The Pro Rata Rule

The rule lives in Section 408(d)(2) of the tax code: for figuring the tax on distributions, all of your individual retirement plans are treated as one contract, and all distributions during the year are treated as one distribution. A Roth conversion is a distribution for this purpose. So the IRS never sees "the $7,500 IRA I just opened at a new brokerage." It sees one big IRA, containing every traditional, SEP, and SIMPLE dollar you have, some of it pretax and some of it after-tax basis.

Think of it as cream in coffee. Your after-tax basis is the cream, your pretax money is the coffee, and the moment both exist, every pour out of the pot comes out blended. There is no spoon that extracts just the cream. The only way to convert cream by itself is to remove the coffee from the pot first, and there is a legal way to do exactly that, covered in the fix section below.

This matters most for the backdoor Roth: a nondeductible IRA contribution followed by a conversion. The whole strategy assumes the conversion is nearly tax-free because you already paid tax on the contribution. That assumption holds only when your combined IRA pot contains nothing but that contribution. One old rollover IRA sitting at a custodian you forgot about breaks it. The overall strategy, including whether converting makes sense for you at all, is covered in our Roth conversion guide. This page is the deep dive on the one rule that wrecks the execution.

The math comes straight off Form 8606. Your total basis (every nondeductible contribution you ever made) is divided by the value of all your traditional, SEP, and SIMPLE IRAs on December 31 of the conversion year, plus whatever you converted or withdrew during the year. The result is the tax-free percentage of your conversion. Everything else is ordinary income.

The timing detail catches people every year: the denominator is the December 31 balance, not the balance on the day you converted. Convert in January with a zero IRA balance and the conversion looks clean for eleven months. Then in November you change jobs and roll a $200,000 401(k) into a rollover IRA. On December 31 the pot holds $200,000 of pretax money, and your January conversion is retroactively poisoned: on those numbers, roughly $7,229 of a $7,500 conversion becomes taxable. The calendar did not protect you. Only the year-end balance matters.

Here is the failure pattern we see most, with round hypothetical numbers.

Now the same facts with one move added. Before doing anything else, Priya rolls the $92,500 rollover IRA into her current employer's 401(k). Employer plans can accept incoming rollovers of pretax IRA money, and only the pretax money is allowed in, which is exactly what makes this work. On December 31 her traditional IRA balance is $0. The pot is just the $7,500 she converted, her basis covers 100% of it, and the conversion is tax-free apart from any earnings between contribution and conversion.

Want to see the math on your own balances before you touch anything? Run them through our Roth conversion calculator first.

The pro rata pot is precise, and both halves of the list matter. The accounts inside it decide how much tax you owe; the accounts outside it are your escape routes.

Two of the exclusions deserve emphasis. First, spouses: the calculation runs per taxpayer, so your spouse's $400,000 rollover IRA has zero effect on your own backdoor Roth. Each spouse files a separate Form 8606, and it is common for one spouse to run the strategy cleanly while the other cannot. Second, inherited IRAs: an IRA inherited from someone other than your spouse stays in its own lane, with its own basis tracked on its own Form 8606, and does not mix with your pot. A surviving spouse who elects to treat an inherited IRA as their own moves it INTO their pot, which is worth knowing before making that election.

Employer plans are the other big exclusion. Your 401(k), 403(b), 457(b), or TSP balance never appears on Form 8606, no matter how large. That asymmetry is not a loophole so much as the design, and it powers both the fix in section 6 and the mega backdoor Roth, which runs entirely inside an employer plan and never touches the IRA pot.

Your basis is not tracked by your brokerage, and it is not on your account statements. It exists in exactly one place: Form 8606, filed with your return for every year you make a nondeductible contribution or take money out of an IRA that has basis. Line 14 of each year's form carries your unused basis forward to the next, forming a paper chain that can run for decades. Break the chain and the IRS's working assumption is that your basis is zero.

Forgetting to file carries a $50 penalty per missed form, and overstating your basis carries a $100 penalty, both waivable for reasonable cause. But the penalties are not the real cost. The real cost is that undocumented basis gets taxed twice: once when you earned the money, and again when it comes out of the IRA with no Form 8606 trail proving you already paid. The repair is filing the missed forms to rebuild the basis history, supported by old returns and contribution records.

Fix 1: Roll the pretax money into an employer plan

This is the standard play, sometimes called a reverse rollover. Move the pretax balance of your traditional and rollover IRAs into your current 401(k) or 403(b), if the plan accepts incoming rollovers (most large plans do, check the summary plan description). The rules only allow pretax money into the plan, which means your after-tax basis is left behind in the IRA by design. That stranded basis is exactly what you want: an IRA holding nothing but basis converts 100% tax-free. Self-employed with no employer plan? A solo 401(k) that accepts roll-ins can serve the same role.

Fix 2: Convert the whole pot in a low-income year

If a low-income year is coming (a sabbatical, a business loss year, the gap between retirement and Social Security), converting the entire pretax balance at a low marginal rate can beat years of tiptoeing around the pro rata rule. You pay the tax once, on purpose, at a rate you chose, and every conversion afterward is clean. Model the bracket math in our Roth conversion calculator and see the full timing playbook in the Roth conversion guide.

Fix 3: Isolate the basis, then convert

The combination move: roll everything pretax into the employer plan (Fix 1), leaving only basis in the IRA, then convert the remainder to Roth tax-free. Done right, this rescues years of stranded nondeductible contributions in one sequence. The execution details matter: the plan rollover must actually complete before December 31 of the conversion year, and the amounts have to be computed precisely, because rolling even a dollar of basis into the plan is not permitted.

Some pro rata exposure cannot be cleared, and pretending otherwise is how people annualize a tax mistake. Skip the backdoor Roth, at least for now, if any of these describe you:

• A large pretax IRA and no plan that will take it. If you hold, say, $400,000 of pretax IRA money, your employer plan refuses incoming rollovers, and you have no self-employment income to open a solo 401(k), a $7,500 backdoor contribution converts about 98% taxable. You would pay high-bracket tax now for a small Roth balance. The math simply is not there.
• SEP or SIMPLE IRA owners who keep contributing. Every new employer contribution refills the pretax pot, so clearing it once does not stay cleared. Restructuring the retirement plan itself usually has to come first.
• Anyone mid-rollover in the conversion year. If a 401(k)-to-IRA rollover is already in flight this year, the December 31 balance will include it. Wait for a calendar year when the pot can actually be empty at year end.

None of this means giving up on Roth money. If your employer plan allows after-tax contributions with in-plan conversions or in-service rollovers, the mega backdoor Roth can move far more than $7,500 a year into Roth without ever touching the IRA pot. And a plain taxable brokerage account, boring as it is, beats a conversion done at the wrong rate.