About the Mega Backdoor Roth Calculator
This mega backdoor Roth calculator shows how much room you have for after-tax (non-Roth) contributions in your 401(k) once your own deferrals and your employer’s contributions are counted. That space can be converted to Roth — through an in-plan Roth conversion or an in-service rollover to a Roth IRA — so it grows tax-free. Enter your pay, your deferrals and what your employer puts in, and the calculator applies the IRS annual additions limit.
It is meant for high earners who already max out their regular 401(k) and are above the Roth IRA income limits, and for anyone checking whether their plan’s after-tax feature is worth using. A projection estimates what the converted money could grow to and how much tax on growth you avoid compared with a taxable brokerage account.
Figures are for 2026 (IRS Notice 2025-67): the section 415(c) limit on total annual additions is $72,000 or 100% of your pay if less, the elective deferral limit is $24,500, and catch-up contributions ($8,000 at 50+, $11,250 at ages 60–63) sit on top of the $72,000 and do not use up after-tax space. Your plan must allow after-tax contributions and in-service conversions or withdrawals for the strategy to work.
With the default inputs, the after-tax contribution room (mega backdoor) is $35,500.00. Change any value above to recalculate instantly.
How to use the mega backdoor roth calculator
- 1Enter your age and annual compensation.
- 2Enter your total 401(k) deferrals, including any catch-up contributions.
- 3Add your employer’s expected match and profit-sharing contributions.
- 4Read the after-tax room and per-paycheck amount, then confirm your plan allows after-tax contributions and in-service conversions.
- 5Adjust years and return to see how much tax-free Roth money the strategy could build.
Formula and method
Section 415(c) caps the total "annual additions" to your 401(k) — your elective deferrals, employer match and profit sharing, and after-tax contributions — at $72,000 for 2026, or 100% of your compensation if that is lower. Catch-up contributions are excluded from this cap, so only the first $24,500 of your deferrals counts. Whatever is left after deferrals and employer money is the space you can fill with after-tax contributions and then convert to Roth.
The projection assumes you contribute the same room at the end of each year and the converted Roth balance compounds at your expected return. Growth in a Roth is tax-free when withdrawn as a qualified distribution, so the "tax avoided" figure is simply that growth multiplied by the rate you would otherwise pay on gains in a taxable account. Plan-specific caps, nondiscrimination (ACP) testing and your actual cash flow can limit the real figure.
- Room
- After-tax contribution space for the year
- 415(c) limit
- Total annual additions limit ($72,000 for 2026)
- r
- Expected annual return
- n
- Number of years you contribute
Worked examples
$200k earner maxing deferrals with $12k employer match
The cap is the full $72,000 because pay exceeds it. Subtracting $24,500 of deferrals and $12,000 of employer money leaves $35,500 of after-tax room — $1,365.38 per biweekly paycheck. Contributed and converted for 15 years at 7%, it could grow to about $892,080, of which $359,580 is tax-free growth.
Age 52 with catch-up contributions
Of the $32,500 deferred, only $24,500 counts toward the $72,000 cap; the $8,000 age-50 catch-up sits on top. That leaves $72,000 − $24,500 − $9,000 = $38,500 of after-tax room, for $80,000 of total contributions ($72,000 plus the catch-up).
Pay below the 415(c) limit
Annual additions cannot exceed 100% of compensation, so the cap is $60,000 rather than $72,000. After $10,000 of deferrals and $3,000 of employer money the technical room is $47,000 — though few people can contribute most of their pay.
Frequently asked questions
What is the mega backdoor Roth limit for 2026?+
The total of employee deferrals, employer contributions and after-tax contributions cannot exceed $72,000 in 2026 (or 100% of your pay if lower). Your after-tax room is $72,000 minus your deferrals (up to $24,500) and employer contributions. Catch-up contributions are on top.
Do catch-up contributions reduce my after-tax room?+
No. Catch-up contributions ($8,000 for ages 50+, $11,250 for ages 60–63 in 2026) are not counted toward the 415(c) annual additions limit, so they do not use up mega backdoor space.
How is a mega backdoor Roth taxed?+
After-tax contributions are made with money you have already paid tax on, so converting them to Roth is tax-free. Any earnings that build up before the conversion are taxable when converted, which is why many plans offer automatic in-plan conversions.
Does my plan have to allow the mega backdoor Roth?+
Yes. The plan must accept after-tax (non-Roth) contributions and allow either in-plan Roth conversions or in-service withdrawals of after-tax money to a Roth IRA. Many plans offer neither, so check your summary plan description.
Is there an income limit for the mega backdoor Roth?+
There is no IRS income limit, which is why high earners use it. However, plans must pass nondiscrimination (ACP) testing, and some plans cap or refund after-tax contributions from highly compensated employees.
What is the difference between a backdoor Roth and a mega backdoor Roth?+
A backdoor Roth is a nondeductible traditional IRA contribution (up to $7,500 in 2026) converted to a Roth IRA. A mega backdoor Roth uses after-tax 401(k) contributions, which can be several times larger.
Tax results are estimates based on published rules and simplified assumptions. They are not tax advice — check official guidance or a tax professional for your situation.