Mega Backdoor Roth Calculator
See how much after-tax money fits in your 401(k) for 2026 under the $72,000 limit, once your deferrals and your employer's contributions are counted, and whether the new Roth catch-up rule applies to you.
2026 limits from IRS Notice 2025-67. Everything runs in your browser. For education only, not tax or investment advice.
You and your plan, 2026
Pre-tax plus Roth, not catch-up; 2026 limit $24,500
For the year, in dollars
Non-elective, true-up
Usually 0
W-2 box 3; decides the Roth catch-up rule and the highly-compensated test
After-tax room in your 401(k) for 2026
$35,500
That is $72,000 for all contributions, less your $24,500 of deferrals and $12,000 from your employer. Converted to Roth, it grows tax-free.
Total limit (415(c))
$72,000
all contributions
Catch-up allowed
$0
from age 50
Most into the plan
$72,000
including catch-up
- With 2025 pay over $160,000 you are a highly compensated employee. If the plan fails its ACP test, part of your after-tax money can be refunded, so the usable room may be less.
- The room is only usable if your plan accepts after-tax contributions and lets you convert them to Roth inside the plan or withdraw them while working. Convert soon after contributing, so little growth is taxed.
How this is worked out. Everything added to your 401(k) in a year counts toward the $72,000 limit for 2026, or 100% of pay if lower: your deferrals, the employer's money and after-tax contributions. Catch-up contributions are extra. After-tax room is what is left.
2026 limits. The 415(c) limit applies per employer (and related employers), while the deferral limit applies to you across all plans. Your plan may cap after-tax contributions as a percentage of pay below this room.
$72,000 covers everything
The 415(c) limitcounts your deferrals, the employer's money and after-tax contributions together. The after-tax room is what is left.
Catch-ups are extra
$8,000 at 50 and $11,250 at 60 to 63 go on top of the $72,000, and above $150,000 of 2025 wages they must be Roth.
Convert quickly
Earnings on after-tax money are taxed when converted, so plans that convert automatically or soon after each paycheck work best.
Check the plan first
The room is only usable if your plan offers after-tax contributions and an in-plan Roth conversion or in-service withdrawal.
Frequently asked questions
What is a mega backdoor Roth?
Putting after-tax (not Roth) money into your 401(k) above the normal deferral limit, then converting it to Roth inside the plan or rolling it out to a Roth IRA. It needs a plan that accepts after-tax contributions and allows in-plan conversions or in-service withdrawals.
How much can I contribute after-tax in 2026?
Everything added to your account counts toward the 2026 limit of $72,000, or 100% of your pay if less: your pre-tax and Roth deferrals (up to $24,500), your employer's match and profit sharing, forfeitures and after-tax money. What is left after the first three is your after-tax room. Catch-up contributions are on top.
Do catch-up contributions reduce the room?
No. Catch-ups sit outside the $72,000 limit. For 2026 they are $8,000 from age 50, and $11,250 for people who are 60 to 63 at the end of the year. A 61-year-old can therefore get up to $83,250 into the plan.
What is the 2026 Roth catch-up rule?
From 2026, if your Social Security wages from the employer were more than $150,000 in 2025, any catch-up contributions you make must be Roth. If your plan has no Roth option, it may not let you make catch-ups at all. Your regular deferrals can still be pre-tax.
Why might my plan refund some after-tax money?
After-tax and matching contributions must pass the ACP nondiscrimination test. If highly compensated employees (2025 pay above $160,000 for the 2026 test) contribute much more than everyone else, the plan refunds part of their after-tax money. Ask your plan administrator whether it has failed the test before.
Sources and references
- IRS Notice 2025-67: 2026 limits for 401(k) plans
- 26 U.S.C. 415(c): the annual additions limit
- 26 U.S.C. 414(v): catch-up contributions, outside 415(c), Roth-only above the wage threshold
- 26 U.S.C. 401(m): the ACP test on after-tax and matching contributions
- IRS Notice 2014-54: splitting after-tax money between a Roth IRA and a traditional IRA
- T.D. 10033: final regulations on Roth catch-up contributions
- Limits encoded as of 27 October 2026 for 2026; 2027 limits had not been published.
Disclaimer: This calculator is for educational purposes only and does not constitute tax or investment advice. It uses the 2026 limits for one employer's plan and does not model your plan's own caps or a failed ACP test. Confirm with your plan administrator.
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