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Home›US›Mega Backdoor Roth Calculator

Mega Backdoor Roth Calculator

Find your 2026 mega backdoor Roth space, project tax-free growth over time, and see how it stacks up against a regular Roth IRA. Covers the $70K 415(c) limit, catch-up contributions, and SECURE 2.0 super catch-up for age 60-63.

Find your 2026 mega backdoor Roth space โ€” what remains after employee and employer contributions hit the 415(c) limit
Plan requirements: Your employer plan must allow (1) after-tax contributions AND (2) in-plan Roth conversion. Check with HR before contributing.
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Used for context โ€” does not affect the 415(c) calculation directly
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2026 elective deferral limit: $23,500 (under 50)
$
Total dollar amount your employer contributes (not a percentage)
Determines your 415(c) total annual limit
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How to Use This Calculator

Your Mega Backdoor Space tab

Enter your annual salary, employee 401(k) contribution, employer match amount, and age group. The calculator subtracts your employee and employer contributions from the 415(c) limit to show how much space remains for after-tax contributions that can be converted to Roth. Requires your plan to allow both after-tax contributions AND in-plan Roth conversion — check with HR.

Tax-Free Growth tab

Enter your annual mega backdoor amount (from Tab 1), years to retirement, and expected return. The calculator projects your Roth balance at retirement alongside total contributions vs growth, and compares against a taxable brokerage account using a 15% long-term capital gains rate.

Regular vs Mega tab

Compare a regular Roth IRA ($7,000/year, 2026 limit) against your mega backdoor amount over the same time horizon. See the multiplier: how many times more tax-free wealth the mega backdoor generates vs maxing a regular Roth IRA.

Share your result

Every input is encoded in the URL. Click Share to send your exact scenario to a financial advisor, spouse, or CPA.

The Formula

The mega backdoor Roth space is determined by the IRS 415(c) total contribution limit, which caps ALL money flowing into a 401(k) in a given year.

Mega Backdoor Space = 415(c) Limit − Employee Elective Deferrals − Employer Match

415(c) Limit (2026):
  Under 50: $70,000
  Age 50–59: $77,500 (standard catch-up)
  Age 60–63: $81,250 (SECURE 2.0 super catch-up)

Example: $70,000 − $23,500 − $9,000 = $37,500 mega backdoor space

Roth Balance = Annual Amount × ((1 + r)^n − 1) / r × (1 + r)
where r = annual return, n = years

After-tax contributions go into the 401(k) alongside pre-tax and employer contributions but count against the same 415(c) ceiling. They grow tax-deferred inside the plan. The in-plan Roth conversion moves them to a designated Roth 401(k) account — from that point, growth is tax-free. Convert quickly to minimize taxable earnings on the after-tax balance.

Example

Sarah — Software Engineer, 35, no catch-up

Sarah earns $150,000/year. Her employer offers a 401(k) with a dollar match of $9,000, and the plan allows after-tax contributions and in-plan Roth conversions.

Your Mega Backdoor Space

415(c) limit (under 50)$70,000
Employee contribution (max)$23,500
Employer match$9,000
Mega backdoor space$37,500

Tax-Free Growth (20 years at 7%)

Annual contribution$37,500
Total contributions$750,000
Roth balance (tax-free)$1,540,000
Taxable brokerage (after 15% LTCG)$1,394,000
Tax-free advantage$146,000

Regular vs Mega (20 years at 7%)

Regular Roth IRA ($7,000/yr)$287,000
Mega backdoor ($37,500/yr)$1,540,000
Multiplier5.4×
Extra tax-free wealth$1,253,000

By using the mega backdoor, Sarah accumulates over $1.54M tax-free by retirement — 5.4 times more than the $287K she would have in a regular Roth IRA. Combined with her pre-tax 401(k), she is putting $63,500/year toward retirement.

FAQ

The mega backdoor Roth is a strategy that lets you contribute far more to a Roth account than the standard Roth IRA limit ($7,000 in 2026). You make after-tax contributions to your 401(k) — above the $23,500 elective deferral limit — and then convert those funds to Roth via an in-plan Roth conversion. The 2026 415(c) total cap is $70,000 (under 50), so depending on your employer match, you may be able to put $30,000–$46,000/year into Roth on top of your regular deferrals.
Yes — two features are required. First, your 401(k) plan must permit after-tax contributions (separate from pre-tax or Roth elective deferrals). Second, it must permit in-plan Roth conversions (or in-service withdrawals to a Roth IRA). Not all plans offer both. Check your Summary Plan Description or ask HR directly. Many large employers — particularly in tech, finance, and consulting — do offer both features.
There are no income limits for after-tax 401(k) contributions or in-plan Roth conversions. This is one of the key advantages over the regular Roth IRA, which phases out at $150,000–$165,000 (single) or $236,000–$246,000 (MFJ) in 2026. The mega backdoor Roth is especially valuable for high earners who are above Roth IRA income limits.
Under the SECURE 2.0 Act (effective 2025), employees aged 60 through 63 can make a larger catch-up contribution to their 401(k) — the greater of $10,000 or 150% of the standard catch-up amount. In 2026, this raises the 415(c) total cap to $81,250 for that age group (vs $77,500 for ages 50-59 and $70,000 for under 50). This super catch-up applies only to the 60-63 age window; at 64, you revert to the standard catch-up.
Yes. The earlier House version of the Build Back Better Act proposed banning the mega backdoor Roth for high earners, but that provision was removed before the bill became law. The One Big Beautiful Budget Act (OBBBA) did not restrict after-tax 401(k) contributions or in-plan Roth conversions. The mega backdoor Roth remains fully legal in 2026 and beyond under current law.

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