Mega Backdoor Roth Calculator 2026 β€” After-Tax 401(k) Contribution Analyzer

Calculate your mega backdoor Roth contribution space and see how much tax-free wealth you can build over time vs a taxable brokerage account.

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Of salary (e.g. 5% match = $10,000 on $200K)
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2026 employee limit: $24,500 ($31,000 if age 50+)
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2026 total limit (employee + employer): $70,000
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Common salaries:
$0
Max Mega Backdoor Contribution
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After-Tax Space Available
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Projected Roth Value at Retirement
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Estimated Lifetime Tax Savings vs Taxable

Year-by-Year Roth Accumulation (10 Years)

Year Annual Contribution Growth Cumulative Roth Balance

How the Mega Backdoor Roth Works

The mega backdoor Roth takes advantage of the gap between the employee elective deferral limit ($24,500 in 2026) and the overall 401(k) addition limit ($70,000 in 2026). The space between your employee contributions plus employer match and the $70,000 ceiling can be filled with after-tax contributions β€” which you then convert to Roth immediately.

The key requirement: your employer's 401(k) plan must allow after-tax contributions AND either in-plan Roth conversions or in-service withdrawals. Without these plan features, the mega backdoor strategy is not available to you.

The Formula

Employer Match = Salary Γ— Match Percentage
After-Tax Space = Total Limit ($70,000) βˆ’ Employee Pre-Tax βˆ’ Employer Match
Max Mega Backdoor = After-Tax Space
Future Roth Value = Annual Contribution Γ— [(1+r)^n βˆ’ 1] Γ· r
Tax Savings vs Taxable = Future Value Γ— Assumed LTCG Rate (15%)

Example

$200K salary, 5% employer match, max employee contribution, 20 years, 7% return:
Employer match: $200,000 Γ— 5% = $10,000
After-tax space: $70,000 βˆ’ $24,500 βˆ’ $10,000 = $36,500/year
Future Roth value: $36,500/year at 7% for 20 years = $1,591,800
Tax saved vs taxable (15% LTCG): ~$239,000 in lifetime tax savings
Plus tax-free withdrawals in retirement!

Who Can Use Mega Backdoor Roth?

You can use this strategy if your employer's plan allows: (1) after-tax 401(k) contributions beyond the pre-tax limit, and (2) in-plan Roth conversions or in-service distributions. Large tech companies (Google, Meta, Amazon, Microsoft) commonly offer this. Government 403(b) plans rarely do. Always verify with your plan documents or HR before contributing.

Extended

Mega Backdoor Roth vs Taxable Brokerage Comparison

Side-by-side long-term projection: tax-free Roth growth vs after-tax brokerage with annual tax drag

Compare the long-term wealth difference between investing through the mega backdoor Roth versus a taxable brokerage account with annual tax drag.

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Synced from main calculator
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Effective annual drag on taxable growth

Roth vs Taxable Brokerage β€” Wealth Comparison

Time Horizon Roth Balance (tax-free) Taxable Balance (after-tax) Roth Advantage

Frequently Asked Questions

What is the mega backdoor Roth?
The mega backdoor Roth is a strategy that allows you to contribute up to $46,500 extra to a Roth account beyond the standard $24,500 employee 401(k) limit. You make after-tax (non-Roth) contributions to your 401(k), then immediately convert them to Roth via an in-plan conversion or roll them to a Roth IRA. The total 2026 401(k) limit is $70,000 (including employee + employer contributions), so your after-tax space equals $70,000 minus your pre-tax contributions and employer match.
Does my employer need to allow mega backdoor Roth?
Yes. Your 401(k) plan must allow two things: (1) after-tax contributions beyond the standard employee limit, and (2) either in-plan Roth conversions or in-service distributions so you can move the money to a Roth IRA. Roughly half of large employer plans support this strategy. Check your plan documents or ask your HR department.
Is there an income limit for mega backdoor Roth contributions?
No. Unlike direct Roth IRA contributions (which phase out above $146,000 single / $230,000 married), the mega backdoor Roth has no income limit. This makes it particularly valuable for high earners who are otherwise shut out of Roth IRA contributions. The only limits are the overall 401(k) addition limit of $70,000 (2026).
What is the tax treatment of mega backdoor Roth?
The after-tax contributions themselves are made with money you have already paid income tax on. When you convert them to Roth immediately (sometimes called "in-plan Roth conversion"), any earnings since the contribution are taxable, but the contribution amount itself is not. This is why converting immediately β€” before earnings accumulate β€” is the recommended approach to avoid any tax on conversion.
How does the mega backdoor Roth compare to a regular taxable brokerage account?
A taxable brokerage account offers more flexibility (no withdrawal restrictions) but at the cost of annual tax drag from dividends and capital gains distributions. In a Roth account, growth is completely tax-free and qualified withdrawals are never taxed. Over 20-30 years, the compounding advantage of tax-free growth in a Roth versus paying 15-20% LTCG tax annually in a taxable account can be substantial β€” often hundreds of thousands of dollars on a large balance.

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