The $23,500 elective deferral limit is not the ceiling on your 401(k) - the $70,000 annual additions limit is. Enter your salary and what the plan has already absorbed to find the after-tax room a mega backdoor Roth can use, and what that room is worth against a plain taxable account.
Situation: Situation: You earn $180,000, contribute $23,500 pre-tax, and your employer adds $9,000. Total used is $32,500.
Rate / rule: Rate: The 2026 415(c) limit is $70,000, so your remaining room is $70,000 - $32,500 = $37,500.
Calculation: Calculation: Contribute $37,500 after-tax, convert immediately to Roth. To max the $23,500 employee elective limit you would first need to top up pre-tax or Roth to $23,500 (it already is), leaving $37,500 of employer + after-tax space.
Situation: Situation: Your plan accepts after-tax dollars but does not permit in-plan Roth conversions.
Rate / rule: Rate: After-tax money that cannot be converted grows tax-deferred, but earnings are taxed as ordinary income on withdrawal - you lose the entire advantage.
Calculation: Calculation: The only workaround is to roll the after-tax sub-account to a Roth IRA after separation from service. During employment the earnings compound as ordinary income.
Situation: Situation: Age 45, retiring at 65. After-tax contributions of $37,500/year, 7% expected return.
Rate / rule: Rate: The 415(c) limit does not apply to a Roth IRA or taxable account, but the IRA is capped at $7,000/year - so $30,500 of your $37,500 has nowhere else to go.
Calculation: Calculation: Mega backdoor Roth grows to $1,537,331 tax-free. The same contributions in a taxable account, after a 15% annual tax drag on growth ($37,500 at 7% becomes 5.95%) and a simplified 8% liquidation cost, reach roughly $1,262,303.
415(c) annual additions limit = $70,000 in 2026. This caps everything going into the plan from all sources combined, with a separate catch-up for age 50+.
402(g) employee elective deferral limit = $23,500 in 2026. This caps only what you elect to defer from salary, pre-tax or Roth.
After-tax contributions are not deferrals, so they are not counted against the $23,500 — only against the $70,000 ceiling. That gap is the entire strategy.
In-plan Roth conversion = moving the after-tax sub-account into the plan's designated Roth account. Because the basis was already taxed, the conversion is not taxable income.
| Limit | 2026 Amount | What It Covers | Catch-Up Age 50+ |
|---|---|---|---|
| 402(g) elective deferral | $23,500 | Your pre-tax + Roth 401(k) elections | +$7,500 |
| 415(c) annual additions | $70,000 | Employee + employer + after-tax | +$7,500 |
| 457(b) if also eligible | $23,500 | Separate limit, government plans only | +$7,500 |
| Traditional / Roth IRA | $7,000 | Separate from any plan | +$1,000 |
| Compensation cap | $350,000 | Salary counted for match and % limits | — |
The difference between the $70,000 and $23,500 lines — $46,500 — is the theoretical maximum after-tax space, but only if your employer contributes nothing. Employer contributions consume that space first.
The clean version of this strategy is sometimes called a “megabackdoor Roth,” and the word that matters is immediate. After-tax dollars sitting in the plan generate pre-tax earnings. When you eventually convert, those earnings are ordinary income. Converting in the same pay period you contribute leaves almost no earnings, so almost nothing is taxable.
Practical check: ask whether the plan allows conversions per pay period, or only once per year. Annual conversions on a full year of earnings create a taxable event that wipes out much of the benefit.
Pro-rata rule does not apply to in-plan conversions of a segregated after-tax sub-account, but it does apply if you roll after-tax money out to a traditional IRA first.
The strategy is real, but availability is far narrower than the internet suggests. Three conditions must all be true, and most plans satisfy fewer than three.
| Plan Feature | Why It Matters | How to Check |
|---|---|---|
| After-tax contributions permitted | Without this there is no space above the elective deferral limit | Summary Plan Description, "Types of Contributions" |
| In-plan Roth conversion allowed | Converts the after-tax sub-account to tax-free growth without a taxable event | Ask HR; some plans allow conversion only at termination |
| Conversion frequency | Per-pay-period conversion keeps taxable earnings near zero | Recordkeeper phone call, not the SPD |
| No ACP test failure | Highly compensated employees can be refunded, undoing the contribution | Plan's annual nondiscrimination testing notice |
The fourth row is the one people forget. If your plan fails the actual contribution percentage test, the recordkeeper refunds after-tax money to highly compensated employees in the following year, and the refund carries its own tax reporting. A plan that allows the feature is not the same as a plan that reliably delivers it.
For a high earner who has already used the obvious accounts, the sequencing question is which dollar goes where first. The ordering below reflects tax efficiency per dollar of contribution.
An immediate, guaranteed return with no market risk. Nothing else competes with free money, so fill this before anything else.
The only triple-tax-advantaged account: deductible in, tax-free growth, tax-free for qualified medical costs. The 2026 family limit is $8,750 with a $1,000 catch-up at 55.
The $23,500 pre-tax or Roth election. Pre-tax is the better comparison against the current marginal rate; Roth is better if you expect higher rates later.
Up to the $70,000 ceiling. No deduction now, but all future growth is tax-free. The advantage over a taxable account is roughly the annual tax drag on growth, compounded.
Unlimited, liquid, and the only option once the plan is full. A 15% annual tax drag on growth is the price, plus a basis calculation at liquidation.
The gap between the after-tax option and the taxable account is not the contribution — both use post-tax dollars. It is entirely the tax drag on growth, compounded over the decades until retirement. On $37,500/year at 7% for 20 years that gap is roughly $275,000.
Three errors convert a tax-free strategy into ordinary income.
A fourth, quieter problem is the state tax on conversion if you move to a different state between contribution and conversion. The conversion basis is federal, but state treatment of after-tax contributions varies — a few states do not recognize the basis and tax the conversion as income.
⚠ Important Disclaimer: After-tax contributions and in-plan Roth conversions depend entirely on your specific plan document and recordkeeper - features described here are permitted by law but are not required in any plan. Contribution limits are 2026 figures and are indexed annually. Projections use a constant rate of return and a simplified tax-drag model; actual results will differ. Confirm plan features with your HR department and consult a tax professional before electing after-tax contributions.