The Mega Backdoor Roth, Explained
A primer on the rules the calculator above is enforcing: what Roth money is worth, the two 401k limits the IRS imposes, what “per employer” actually means, and the mechanics of the 415(c) cap that makes mega backdoor contributions possible. For a deeper dive, read the full mega backdoor Roth guide.
Why Roth Money Is Different
Investment gains in a Roth account are never taxed. In a pre-tax account, every dollar of growth will be taxed as ordinary income when you withdraw it. Over 20–30 years of compounding, this difference is substantial.
In retirement, Roth withdrawals don’t count as income. This means they don’t push you into higher tax brackets, don’t increase your Medicare premiums, and don’t trigger taxes on Social Security benefits.
Roth contributions (not earnings) can be withdrawn at any time without taxes or penalties. This makes Roth money more flexible than pre-tax retirement savings, which are locked until 59½.
The Two 401k Limits
The IRS imposes two separate limits on 401k contributions. Most people only know about the first. The mega backdoor Roth exploits the gap between them.
$24,500
IRC § 402(g). The total pre-tax and Roth contributions you can make across all employers in a calendar year. This is a personal limit that follows you, not the plan.
$72,000
IRC § 415(c). The total of all contributions at one employer: your elective deferrals + employer match + after-tax contributions. Each unrelated employer has its own independent bucket.
How the 415(c) Limit Actually Works
Internal Revenue Code § 415(c) caps the “annual additions” that any one defined contribution plan can credit to a single participant. For 2026 that cap is $72,000 (it indexes annually for inflation). The annual additions bucket is the sum of:
- Elective deferrals — your pre-tax and Roth contributions
- Employer contributions — matching, non-elective, and profit-sharing contributions
- Employee after-tax contributions — the piece you’re trying to maximize for the mega backdoor
- Forfeiture allocations — unvested amounts from former employees that get redistributed
A few items deliberately sit outside the 415(c) bucket: age-50 catch-up contributions, rollovers from other plans, loan repayments, and the gain on after-tax contributions that you later convert to Roth. That last point matters: you contribute after-tax dollars (which count) and then convert them to Roth (which doesn’t separately count), so the conversion itself doesn’t eat your 415(c) room.
The remaining headroom after your elective deferrals and your employer’s match is what you can stuff into the after-tax bucket. In the maximum case — full $24,500 elective with no employer match — that’s up to $47,500 per employer of after-tax contributions that you can then convert to Roth.
Sources: 26 U.S.C. § 415 (Cornell LII), IRS Retirement Topics — 401(k) and Profit-Sharing Plan Contribution Limits.
What “Per Employer” Really Means
The 415(c) limit is “per employer,” not per job and not per plan. The IRS treats certain related companies as a single employer under IRC § 414(b), (c), (m), and (o). If two entities fall into the same controlled group or affiliated service group, they share one 415(c) bucket between them, even if they run separate 401(k) plans.
Two W2s at the same company — or at a parent and its wholly-owned subsidiary — share a single $72,000 cap. Controlled groups under § 414(b)/(c) are aggregated as one employer for this purpose.
A W2 at Company A and a W2 at Company B (no common ownership) gives you two independent 415(c) caps. The $24,500 elective deferral limit is still shared across both, but the after-tax room is doubled.
Self-employment income through a solo 401(k) creates its own 415(c) bucket, as long as the businesses aren’t in an affiliated service group. Many high earners stack a W2 plan with a solo 401(k) to double up after-tax room.
Professional service firms (medical, legal, consulting) where you own part of the business and provide services together with another entity can be treated as one employer under § 414(m), even without ownership overlap. Get this checked by a tax pro.
Sources: 26 U.S.C. § 414 (controlled group / affiliated service group rules), IRS One-Participant 401(k) Plans.
Coordinating the Elective Deferral Limit Across Jobs
The $24,500 elective deferral cap is set by IRC § 402(g) and applies to you, not your plan. Combined pre-tax and Roth deferrals across every 401(k), 403(b), SARSEP, and SIMPLE IRA you participate in must stay under it for the calendar year. Your employer doesn’t know about your other employer’s 401(k) — it’s your responsibility to coordinate.
If you accidentally over-defer, the excess plus the earnings on it must be withdrawn by April 15 of the following year to avoid double taxation. The optimizer above models this constraint chronologically across all jobs so you stay under the cap by design.
Sources: 26 U.S.C. § 402(g) (Cornell LII), IRS — 401(k) and Profit-Sharing Plan Contribution Limits.
What This Looks Like at Retirement
Take a single year’s contributions and let them compound at 7% annual returns for 25 years. When you withdraw from a pre-tax account, those dollars are taxed as ordinary income at your marginal rate. Roth withdrawals are completely tax-free.
One year’s contribution: $24,500
After 25 years: $132,972
Tax on gains at 24% marginal: −$26,033
$106,939
you keep
Every dollar withdrawn is taxed at your marginal rate
One year’s contribution: $24,500
After 25 years: $132,972
$132,972
you keep
Same contribution, but withdrawals are tax-free
One year’s contribution: $72,000
After 25 years: $390,775
$390,775
you keep
Nearly 3x the tax-free retirement savings
Hypothetical illustration. Shows one year of contributions compounding over 25 years at 7% annual returns. Pre-tax gains taxed at 24% marginal rate (original contributions are not double-taxed). Employer match not included. Actual results vary with investment performance and tax rates.
How the Mega Backdoor Roth Works
The strategy has two parts: make after-tax contributions to your 401k (beyond the elective limit), then convert them to Roth. The “backdoor” name comes from the fact that you’re getting money into Roth through a side door, bypassing the normal contribution limits.
First, contribute the full $24,500 in pre-tax or Roth deferrals. These are the standard contributions most people make.
Your plan must support after-tax contributions (distinct from Roth). These come from your paycheck after income tax, and fill the gap between your elective + match and the $72,000 cap.
Convert after-tax dollars to Roth via an in-plan Roth conversion. Do this as soon as possible after each contribution to minimize taxable earnings in the after-tax account.
Does Your Plan Actually Allow This?
Two plan features need to be present for a mega backdoor Roth to work, and they’re not the same thing as “does my 401k have a Roth option.” Check your Summary Plan Description (SPD) for both:
A separate contribution source from Roth. Look for “voluntary after-tax,” “non-Roth after-tax,” or “employee after-tax” in your plan documents. If you only see pre-tax and Roth options, your plan doesn’t support this.
After-tax money needs to move to Roth quickly to minimize taxable gains. The plan needs either in-plan Roth conversions or in-service withdrawals to a Roth IRA. Automatic conversions (“mega backdoor on autopilot”) are the gold standard.
Why This Is Hard to Plan
The math seems simple on paper, but in practice there are several things that make it tricky to get right:
If you set elective percentages too high, you hit the $24,500 limit before December. Once contributions stop, so does your employer match for those paychecks. Unless your employer offers a true-up provision, that match is lost.
The elective limit is shared across all employers. If you change jobs mid-year, you need to track how much you already contributed and adjust at the new employer to avoid exceeding the limit.
Each employer withholds federal income tax as if they are your only employer. With multiple jobs or large after-tax contributions reducing take-home pay, your withholding may not match your actual tax liability.
Contribution percentages interact with pay frequency, bonus timing, and RSU vests. A percentage that works for regular paychecks may push you over limits when a bonus hits.
The calculator above handles all four of these by processing every paycheck chronologically and enforcing both limits across all jobs as they accrue.
This page is informational and not tax or legal advice. Citations link to the Internal Revenue Code and IRS publications — verify current limits, which adjust annually for inflation, on irs.gov.