The Backdoor Roth and the Pro-Rata Rule That Quietly Taxes It

Why the back door exists at all

Roth IRAs have income limits. Above a certain modified adjusted gross income, direct Roth IRA contributions phase out and then disappear entirely. Traditional IRA contributions have no income limit — only the deduction phases out for people covered by a workplace plan.

That asymmetry is the whole opening. A high earner can always make a nondeductible traditional IRA contribution, and since 2010 there has been no income limit on conversions from traditional to Roth. Put the two together and you reach a Roth IRA through a door the income limits do not guard.

Congress has been aware of this for over a decade and has repeatedly declined to close it, even acknowledging the strategy in committee reports. The phase-out ranges are adjusted annually, so check the current figures before assuming you are excluded — some people run the backdoor unnecessarily.

The mechanics, step by step

1. Contribute to a traditional IRA. For 2026 the limit is $7,500, plus a $1,100 catch-up if you are 50 or older. Make it nondeductible — do not claim a deduction for it.
2. Leave it in cash briefly. Most people wait a few days for the contribution to settle. Any earnings during this window are taxable on conversion, so a short gap keeps the taxable amount near zero.
3. Convert to a Roth IRA. Your custodian will have a conversion form or a button. Choose to have no tax withheld — you want the full amount to land in the Roth.
4. File Form 8606. This is not optional and it is where most self-managed backdoor Roths go wrong. Part I reports the nondeductible contribution and establishes your basis; Part II reports the conversion. Skip it and the IRS has no record that you already paid tax on those dollars, and you can end up taxed twice.

Executed cleanly with no other IRA balances, the taxable amount is zero or a few dollars of interest, and you have moved $7,500 into a Roth despite being over the income limit.

The pro-rata rule, which is where it breaks

Here is the rule that turns a clean maneuver into an expensive one.

When you convert, you may not designate which dollars are converting. The IRS treats all of your traditional, SEP and SIMPLE IRAs as one single account, measured by their total balance on December 31 of the year of conversion. Your conversion is deemed to pull proportionally from pre-tax and after-tax money across that whole aggregate.

The standard analogy is cream in coffee. Once stirred, you cannot pour out only the cream.

The proportion is simply your total after-tax basis divided by your total IRA value:

• An engineer earning $210,000 has a rollover IRA holding $142,500 from a former employer's 401(k), all pre-tax.
• She contributes $7,500 nondeductible and converts that $7,500 to a Roth, expecting no tax.
• Her total IRA value is $150,000. Her after-tax basis is $7,500 — exactly 5%.
• So only 5% of the conversion, $375, is treated as a return of basis. The other $7,125 is ordinary taxable income.
• At a 32% marginal rate, that is roughly $2,280 in unexpected federal tax, plus state.

And the remaining basis does not vanish — $7,125 of basis stays spread across her IRAs, to be recovered in fractions over future distributions. She has created a permanent accounting burden in exchange for a surprise tax bill.

You cannot convert only the after-tax dollars. The IRS blends every traditional, SEP and SIMPLE IRA you own into one pot and takes its share proportionally.

Three details that change the calculation

The measurement date is December 31, not the conversion date. Emptying your rollover IRA in February does not help if you roll money back in before year end. Conversely, a pre-tax IRA you clear out in November still leaves you clean for a conversion done in March, as long as the December 31 balance is zero.

Roth IRAs and inherited IRAs are excluded from the aggregation. So are 401(k), 403(b) and 457 balances — employer plans are not part of the IRA pot at all. This is the key to the fix below.

Your spouse's IRAs are not included. The calculation is per individual, even on a joint return. A couple where one spouse holds a large rollover IRA and the other holds none can run a clean backdoor Roth for the second spouse.

The fix: the reverse rollover

Because employer plans are excluded from the pro-rata calculation, the standard solution is to move the pre-tax IRA money out of the IRA system entirely.

Most 401(k) plans accept incoming rollovers from IRAs. Roll your rollover IRA into your current employer's 401(k), and by December 31 your traditional IRA balance is zero. The pro-rata rule then has nothing to work with, and the backdoor Roth converts cleanly.

Before doing it, weigh what you give up. A 401(k) has a fixed investment menu where an IRA has the whole market. Plan fees may be higher — or, in a large plan with institutional share classes, lower. Creditor protection under ERISA is generally stronger in a 401(k). And 401(k)s do not permit qualified charitable distributions, which matters if you are near or past 70½.

If your plan does not accept incoming rollovers, or the trade-offs are unattractive, the honest answer is often that the backdoor Roth is not worth it for you. Paying thousands in tax to move $7,500 into a Roth is rarely a good trade.

A note on the step transaction doctrine

For years, commentators warned that converting immediately after contributing might be recharacterized as a direct Roth contribution you were not eligible to make. That concern has largely faded. Congress explicitly referenced the strategy in the legislative history of the 2017 tax law without prohibiting it, and the IRS has not challenged it. Most practitioners no longer recommend an artificial waiting period, though some still prefer a short gap out of caution.

The other back door, briefly

The mega backdoor Roth is a different and much larger strategy that lives inside a 401(k), not an IRA. It requires a plan that permits after-tax (not Roth) contributions beyond the $24,500 deferral limit, plus either in-plan Roth conversions or in-service withdrawals. Where available it can move substantially more money per year into Roth treatment. It has nothing to do with the pro-rata rule described here — and most plans do not offer it, so check your summary plan description before getting attached to the idea.