Mega Backdoor Roth Calculator (2026)

Your Mega Backdoor Roth (MBR) space is the gap between the IRS's § 415(c) annual additions limit ($72,000 in 2026) and the sum of your employee elective deferral plus your employer's contributions. This calculator tells you exactly how much after-tax room you have — and what it's worth over a career.

How the MBR space calculation works

The IRS sets two separate contribution limits for 401(k) plans:

• Employee elective deferral limit — $24,500 (2026).¹ Caps the amount you contribute as pre-tax or Roth employee deferrals.
• § 415(c) annual additions limit — $72,000 (2026).¹ Caps the total added to the account from all sources: employee deferrals + employer contributions + after-tax employee contributions.

The Mega Backdoor Roth lives in the gap between those two numbers:

Example: $200K base salary, 6% employer match ($12,000/yr). MBR space = $72,000 − $24,500 − $12,000 = $35,500.

For higher salaries with larger employer matches, the remaining space shrinks. For lower salaries or smaller matches, it's larger.

Catch-up contributions for ages 50+ (above the $72K limit)

Catch-up contributions are layered on top of the $72,000 § 415(c) limit — they do not reduce your MBR space:

• Ages 50–59 and 64+: $8,000 additional catch-up in 2026¹
• Ages 60–63 (SECURE 2.0 "super catch-up"): $11,250 in 2026²

A 55-year-old with $35,500 of MBR space can put in a 2026 grand total of $72,000 + $8,000 = $80,000.

High-earner Roth catch-up rule (effective 2026)

Under SECURE 2.0 § 603, starting in 2026, employees age 50 or older whose prior-year FICA wages exceeded $145,000 must make all 401(k) catch-up contributions to a Roth account — not pre-tax.³ For most senior tech employees this threshold is crossed easily. The dollar amount of the catch-up doesn't change; what changes is the tax treatment (you pay income tax now rather than at withdrawal).

Steps to use your MBR space

1. Confirm after-tax contributions are available. Log into your 401(k) portal — Fidelity NetBenefits, Vanguard, Schwab — and look for a contribution type called "after-tax" (distinct from "Roth" and "pre-tax"). If you only see two options, MBR is not available at your employer.
2. Enable automatic in-plan Roth conversion. Most plans allow automatic daily or monthly conversion of after-tax contributions to Roth 401(k). This is critical: after-tax dollars that sit in the plan without converting accrue taxable earnings.
3. Set your after-tax contribution election. Usually expressed as a % of salary. If your MBR space is $35,500 and base is $200K, that's approximately an 17.75% after-tax election.
4. Recalculate every January. The IRS limits adjust annually; employer contributions change with raises and plan updates. Update your elections each new year.