Mega Backdoor Roth Calculator: How Much After-Tax Space Do You Have?

Your after-tax room is the IRC §415(c) total annual additions limit, $72,000 for 2026, minus your own elective deferral (up to $24,500) and any employer match or profit-sharing contribution. For example, $72,000 − $24,500 deferral − $10,000 match = $37,500 of after-tax space you can contribute and convert to Roth. Catch-up contributions ($8,000 at age 50+, or $11,250 at ages 60-63) are allowed in addition to the $72,000 ceiling, so they don't reduce your after-tax room. Your plan's own terms can impose lower caps, so the plan document controls.

Not all plans do, and this is the single biggest gating factor. Your plan must (1) allow voluntary after-tax (non-Roth) contributions, a separate source from Roth deferrals, and (2) allow either in-plan Roth conversions or in-service withdrawals so you can move the after-tax money to Roth quickly. Ask HR or your plan administrator for the Summary Plan Description and look for 'after-tax contributions' and 'in-plan Roth rollover' or 'in-service distribution' language. If your plan lacks these features, the strategy isn't available this year, though self-employed owners can build a Solo 401(k) that includes them.

No. The IRA pro-rata rule aggregates all your traditional IRAs, but it applies to IRAs, not to after-tax employee contributions inside a 401(k). Your 401(k) after-tax source is accounted for separately, so a large rollover IRA that would wreck a backdoor Roth IRA has no effect here. The only 'pro-rata' concern is inside the plan: when you convert, your after-tax basis converts tax-free, but any earnings that accrued before the conversion are taxable as ordinary income. That's why you convert as soon as each after-tax contribution posts.

Both are valid routes to Roth. An in-plan Roth conversion keeps the money inside your 401(k) and can often be automated so every after-tax dollar converts the moment it posts. An in-service rollover moves the money out: under IRS Notice 2014-54, you can direct the after-tax basis to a Roth IRA tax-free and send any pre-tax earnings to a Traditional IRA, deferring tax on them. A Roth IRA gives you more investment flexibility and no lifetime required minimum distributions; the in-plan route wins on speed and simplicity. Which is better depends on your plan's features and where you want the money to live.

Indirectly, yes. Catch-up contributions are allowed in addition to the $72,000 §415(c) limit, so deferring an extra $8,000 (age 50+) or $11,250 (ages 60-63) doesn't eat into your after-tax room. With the age-50 catch-up, your total deferral can reach $32,500 in 2026 and your effective all-in ceiling is about $80,000. The catch-up dollars themselves go into your regular deferral bucket (pre-tax or Roth), not the after-tax bucket, but they let you save more overall without shrinking the space this calculator measures.

For higher earners who have already captured the full employer match, maxed their elective deferral, and still have cash to invest, it's usually one of the best remaining moves, it can shelter $30,000-$40,000 a year of growth from tax for life, with no income limit to qualify. It's less compelling if your plan doesn't support it, if cash flow is tight, or if you still need a current-year deduction (after-tax contributions don't give you one). The deciding factors are your plan's features, your marginal bracket, and your surplus cash flow.