Mega Backdoor Roth Calculator

2026 rules

Find how much after-tax money your 401(k) can take this year for a mega backdoor Roth.

Your numbers

Salary

Your pre-tax and Roth 401(k) deferrals

2026 limit: $24,500 (catch-up contributions don't count here).

Employer match and profit sharing (% of salary)

Plan's after-tax limit (% of salary), if any

Leave at 0 if the plan has no separate cap.

Result

After-tax room for a mega backdoor Roth$37,500

2026 total limit (section 415)$72,000
Your deferrals-$24,500
Employer contributions-$10,000
After-tax contributions allowed$37,500
Converted to Roth each year$37,500

The plan must allow after-tax contributions and in-plan Roth conversions or in-service withdrawals. Convert quickly so little growth is taxable.

How it works

All contributions to one employer's plan, yours and the employer's, are capped at $72,000 in 2026 (catch-up contributions are on top). Whatever room is left after your $24,500 of deferrals and the employer's match can go in as after-tax contributions.

Those after-tax dollars become Roth money when the plan converts them in-plan or lets you roll them to a Roth IRA. Converting soon after each contribution keeps the taxable growth near zero.

2026 figures: IRS Rev. Proc. 2025-32 (2026 inflation adjustments); IRS Notice 2025-67 (2026 retirement plan limits); CMS 2026 Medicare Parts A & B premiums and deductibles.

Questions

Does every 401(k) allow it?

No. The plan must accept after-tax contributions and allow in-plan Roth conversions or in-service withdrawals. Check your summary plan description.

Can highly paid employees use it?

Testing rules can limit after-tax contributions for highly compensated employees ($160,000 or more in 2025 pay). Some plans refund part of them after the year.