A pre-tax rollover IRA in the pot makes a backdoor conversion mostly taxable — unless you clean it out before year-end.

One pot, no matter how many accounts

The rule’s legal home is Section 408(d)(2) of the Internal Revenue Code, which tells the IRS to treat all of an individual’s traditional IRAs as a single contract when taxing a distribution — and a Roth conversion counts as a distribution. Your nondeductible contribution may sit in a brand-new account at one brokerage and the rollover money at another; for pro-rata purposes there is one pot.

Three account types fall into it: traditional IRAs — including rollover IRAs, which are simply traditional IRAs funded from an old workplace plan — plus SEP IRAs and SIMPLE IRAs. Three stay out. Money still inside a 401(k), 403(b) or 457 plan is invisible to the calculation, the detail that powers the cleanest fix below. Roth IRAs are excluded, since the whole pre-tax versus Roth distinction is that Roth dollars already paid their tax, and inherited IRAs are kept separate from your own retirement money. And the pot is personal: the math runs per person, so your spouse’s IRAs never enter your calculation.

The Form 8606 fraction and the December 31 snapshot

The arithmetic is spelled out in the instructions for Form 8606, which tracks after-tax IRA money for life. The tax-free share of a conversion equals your basis — the running total of nondeductible contributions you have reported — divided by the December 31 value of all your traditional, SEP and SIMPLE IRAs for the conversion year, plus whatever you converted or withdrew during that year. Line 6 of the form asks for that year-end balance — the IRS does not care what your accounts held on the day you converted, only what they hold on December 31.

That timing cuts both ways. Convert in March while a six-figure rollover IRA sits in your name and nothing is lost yet — you have until New Year’s Eve to move the pre-tax money somewhere the rule cannot see, and the conversion you already made becomes retroactively tax-free. The trap also springs in reverse: roll an old 401(k) into an IRA in November, after a textbook backdoor in January, and you have quietly poisoned a settled conversion.

A worked example: Maya’s $67,500 rollover IRA

Consider Maya, who, like millions of departing employees, rolled an old 401(k) into a rollover IRA: $67,500, every dollar pre-tax. Her income now sits above the Roth phase-out, so she follows the tutorial — a $7,500 nondeductible contribution to a fresh traditional IRA, converted the following week, supposedly tax-free.

Form 8606 disagrees. Her basis is $7,500. The denominator is the December 31 value of her IRAs — the $67,500 rollover — plus the $7,500 conversion: $75,000 in total. The tax-free fraction is $7,500 divided by $75,000, or 10%. Of the $7,500 she converted, only $750 escapes tax; the other $6,750 lands on her return as ordinary income, and in the 22% bracket that is roughly $1,485 of federal tax on a move the tutorials promised was free.

Form 8606 math	Convert as-is	Reverse rollover first
Nondeductible basis	$7,500	$7,500
IRA value on December 31	$67,500	$0
Plus conversions during the year	$7,500	$7,500
Tax-free fraction	$7,500 ÷ $75,000 = 10%	$7,500 ÷ $7,500 = 100%
Tax-free portion of the conversion	$750	$7,500
Taxable ordinary income	$6,750	$0
Federal tax at 22%	≈ $1,485	$0

Now the alternative: before December 31, Maya moves the $67,500 of pre-tax money into her current employer’s 401(k) through a reverse rollover. Her year-end traditional IRA balance drops to zero, the denominator shrinks to the $7,500 she converted, and the fraction becomes 100% tax-free. Same contribution, same conversion, $1,485 of difference — decided entirely by the order of operations.

Three exits before December 31

The first and usually cheapest exit is Maya’s: the reverse rollover. Move the pre-tax IRA money into your current employer’s 401(k) — or into a Solo 401(k) if you have self-employment income — provided the plan accepts incoming rollovers, which is a plan-document question rather than a legal right, so confirm first. One asymmetry works entirely in your favor: workplace plans may only accept pre-tax dollars, so your nondeductible basis stays behind in the IRA — exactly what you want, a small account holding nothing but basis, ready to convert clean. While the plan documents are open, check for after-tax contributions with in-plan conversions — the lever behind the much larger mega backdoor Roth.

The second exit is to convert everything: take the whole pre-tax balance to Roth, pay ordinary income tax now — in one year, or spread across several by converting in slices — and emerge with a permanently clean slate. It is a real bill, but it can make sense when the balance is modest or your bracket unusually low, and it ends the problem for good.

The third exit is to accept the proportion: nothing forbids a backdoor with a mixed pot, and each attempt simply becomes a partial Roth conversion, mostly pre-tax and taxed accordingly. For some savers that trade is still worth making — but it should be a decision taken with the fraction in front of you, not a surprise on next April’s return.

The rule is arithmetic, not a penalty

Two pieces of comfort survive. First, your nondeductible contribution is never taxed twice. The basis reported on Form 8606 carries forward until all of it has come out tax-free; in Maya’s convert-as-is scenario, the $6,750 of unused basis shelters a slice of every future withdrawal or conversion. Second, the rule creates no tax out of thin air: the rollover money was always pre-tax and was always going to be taxed once — the rule merely refuses to let you choose which dollars go first.

What it does punish is sequencing. The backdoor Roth is a calendar game: inventory every traditional, SEP and SIMPLE IRA before you contribute, clear the pre-tax money before December 31 of the year you convert, and file Form 8606 for every year you add nondeductible money. Do those three things in order, and the trap inside the backdoor never springs.

Sources

• IRS — Instructions for Form 8606 — the pro-rata calculation: nontaxable fraction = basis ÷ (December 31 IRA value plus the year’s distributions and conversions).
• Cornell Law — 26 U.S. Code § 408(d)(2) — statutory aggregation of an individual’s traditional, SEP and SIMPLE IRAs as one contract.
• IRS — Notice 2025-67 — 2026 limits: $7,500 IRA contribution ($8,600 with the age-50 catch-up); Roth IRA MAGI phase-outs of $153,000–$168,000 single and $242,000–$252,000 married filing jointly.

Limits and phase-outs are the IRS-published 2026 amounts; the worked example is illustrative and assumes a single marginal rate. This is general information, not individualized tax advice.