"Backdoor Roth" and "mega backdoor Roth" are two different maneuvers that happen to share a name. Both are set up the same way in FIREproof - as a Cash Flow Priority, not as an adjustment - and you can reach both from three places: the Cash Flow step, the Tax Strategy step's "Add Tax Strategy" picker, or the Job Details section of the Job itself. Whichever door you use, one goal is stored, and it shows up in all three.

The Job door is the shortest: open the Job on the Income step, expand Job Details, and use Set up 401k contributions, Set up Backdoor Roth, Set up Mega Backdoor Roth, or Set up other contributions for anything else that comes out of that paycheck. Each opens the goal form with that job already linked; the three guided ones also narrow the account list to that person's eligible accounts, so the only thing left to answer is which account. If you do not have that account yet, the form says so and offers Create this account now - you fill it in, close it, and land back on the goal with the new account selected. Saving (or abandoning) the goal returns you to the Job.

Backdoor Roth IRA

In the real world, a high earner cannot contribute to a Roth IRA directly, so they contribute to a Traditional IRA without taking the deduction and immediately convert it. The two steps exist purely to get around the income limit; the money is post-tax on the way in and Roth on the way out.

In FIREproof, point a Cash Flow Priority at your Roth IRA and tick Backdoor Roth. The engine models the real IRS income rules for IRA contributions, so the flag is what makes the maneuver work the way it does in real life:

• Earned income is required. An IRA contribution needs compensation - wages (a Job or Take-Home Salary) or self-employment earnings - and cannot exceed it. A retired household's surplus cannot fund an IRA, and the year detail says so when this binds. Under married-filing-jointly, combined household compensation covers both spouses' IRAs (the spousal IRA).
• Direct Roth IRA contributions phase out by income. Above the IRS MAGI band a plain Roth IRA goal funds a reduced amount, then nothing, with the reason in the year detail. The Backdoor Roth flag routes the contribution through a nondeductible Traditional IRA and converts it immediately - conversions have no income limit, so the flagged goal funds the full IRA limit at any income.
• The pro-rata rule is modeled. If you hold pre-tax money in a Traditional IRA, part of each backdoor conversion is taxable ordinary income, exactly as Form 8606 computes it, and the year detail reports it. With no pre-tax IRA money the conversion is fully tax-free.
• Traditional IRA contributions are deducted when you qualify. With no workplace plan the deduction is unlimited by income; when you (or your spouse) are covered by a 401(k)-type plan it phases out by MAGI, and the nondeductible remainder is tracked as basis so those dollars are never taxed twice on the way out.
• The conversion is instant, with zero earnings. No taxable gain accrues between the contribution and the conversion.
• The annual IRA limit still applies. A goal asking for more than the IRA limit contributes the limit and the remainder flows to the next priority. If you meant to save far more than that in a Roth, you want the mega backdoor below.
• One limit covers both IRA types. Your Traditional and Roth IRAs share a single annual contribution limit per person, so two goals pointed into them split one allowance rather than getting one each - the second funds only what the first leaves. When you set up a goal alongside an existing one, the goal editor names the goal it shares the limit with. A mega backdoor rollover is exempt: it is not an IRA contribution, so it does not consume this limit.

Mega backdoor Roth

This is a different maneuver with a much larger ceiling. Some 401(k) plans let you contribute after-tax dollars on top of your regular employee deferral, then convert those dollars to Roth inside the plan. The employee deferral limit does not apply to them - the ceiling is the total plan limit, which covers your deferrals, your employer's match, any non-elective employer contribution, and this after-tax money together.

In 2026 that is $24,500 of employee deferral inside a $72,000 total plan limit, so a maxed-out saver whose employer adds nothing has roughly $47,500 of after-tax room. Both figures rise with age: at 50 to 59 (and 64+) each gains a $8,000 catch-up, and at 60 to 63 the enhanced catch-up is $11,250.

Two ways the money leaves the plan

After-tax 401(k) dollars have two legal exits, and FIREproof supports both. Which one you have depends on your plan, so pick the account that matches how yours actually works.

• In-plan Roth rollover - the money moves into your plan's Roth 401(k) sub-account and stays inside the employer plan.
• In-service rollover to a Roth IRA - the older and arguably more common route. The money leaves the plan and lands in your Roth IRA. This is a rollover, not an IRA contribution, so the annual IRA limit does not apply to it. That is why a $34,000 mega backdoor goal into a Roth IRA is not cut down to the IRA limit the way an ordinary Roth IRA goal is.

Either way, the ceiling is the same: your employer plan's total limit, which the linked Job identifies. You never name a second account - FIREproof measures the limit against the plan that Job pays into, and the account you pick is only where the dollars end up. An ordinary Roth IRA goal running alongside a rollover into the same Roth IRA does not reduce your plan room, and the rollover does not use up your IRA limit.

Setting it up

1. Go to Cash Flow and click Add Priority (or use Set up Mega Backdoor Roth from the Tax Strategy picker or from the Job's Job Details section, either of which opens the same form pre-filled).
2. Choose your Roth 401(k) or your Roth IRA - whichever exit route your plan offers - and link it to the Job whose plan it belongs to. Both are required: the limit being filled belongs to that employer's plan.
3. Tick Mega Backdoor Roth (after-tax contributions). The checkbox only appears once the account and the job are both set.
4. Enter an amount, or pick Maximize Contribution to fill whatever room the plan has left after your deferrals and your employer's money.

How the money moves

• It comes out of your paycheck, the same way a deferral does, and it reduces your take-home pay. You will see it in the year detail by expanding the job's row under Income, as "Mega Backdoor Roth (after-tax)".
• It does not reduce your taxable wages. That is the whole point of after-tax money - you pay income tax on it now so it never gets taxed again.
• It does not earn employer match. Match applies to elective deferrals only.
• It is converted or rolled over immediately, with zero earnings. No taxable gain accrues in the after-tax bucket, so there is no second tax event and no basis to track. (In reality a Roth IRA rollover splits any earnings off to a Traditional IRA under Notice 2014-54; with zero earnings there is nothing to split.)

Where to see it in your results

Open the Year-by-year analysis on a working year and look for the Roth Activity category. It lists every dollar that landed in a Roth account that year and what the move cost in tax, so a conversion (taxed as ordinary income) sits directly beside a mega backdoor contribution (no further tax). The cash itself is also counted under Withdrawals and Deposits; the category restates it so the Roth picture is in one place.

Which one do I want?

• Roth IRA goal - a few thousand dollars a year, no employer involved, capped by the IRA limit.
• Mega backdoor Roth - potentially tens of thousands a year, requires an employer plan that offers after-tax contributions plus one of the two exit routes above, capped by the total plan limit rather than by the IRA limit.

They are not mutually exclusive. A household maxing both would carry two goals: an ordinary one into the Roth IRA, and a job-linked flagged one into the Roth 401(k) or the Roth IRA. Pointing both at the same Roth IRA is fine - the IRA limit binds the ordinary goal only.