High earners use the mega backdoor to push up to $46K into Roth via after-tax 401(k) contributions + in-plan conversion. Compute headroom available given existing contributions and employer match.
Detailed instructions, formula notes, and US-context guidance shown in the calculator above.
Disclaimer: Estimate only. Consult a qualified professional for decisions with major financial, legal, or health consequences.
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Calculator information
📋 How to use this calculator
- Enter age (under 50, 50+, or 60-63 for super catch-up).
- Enter annual salary.
- Enter your pre-tax + Roth 401(k) contributions for the year.
- Enter employer match (dollar amount).
- Review remaining after-tax 401(k) contribution headroom (mega backdoor amount).
- Plan in-plan Roth conversion or in-service distribution to Roth IRA.
🧮 Mega Backdoor Roth (415(c) Total Limit, 2026)
After_tax_cap = 415c_total - Employee_pretax_Roth - Employer_match
- 415(c) total 2026: $70,000 (under 50) / $77,500 (50+) / $81,250 (60-63 super catch-up)
- Employee pre-tax + Roth limit 2026: $23,500 base + $7,500 catch-up at 50+
- Employer match counts against 415(c), not employee limit
- After-tax contributions: post-tax money, but earnings still pre-tax until converted
- In-plan Roth rollover or in-service distribution → Roth IRA converts after-tax + earnings
Plan must offer (1) after-tax contributions AND (2) in-plan Roth conversion OR (3) in-service distribution. Not all 401(k) plans support both. Check Plan Document or call HR/benefits. The earlier you convert, the less taxable earnings build up between contribution and conversion.
💡 Worked example: Tech worker, 35, $200K salary, plan supports mega backdoor
Given:- Age: 35 (no catch-up)
- Salary: $200,000
- Pre-tax 401(k) contribution: $23,500 (max)
- Employer match: 50% of first 6% = $6,000
- 415(c) total cap (under 50): $70,000
Steps:- 415(c) - pre-tax - match = $70,000 - $23,500 - $6,000 = $40,500
- Contribute $40,500 in after-tax 401(k) over the year
- Each pay period (or annually), convert to Roth via in-plan rollover
- Total annual Roth funded: $23,500 + $40,500 = $64,000
- 30-yr growth at 7% on full Roth flow: ~$5.3M tax-free at 65
Result: $40,500/yr additional Roth contributions via mega backdoor. With market growth, this builds millions of tax-free retirement assets vs being stuck at the $7,000 Roth IRA limit.
❓ Frequently asked questions
Does my 401(k) plan allow mega backdoor Roth?
Check three things: (1) Does the plan accept AFTER-TAX (non-Roth) contributions beyond the regular pre-tax/Roth limit? (2) Does it offer IN-PLAN ROTH CONVERSION (Roth in-plan rollover)? (3) OR does it offer IN-SERVICE DISTRIBUTION (rolling out to Roth IRA while still employed)? Either #2 or #3 makes mega backdoor work. Common plans that support: Google, Meta, Amazon, Microsoft, most Fortune 500 tech. Many smaller employers don't. Call benefits or read the Summary Plan Description (SPD).
What's the difference between Roth 401(k) and mega backdoor Roth?
Roth 401(k) is the post-tax version of regular 401(k), subject to the SAME $23,500 employee limit. Mega backdoor uses the AFTER-TAX bucket — a third contribution type, allowed up to the $70K total 415(c) limit. After conversion, after-tax 401(k) money becomes Roth and grows tax-free thereafter. Mega backdoor effectively lets you put much more into Roth annually than regular limits allow.
What's the deadline to do the conversion?
There's no hard deadline, but earlier = better. Earnings on after-tax contributions are TAX-DEFERRED, not tax-free. If you contribute $40K after-tax and don't convert for 5 years, by then maybe $50K with earnings. The $40K converts to Roth tax-free; the $10K of earnings converts as taxable income. Convert each pay period (or annually) to minimize the taxable earnings portion. Some plans auto-convert each pay period — easiest setup.
Can I do mega backdoor if I'm self-employed?
Yes — Solo 401(k) plans from Fidelity, Schwab (limited), E*TRADE, MySolo401k allow after-tax contributions and in-plan conversion. As a self-employed sole prop with $200K net business income, you can shelter up to $70K total in Solo 401(k). Setup cost: $0-1,500 depending on custodian. Custom Solo 401(k) plan documents (~$500) often offer more flexibility than off-the-shelf Fidelity Solo 401(k).
Are mega backdoor Roth conversions reported to IRS?
Yes — Form 5498 (employer reports contributions), Form 1099-R (reports in-plan conversions). The taxable portion (earnings on after-tax that converted) shows on your 1040 as ordinary income. The non-taxable portion (your original after-tax contributions) reduces basis. No 10% penalty for in-plan Roth conversion. Once in Roth, 5-year rule applies before tax-free withdrawal of earnings (under 59.5).
📚 Sources & references
Last updated: May 23, 2026