Solo 401k Mega Backdoor Roth 2026 After-Tax Calculator
A solo 401k mega backdoor Roth 2026 lets self-employed savers stack employee deferral, employer profit-share, and after-tax dollars up to the IRS $70,000 415(c) annual addition limit, then convert the after-tax bucket to Roth tax-free.
| Employee deferral (pre-tax or Roth) | — |
| Age-based catch-up | — |
| Employer profit-share (20–25% net SE) | — |
| After-tax non-Roth contribution | — |
| Total annual addition | — |
| Mega backdoor Roth conversion | — |
A solo 401k mega backdoor Roth 2026 lets self-employed savers stack employee deferral, employer profit-share, and after-tax dollars up to the IRS $70,000 415(c) annual addition limit, then convert the after-tax bucket to Roth tax-free. The 2026 employee deferral cap is $23,500, with a $7,500 catch-up at age 50+ and an enhanced $11,250 catch-up at ages 60–63 under SECURE Act 2.0.
How the Three Buckets Stack in 2026
Bucket 1: employee elective deferral up to $23,500 (pre-tax or Roth). Bucket 2: employer profit-share, calculated as 25% of W-2 wages for S-Corp owners or roughly 20% of net SE earnings (after half-SE-tax) for sole proprietors. Bucket 3: after-tax non-Roth, equal to $70,000 minus buckets 1 and 2. Catch-ups sit outside the $70,000 cap and extend the total to $77,500 (age 50+) or $81,250 (ages 60–63).
Converting the After-Tax Bucket to Roth
The after-tax bucket only becomes "mega backdoor Roth" once you execute an in-plan Roth conversion or in-service rollover to a Roth IRA. Most off-the-shelf solo 401k providers (Fidelity, Schwab, Vanguard) do not permit after-tax contributions — you need a custom prototype or self-directed solo 401k plan document that allows after-tax sub-account and in-plan Roth conversions. Convert quickly to avoid earnings becoming taxable on conversion.
Spousal Doubling and S-Corp Strategy
If your spouse is on payroll, they can run a parallel solo 401k and double the household limit toward $140,000+ in 2026 annual additions. S-Corp owners must base employer contribution on W-2 wages, not distributions, so balancing reasonable salary against profit-share is critical. Sole proprietors use net SE earnings minus half the SE tax as the contribution base per IRS Publication 560.
Common Solo 401k Mistakes
(1) Using a plan that doesn't allow after-tax contributions — you lose the mega backdoor entirely. (2) Forgetting that employee deferral ($23,500) is an aggregate limit across all 401k plans, including a day-job W-2 plan. (3) Missing the December 31 plan-establishment deadline for solo 401k (SECURE Act lets sole proprietors set up by the tax filing deadline for prior-year employer contributions only). (4) Skipping Form 5500-EZ once plan assets exceed $250,000.
Last updated May 2026. Sources: IRS Notice 2024-80 (2026 inflation adjustments), Publication 560, SECURE Act 2.0 Section 109.