⚡ Interactive Tax-Optimization Tool

Mega Backdoor Roth Contribution Calculator

Determine your exact remaining after-tax 401(k) space for a Mega Backdoor Roth conversion after accounting for employee deferrals, employer matches, and IRS compensation limits.

🧮 Enter Your 401(k) Plan Details

Updated with official IRS 2026 contribution limits (Section 415(c) & Section 401(a)(17)).

2026 IRS Cap: $355k
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IRS Rule: Under IRC Section 401(a)(17), 2026 employer matches are capped at compensation up to $355,000.

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Standard 2026 employee elective deferral (Traditional 401(k) + Roth 401(k) combined).

Employer Match Formula Look up company match →
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%
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Your 401(k) Breakdown Total Cap: $71,000
Available After-Tax Contribution Room
$41,600
Eligible for Mega Backdoor Roth Conversion
Section 415(c) Bucket Allocation $71,000 Max
1. Elective (34%) 2. Match (8%) 3. After-Tax (58%)
IRS Overall Limit (Sec. 415c): $71,000
(-) Employee Deferral: -$24,000
(-) Employer Match: 50% on first 6% of $180,000
-$5,400
(=) Remaining Mega Backdoor Space: $41,600
Follow Step-by-Step Setup Guide
🗓️ Per-Paycheck Allocation Payroll Deductions

To hit your maximum after-tax limit evenly throughout the tax year without under-contributing or front-loading, set your payroll deduction to:

Bi-Weekly (26 Paychecks/yr) $1,600.00 every 2 weeks
Semi-Monthly (24 Paychecks/yr) $1,733.33 twice per month (15th & 30th)
💡 True-Up Tip: If your company offers a 401(k) true-up provision, you can max out earlier in the year without forfeiting your employer match.
Buck by Buck • 401(k) Summary
Mega Backdoor Roth Plan
After-Tax Space $41,600
Overall 415(c) Cap $71,000
Employee Deferral $24,000
Employer Match $5,400
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How the 2026 Mega Backdoor Roth Contribution Limits Work

The Mega Backdoor Roth strategy allows high-income earners to exceed the standard $24,000 elective deferral limit and contribute up to the total IRS Section 415(c) limit ($71,000 for under age 50 in 2026) into tax-free Roth accounts.

TL;DR Calculation Formulas: Maximum After-Tax 401(k) Space

Maximum After-Tax Capacity = Section 415(c) Limit - Employee Elective Deferrals - Employer Match
Eligible Employer Match Base = Min(Annual Salary, Section 401(a)(17) Compensation Cap)
Per-Paycheck Contribution Target = Maximum After-Tax Capacity / Remaining Pay Periods in Tax Year

🏛️ IRS Section 401(a)(17) Salary Cap

Even if your compensation is $500,000, federal law caps the maximum salary recognized for 401(k) matching at $355,000 in 2026. Any percentage match formula only calculates up to this ceiling.

🔄 Automatic In-Plan Conversions

After making voluntary after-tax contributions, you must immediately convert them to a Roth 401(k) or Roth IRA. Doing this automatically prevents earnings from growing in a taxable pre-tax bucket.

📥 Free Printable PDF Guide 1-Page Printable PDF

Download the Mega Backdoor Roth Decision Tree (PDF)

Plan compliance checklist and 401(k) waterfall verification flowchart. Keep on hand when reviewing plan summary documents or speaking with HR.

  • ✓ Clean, 1-page printable reference cheat sheet
  • ✓ Instant direct access and email delivery
  • ✓ Updated with current statutory limits & guidelines

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Discussion

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Does your employer plan allow in-plan Roth conversions or in-service withdrawals?

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