Mega Backdoor Roth Calculator (2026)
Estimate how much after-tax 401(k) space might remain after employee deferrals and employer contributions—if your plan allows after-tax deferrals and Roth conversions or rollovers.
Headroom inputs
How to Use the Mega Backdoor Estimate
Start from the same employee and employer dollars you would enter for combined limit planning. The result is only the arithmetic remainder—not a determination that your plan permits after-tax deferrals.
- Confirm plan features. You need after-tax contributions plus either in-plan Roth conversion or a rollover path.
- Model tax on the conversion. Converted after-tax earnings (not basis) are generally taxable—this site does not file your return.
How to Read the Results
Pro-rata and tracking
IRA aggregation rules and plan accounting can change conversion taxation; this calculator does not model those details.
What to Do Next
How We Reviewed This Tool
Tool-Level Methodology
- Defined mega backdoor Roth as remaining Section 415 room after elective deferrals and employer contributions, matching how most practitioners explain the spillover concept.
- Separated arithmetic headroom from plan-feature requirements so users do not confuse math with permissioning.
- Routed readers to Roth comparison and rollover tools when they want to move from room estimation to execution planning.
Assumption Review
- The plan must explicitly allow after-tax contributions and a conversion or rollover path; the calculator cannot detect plan features.
- Taxation of conversion is not modeled here beyond educational reminders about basis versus earnings.
- Pro-rata IRA rules and plan-level tracking nuances are intentionally out of scope for this lightweight estimator.
Update Log
- Introduced the mega backdoor page to cover after-tax 401(k) intent not previously isolated as its own tool.
- Synchronized headroom math with the Section 415 combined calculator.
- Emphasized plan-document confirmation because mega backdoor availability is highly employer-specific.
Who Mega Backdoor Roth Is For
High earners who already max pre-tax or Roth elective deferrals and still have Section 415 room sometimes use after-tax contributions as a spillover bucket—when the plan supports it.
Frequently Asked Questions
What is a mega backdoor Roth?
Mega backdoor Roth usually refers to making after-tax contributions inside a 401(k) up to remaining space under the plan's Section 415 limit, then converting those dollars to Roth within the plan or rolling them to a Roth IRA, depending on plan rules.
Does every employer offer mega backdoor Roth?
No. The plan must allow after-tax contributions and either in-plan Roth conversions or appropriate rollover features. Many plans do not offer the full stack.
Is this mega backdoor calculator tax advice?
No. It estimates remaining contribution room using a simplified Section 415 model. Actual taxation of conversions depends on your full tax picture.
How we document this page (E-E-A-T)
Experience. This page is written for U.S. workers navigating real payroll and plan rules, not abstract theory. Where we simplify, we say so explicitly so you can escalate to your plan’s summary plan description (SPD) or recordkeeper.
Expertise. 401lcalculator.com is an independent retirement planning tool site founded by David Jones (calculator methodology specialist). Limits and formulas are checked against IRS retirement-plan notices and SECURE 2.0 framing. Numeric caps align with our 2026 limits reference page, which cites Notice 2025-67.
Authoritativeness. For any conflict between this calculator and the IRS or your plan, the IRS and your plan win. Primary sources:
- IRS — Retirement Topics, 401(k) and profit-sharing plan contribution limits
- IRS Notice 2025-67 (PDF) — official 2026 dollar announcements.
Mega backdoor reality. Arithmetic headroom does not mean your plan offers after-tax deferrals, in-plan Roth conversions, or favorable rollover timing. Those features are SPD-level facts.
Trustworthiness. This tool is not individualized tax, legal, or wealth-management advice. Plan documents, payroll settings, and your full tax return facts can change outcomes. We publish calculator methodology in the sections above so you can compare our framing with your plan administrator or a licensed professional.
For one-on-one guidance, consult a CFP® professional. All math runs locally in your browser; see our privacy policy.
Corrections. If you believe a limit or IRS reference is out of date after publication, please contact us with a primary source link so we can verify and update.
- Reviewed by David Jones
- Limits Updated for 2026 IRS contribution caps
- Formulas Verified quarterly
Review & Methodology
- Reviewed by David Jones (calculator methodology).
- Updated for 2026 IRS contribution limits (refreshed after each annual IRS notice).
- Core calculator formulas are re-tested quarterly; limit-driven logic is checked when IRS guidance changes.
- Educational projections only — not investment, tax, or wealth-management advice. Calculations run locally in your browser.