Mega Backdoor Roth Alternatives: Where $24K Goes Instead

Priya is a 38-year-old software engineer in Austin, Texas, single, earning $250,000 in salary and RSUs. She maxes her 401(k) elective deferral, gets a 5% employer match, and after taxes and her match she has about $24,000 a year left over she wants to invest for retirement. She read about the mega backdoor Roth — the move that lets you stuff after-tax dollars into your 401(k) and convert them to Roth, sheltering up to the $72,000 §415(c) limit. So she called her plan administrator. The answer: “Our plan doesn’t allow after-tax contributions.” Door closed.

That answer is the rule, not the exception. The IRS estimates well under half of large 401(k) plans permit both after-tax contributions and in-service withdrawals — the two features the mega backdoor requires. So the real question for Priya, and for most high earners, isn’t “how do I do the mega backdoor?” It’s “the mega backdoor is impossible — where does my $24,000 go instead?” Here is the ranked answer, with the math.

The fallback ladder, in order

When the after-tax bucket is closed, deploy fresh dollars in this exact sequence. Each rung is more tax-efficient than the one below it, so you fill them top-down:

1. Finish the Roth 401(k) elective deferral — up to $24,500 (2026). If you’re not already maxing the $24,500 §402(g) limit, this is dollar one. It’s the same tax-free-growth bucket the mega backdoor targets, just capped lower.
2. Fund the HSA to the family limit — $8,750 (or $4,400 self-only). The only triple-tax-advantaged account in the code. Used as a stealth retirement account, it beats everything below it.
3. Backdoor Roth IRA — $7,500 (+$1,000 catch-up at 50+). Needs no plan feature, so it stacks on top regardless of your 401(k).
4. Direct-indexed taxable brokerage — everything that’s left. No contribution cap, full liquidity, and direct indexing minimizes the tax drag that makes taxable the last resort.

For Priya, the order matters because her $24,000 of savings doesn’t fit in one bucket. The ranking tells her which dollars buy tax-free growth and which ones land in the only place left that charges tax every year.

Rung 1: Max the Roth 401(k) deferral first ($24,500)

The mega backdoor exists to push more Roth money into your 401(k) than the standard deferral allows. So before you reach for any substitute, confirm you’ve actually maxed the standard deferral itself. For 2026 the §402(g) employee deferral limit is $24,500 (plus an $8,000 catch-up at 50+, and an $11,250 super catch-up at ages 60–63 under SECURE 2.0 §109). If your plan offers a Roth 401(k) option — most now do — route that full $24,500 to the Roth side.

Why Roth over traditional here? Priya is in the 35% federal bracket (single, $250,526–$626,350 for 2026). That argues for the traditional deduction today — but she’s 38 with decades of compounding ahead, and tax-free growth on 27 years of returns usually wins for a long horizon. The honest answer: split it if you want a hedge. The point that’s not debatable is that the $24,500 deferral — in any flavor — comes before a single brokerage dollar, because it’s sheltered space you can never get back once the year closes.

Rung 2: The HSA is the best account nobody calls a retirement account

If Priya is enrolled in a high-deductible health plan, the Health Savings Account is the single most tax-efficient bucket available to her — better than the Roth, better than the traditional 401(k). It is the only triple-tax-advantaged account in the Internal Revenue Code (IRC §223):

• Deductible going in — contributions reduce taxable income like a traditional 401(k).
• Tax-free growth — investments compound with no annual tax drag, like a Roth.
• Tax-free coming out for qualified medical expenses — forever, at any age.

The 2026 HSA limits are $8,750 family and $4,400 self-only (plus a $1,000 catch-up at 55+), per IRC §223(b). The stealth-retirement move: pay current medical bills out of pocket, save the receipts, and let the HSA grow invested for 25 years. Because there’s no deadline to reimburse yourself, you can withdraw decades later, tax-free, against those old receipts. And after age 65, withdrawals for any purpose are penalty-free — taxed at ordinary rates exactly like a traditional IRA, so the worst case is “as good as a 401(k),” and the best case is tax-free forever.

That asymmetry is why the HSA outranks the brokerage and even a backdoor Roth IRA for the next available dollars. There is no version of the HSA that does worse than a taxable account.

Rung 3: The backdoor Roth IRA stacks on top of everything

At $250,000, Priya is far above the 2026 Roth IRA income phase-out (single $150,000–$165,000), so she can’t contribute to a Roth IRA directly. The backdoor Roth fixes that, and it needs no plan feature — it’s entirely separate from her 401(k). She contributes $7,500 to a nondeductible traditional IRA, then converts it to Roth. There’s no income limit on conversions (per the Roth conversion rules), so the maneuver is fully legal.

The one trap is the pro-rata rule under IRC §408(d)(2): if she holds any pre-tax IRA balance — a rollover IRA from an old job, a SEP, a SIMPLE — the IRS treats every conversion dollar as a blend of pre-tax and after-tax, and a slice of the conversion becomes taxable. The clean workaround is to roll those pre-tax IRA balances into her current 401(k) before December 31, leaving a $0 pre-tax IRA balance so the conversion is fully tax-free. That’s another $7,500 of Roth space per year that exists whether or not her plan ever offers after-tax contributions.

Rung 4: A direct-indexed taxable account for the rest

After the Roth 401(k), HSA, and backdoor Roth IRA are full, the remaining dollars land in a taxable brokerage — the only rung that gets taxed every year. There is no contribution cap and full liquidity, but you owe tax on dividends annually and 15% or 20% long-term capital gains plus the 3.8% NIIT on realized gains. For Priya, whose MAGI is well over the $200,000 single NIIT threshold (IRC §1411), the effective LTCG rate is 18.8% on gains in the 15% LTCG band and 23.8% at the top.

Direct indexing is what makes this rung tolerable. Instead of buying a single S&P 500 fund, you hold the individual stocks that make up the index in a separately managed account. When any one stock dips, the manager sells it at a loss, books the tax loss to offset other gains, and buys a similar stock to keep your exposure intact. Those harvested losses offset her RSU gains and up to $3,000 of ordinary income a year, with the excess carried forward. In practice, direct indexing can add roughly 1–2% of after-tax return a year in the early years for a high earner with gains to offset — which is exactly why it beats a plain index fund in a taxable account, even though it can’t beat a Roth.

The math: where Priya’s $24,000 actually goes

Priya already maxes her $24,500 deferral, so rung 1 is done. Her family is on a self-only HDHP, so her HSA limit is $4,400. That leaves the ladder to absorb the $24,000:

The ladder routes $11,900 of the $24,000 into tax-free or triple-tax-free space and only $12,100 into the taxed account. Compare that to the lazy default — dumping all $24,000 into a brokerage. Over 25 years at a 7% return, the $11,900 in Roth/HSA space compounds to roughly $64,600 entirely tax-free. The same dollars in a taxable account, after annual dividend tax and an 18.8% hit on the final gain, would net closer to $54,000 — a difference of more than $10,000 from sequencing alone, on just one year’s contributions. Repeat that every year and the gap compounds into six figures.

What most people miss: how much space the missing bucket really costs

The instinct is to shrug off a missing after-tax option as a minor inconvenience. It isn’t. The after-tax bucket can hold up to the §415(c) gap — the $72,000 total 401(k) limit (2026), minus your $24,500 elective deferral, minus any employer match. For Priya with a 5% match on her $250,000 — about $12,500, well under the §401(a)(17) compensation cap — the after-tax space she can’t use is about $35,000 a year.

That’s $35,000 that would have grown tax-free in Roth space and is now forced into a taxable account. Over 20 years at 7%, $35,000 a year compounds to roughly $1.4 million. In a Roth, every dollar of that is tax-free. In a taxable account, the embedded gains face 18.8–23.8% on the way out. The cost of the missing bucket isn’t the inconvenience — it’s a multi-six-figure tax difference over a career. That’s exactly why the fallback ranking is worth getting right instead of defaulting to a brokerage.

The myth: “no after-tax option means no Roth strategy”

The most common misread is that losing the mega backdoor means losing tax-free growth entirely. False. Stack the rungs that need no plan feature and a high earner still shelters a large slug of fresh contributions every year:

• Roth 401(k) deferral: $24,500
• HSA (family): $8,750
• Backdoor Roth IRA: $7,500
• Total tax-advantaged space without any after-tax 401(k): over $40,000/year

The mega backdoor would add the ~$35,000 §415(c) gap on top of that. Losing it hurts — but it doesn’t leave you with “just a brokerage account.” It leaves you with a $40,000+ tax-advantaged runway and one taxed account for the overflow. Most people never fill even the first $40,000.

Before you settle for the fallbacks: push on the plan

One step worth taking before you accept the ranking: ask whether your employer will add after-tax contributions and in-service rollovers to the plan. These are plan-design choices the employer can amend, not statutory bars. At a tech company full of high earners, an HR or benefits team that hears the request from several employees may add the feature at the next plan amendment cycle. It costs nothing to ask, and if it lands, the mega backdoor reopens. If the answer stays no — which it will for most plans — the ladder above is your move.

The decision lever

A closed after-tax bucket doesn’t change whether you save — it changes where the next dollar lands. Fill in strict order: Roth 401(k) deferral to $24,500, HSA to $8,750 family ($4,400 self-only), backdoor Roth IRA to $7,500, then a direct-indexed taxable account for the rest. The lever you control is the sequence. Every dollar you route into the top three rungs before touching the brokerage is a dollar that never pays the 18.8–23.8% capital-gains-plus-NIIT toll — and over a career, getting that order right is the difference between a taxable account and a tax-free one for tens of thousands of dollars.