The Backdoor Roth and the Pro-Rata Rule That Eats It (Form 8606)
July 22, 2026 · Josh Pickett, EA
You contribute $7,000 to a nondeductible traditional IRA, convert it to Roth a week later, and expect to owe zero tax on the conversion. Then your CPA runs Form 8606 and tells you $6,763 of that conversion is taxable. What happened? You had $200,000 sitting in a rollover IRA from an old 401(k), and the pro-rata rule dragged it into the math.
This is the mistake that derails more backdoor Roths than any other, and it is entirely avoidable, but only if you understand it before you convert, not in April when the 1099-R shows up.
What is a backdoor Roth IRA?
A backdoor Roth is a two-step maneuver that lets high earners fund a Roth IRA despite being over the income limits. You make a nondeductible contribution to a traditional IRA, then convert that traditional IRA to a Roth.
The Roth IRA has a direct-contribution income phase-out. For 2026, single filers phase out between $153,000 and $168,000 of modified AGI, married-filing-jointly between $242,000 and $252,000 (the amounts are inflation-adjusted annually under §408A). But there is no income limit on converting a traditional IRA to Roth: the income cap on conversions was repealed for tax years after 2009. The backdoor exploits that asymmetry: contribute where there is no income limit (a nondeductible traditional IRA under §408(o)), then convert.
The contribution limit for 2026 is $7,500, or $8,600 if you are 50 or older (§219(b)). That is the amount you can move per year through the backdoor.
Does the backdoor Roth actually work?
Yes. It is a well-established strategy that the IRS has effectively acknowledged. Congress referenced it in the legislative history of later tax bills, and the mechanics rely entirely on real code provisions. What the IRS has not blessed is any workaround for the pro-rata rule, which is where most people get burned.
The conversion itself is not a loophole in the pejorative sense. You are converting after-tax dollars, so the contribution portion is not taxed again. The problem is that the tax code will not let you cherry-pick which dollars you convert.
What is the pro-rata rule?
The pro-rata rule (§408(d)(2)) says that when you convert or distribute from a traditional IRA, the IRS treats all your traditional IRA dollars as one commingled pot. You cannot elect to convert only your after-tax basis: every conversion comes out proportionally, part basis, part pre-tax.
The rule aggregates every traditional, SEP, and SIMPLE IRA you own as of December 31 of the conversion year. It does not include Roth IRAs, and it does not include balances still inside an employer 401(k) or 403(b).
The taxable percentage of your conversion equals:
(Total pre-tax IRA balance) ÷ (Total IRA balance including basis)
Here is the earlier example, run through the numbers. It uses a $7,000 contribution, the prior-year limit, purely as an illustration; the mechanics are identical at the current limit:
| Item | Amount |
|---|---|
| Nondeductible contribution (basis) | $7,000 |
| Existing rollover IRA (pre-tax) | $200,000 |
| Total IRA value at year-end | $207,000 |
| After-tax percentage | $7,000 ÷ $207,000 = 3.38% |
| Tax-free portion of $7,000 conversion | $237 |
| Taxable portion of conversion | $6,763 |
You wanted a tax-free $7,000 Roth. You got a $6,763 taxable event, and $6,763 of basis is left stranded in the traditional IRA to be tracked for years. The usual culprit is a rollover IRA from an old 401(k) that the owner moved years ago and forgot exists.
How does Form 8606 track all this?
Form 8606 is the IRS form that reports nondeductible IRA contributions and calculates the taxable portion of any conversion or distribution. It is the only place your after-tax basis is recorded. The IRS does not track it for you.
The critical lines:
- Line 1: nondeductible contributions made for the year.
- Line 2: total basis carried over from prior years.
- Line 6: total value of all traditional, SEP, and SIMPLE IRAs as of December 31 (this is what triggers pro-rata).
- Line 14: basis remaining, carried forward to next year.
- Line 18: the taxable amount that flows to your Form 1040.
File a Form 8606 for every year you make a nondeductible contribution, even in years you do not convert. Under §6693(b), failure to file the form carries a $50 penalty per instance, and, worse than the penalty, losing the basis record means you can end up paying tax twice on the same dollars. When I amend returns to reconstruct basis, the recurring problem is missing 8606s from years past: the basis was real, but there is no paper trail to prove it, and reconstructing it means digging up decade-old contribution records.
Note the timing trap: line 6 uses your December 31 balance, not the balance on the conversion date. If you convert in March and then roll a 401(k) into an IRA in November, that November balance still poisons the pro-rata calculation for the whole year.
How do you avoid the pro-rata rule?
The cleanest fix is to have zero pre-tax IRA dollars on December 31 of the conversion year. There are a few ways to get there.
Roll pre-tax IRA money into your employer 401(k). Balances inside a 401(k) or 403(b) are not counted in the pro-rata calculation. If your plan accepts incoming rollovers (many do), move the rollover IRA into the 401(k) before December 31, leaving only your fresh nondeductible contribution in the traditional IRA to convert cleanly. This is the workhorse solution.
Convert everything and eat the tax. If the pre-tax balance is small, sometimes it is worth converting the whole thing, paying tax once, and starting future backdoor conversions with a clean $0 pre-tax base.
Consider a spousal split. The pro-rata rule is applied per individual, not per couple. A spouse with no pre-tax IRA can run a clean backdoor even if you cannot.
Watch the SEP/SIMPLE trap. Self-employed clients with a SEP-IRA are frequently surprised: SEP and SIMPLE balances are counted. A solo 401(k) instead of a SEP can keep the IRA side clean.
What about the step-transaction risk?
Some advisors historically worried that converting immediately after contributing could be recharacterized under the step-transaction doctrine. In practice this concern has faded: there is no statutory waiting period, and the IRS has not challenged same-week conversions. I do not tell clients to wait an arbitrary number of days; I tell them to make sure the pro-rata math is clean, which is the risk that actually matters.
One real consequence of timing: contributions and conversions are reported for different tax years depending on when they occur. A contribution for 2026 can be made up to the April 2027 deadline, but a conversion is always reported in the calendar year it happens. Keep the two straight, because they land on Form 8606 in different years.
Tax positions depend on your specific facts and applicable state rules (a few states do not conform to federal Roth treatment). Confirm the plan-rollover option with your 401(k) administrator, and coordinate with your tax preparer before you convert, not after.
Sources
- IRC §408A: Roth IRAs and direct-contribution income limits
- IRC §408(o): nondeductible contributions to traditional IRAs
- IRC §408(d)(2): pro-rata (aggregation) rule for IRA distributions
- IRC §219(b): annual IRA contribution limits
- IRC §6693(b): penalty for failure to file Form 8606
- IRS Form 8606 and its instructions
- IRS Form 1099-R
- IRS Publication 590-A and 590-B: traditional and Roth IRA contributions, conversions, and distributions