Chapter 8: The backdoor and the mega backdoor
Maya earns $340,000 a year, and for a single filer the Roth IRA's front door swings shut between $153,000 and $168,000 of income. She still moved $27,500 into Roth accounts last year, signed every form honestly, and triggered almost no tax doing it. The route runs through two side doors Congress has left open for years: the backdoor Roth IRA for $7,500, and the mega backdoor for the other $20,000. Both are legal, both are routine at every major brokerage, and both fail expensively when one rule goes unread. This chapter is that rule, plus the recipe.
Why the side doors exist
The income limits in Chapter 7's table apply to exactly one action: contributing to a Roth IRA directly. Two neighboring actions have no income limit at all. Anyone with earned income may contribute to a traditional IRA, even if their income is too high to deduct it; that creates a nondeductible contribution, money that goes in after tax. And since 2010, anyone may convert traditional IRA money to a Roth, at any income. Chain the two together, a nondeductible contribution followed by a prompt conversion, and you have rebuilt the Roth contribution the phaseout took away. The IRS has acknowledged the maneuver in plain terms. It is a paperwork path, not a loophole on borrowed time.
One clarification saves a lot of confusion: the Roth 401(k) box on your paycheck has no income limit and needs no backdoor. These doors exist for IRA dollars, and for 401(k) space above the deferral cap.
The backdoor, step by step
The whole move takes about thirty minutes a year. In screenshots-in-words:
- Contribute $7,500 in cash to a traditional IRA. At your brokerage, open (or reuse) a traditional IRA and transfer $7,500 from your bank. Leave it in the settlement fund; do not buy investments yet. You will claim no deduction for this contribution at tax time, which is what makes it nondeductible.
- Convert it to the Roth a few days later. Once the deposit settles, use the "convert to Roth IRA" option and move the entire balance. Converting promptly matters because only growth between contribution and conversion is taxable. A few days in a money market might add $4 of interest, so the taxable amount is $4, not $7,500.
- Invest inside the Roth. Now buy the fund. From this point the money compounds tax-free forever, exactly as if it had walked in the front door.
- File Form 8606 with your return. This one-page form reports the $7,500 of basis, money you already paid tax on, and proves the conversion owes almost nothing. Skip the form and the IRS has no record that tax was ever paid, which sets up double taxation. The form is the move; the clicks are just logistics.
Done cleanly, the backdoor turns a phased-out contribution into a routine annual habit. The danger is one rule that most articles mention in a footnote and most people learn from a tax bill.
The pro-rata trap, worked exactly
The pro-rata rule says the IRS treats all of your traditional, SEP, and SIMPLE IRAs as one pot. When you convert any amount, you cannot choose to convert just the after-tax dollars. Every conversion carries pre-tax and after-tax money out in proportion to the whole pot.
A friend of Maya's tries the backdoor with an old rollover IRA in the background
She has $93,000 of pre-tax money in a rollover IRA from an old job, contributes a fresh $7,500 nondeductible, and converts the $7,500 expecting a tax-free move. The IRS sees one pot of $100,500, of which only $7,500 is after-tax basis. So her conversion is 93,000 divided by 100,500, or 92.5%, pre-tax: $6,940 of the $7,500 is taxable income, and only $560 comes through tax-free. In her 32% bracket that is a $2,221 bill she never planned for, and the remaining $6,940 of basis stays stranded in the IRA, tracked on Form 8606 for years of partial conversions to come.
The timing rule makes the trap retroactive. Form 8606 runs the pro-rata math using your IRA balances on December 31 of the conversion year, not on the day you convert. Execute a perfectly clean backdoor in February, then roll an old 401(k) into a rollover IRA in October, and that October balance reaches back and poisons February's conversion. Any year you convert, your traditional IRA pot needs to be empty of pre-tax money by New Year's Eve.
The fix: roll the pre-tax money into a 401(k)
The pro-rata pot counts IRAs only. Employer plans stay out of the math, which creates the standard repair: move the pre-tax money into one before year-end.
Maya's friend asks her current 401(k) whether it accepts roll-ins (most large plans do), then rolls the $93,000 of pre-tax IRA money into it. Basis cannot make that trip, which is exactly the point: the nondeductible $7,500 stays behind as the only IRA money left. By December 31 her IRA pot is 100% basis, the conversion is clean, and every future year's backdoor is too. The roll-in costs nothing in tax, and the main diligence is making sure the 401(k)'s funds are decent, since the money will live there.
If no plan will take the money, the honest options narrow: convert the whole pre-tax balance and pay tax on all of it, which only makes sense in a low-income year (Chapter 9's territory), or skip the backdoor entirely. Paying 32% on $93,000 to enable a $7,500 maneuver is the tax tail wagging a very large dog.
The mega backdoor: the same trick, ten times the size
Your $24,500 deferral is only one of a 401(k)'s ceilings. Plans also carry a much higher overall cap on everything that enters in a year: your deferrals, the employer match, and a third bucket most people have never noticed called after-tax contributions. The mega backdoor fills that third bucket and converts it to Roth, sheltering amounts the IRA-sized backdoor could never reach.
It only works if the plan offers two specific features, and many plans offer neither:
- After-tax contributions, a contribution type separate from both pre-tax and Roth deferrals, available above the $24,500 line.
- A way out while you still work there: an in-plan Roth conversion, or in-service rollovers to a Roth IRA. Without an exit, after-tax money sits earning taxable-someday gains, which defeats the purpose.
Search your plan documents for those two phrases, or ask the administrator both questions directly. If the answer is yes twice, the move is mechanical: contribute after-tax dollars from each paycheck and convert them immediately, ideally with the plan's automatic-conversion setting, so no taxable earnings build up between the two steps. There is no pro-rata problem here, because IRA balances never enter the math.
Maya's plan has both features. She defers the full $24,500 pre-tax at her 35% bracket, collects her match, and then routes $20,000 a year of after-tax contributions through the automatic in-plan conversion. Each payroll's after-tax dollars become Roth dollars the same week, so her conversions add a few dollars of taxable income a year, not thousands. At 7% returns, fifteen years of this builds about $502,580 of Roth money. The same $20,000 a year in her taxable account, with dividends and the final gain taxed at her 18.8% investment rates, would finish around $40,000 poorer after tax. The mega backdoor is the largest recurring tax shelter available to her, and it required one form and one checkbox to set up.
Is it worth the paperwork
The value is real, but it arrives slowly
The backdoor shelters $7,500 a year. In year one, the benefit versus a taxable account is the tax on one year of dividends, maybe $25. The payoff compounds: by the time a decade of contributions is growing, the savings run to thousands, and the cost stays fixed at thirty minutes and one Form 8606 a year. Do it if your IRA pot is clean or cleanable through a 401(k) roll-in. Skip it, without guilt, if a large pre-tax IRA has nowhere to go; fighting the pro-rata rule costs more than the door is worth. The mega backdoor moves bigger dollars and earns a bigger verdict: if your plan supports automatic conversion, it is close to free money for high savers, and if the plan lacks the features, there is no workaround. Either way, neither door comes before Chapter 7's match and HSA.
Before any backdoor conversion, check one number: your combined traditional, SEP, and SIMPLE IRA balance projected to December 31. If it will not be zero, fix that first with a 401(k) roll-in or do not convert. Then contribute, convert within days, and file Form 8606 every single year.
Where people go wrong
- Converting with a forgotten rollover IRA in the background. The pro-rata rule counts every traditional, SEP, and SIMPLE IRA you own, not just the one you funded this year.
- Rolling an old 401(k) into an IRA in the same year as a backdoor. The December 31 test makes a clean spring conversion retroactively taxable.
- Contributing but never converting. The $7,500 sits in a money market for years, building exactly the pre-tax earnings the prompt conversion was meant to avoid.
- Skipping Form 8606. Without it the IRS assumes the conversion is fully taxable, and you pay tax twice on the same dollars.
- Confusing the Roth 401(k) with the mega backdoor. Roth deferrals have no income limit and fit inside $24,500; the mega backdoor is the separate after-tax bucket above it.
Key takeaways
- Income limits block direct Roth IRA contributions, but nondeductible contributions and Roth conversions have no income limit; chained together they are the backdoor.
- The recipe: contribute $7,500, convert within days, invest in the Roth, file Form 8606. Done cleanly, the taxable amount is pocket change.
- The pro-rata rule treats all your IRAs as one pot on December 31: with $93,000 of pre-tax money alongside, a $7,500 conversion is 92.5% taxable, $6,940 of surprise income.
- The standard fix is rolling pre-tax IRA money into a current 401(k) before year-end, leaving only basis behind to convert.
- The mega backdoor needs two plan features, after-tax contributions and an in-plan conversion or in-service rollover; Maya runs $20,000 a year through hers, worth roughly $40,000 over fifteen years versus taxable investing.
Sources: IRS: Publication 590-A · IRS: Publication 590-B · IRS: About Form 8606 · IRS: 2026 Tax Inflation Adjustments · Finvest Personal Finance Guide