Dollar Almanac

Mega Backdoor Roth Capacity Calculator

Updated

A mega backdoor Roth runs on whatever room is left under the annual additions limit once your deferrals and your employer's contributions have taken their share.

Your after-tax room is the annual additions limit minus your elective deferrals and everything your employer put in. Catch-up contributions sit outside that limit rather than inside it.

Pre-tax and Roth together. Not catch-up, which sits outside this limit.

Match and profit sharing, for the whole year rather than to date.

Plan accepts after-tax contributions
Plan allows in-plan Roth conversion

After-tax room left

$32,500.00

Annual additions ceiling
$72,000.00
Your elective deferrals
−$24,500.00
Employer contributions
−$15,000.00
Catch-up, on top of this
$0.00
Elective deferral room left
$0.00

info: Employer money comes out of the same ceiling

Match and profit sharing count against the annual additions limit alongside your own contributions. A better match leaves less after-tax room, which is the opposite of what most people expect, and it is why this figure moves during the year.

What this assumes

Assumption set limits 2026

Age reached this year
40
Plan permits after-tax
Yes, as entered
Catch-up treatment
Outside the annual additions limit
Limits
Notice 2025-67

How this works

The annual additions limit under §415(c) caps everything that goes into a defined contribution plan in a year: your elective deferrals, whether pre-tax or Roth; your employer’s match; any profit sharing; and any after-tax contributions on top. One ceiling, four kinds of money.

That is a different figure from the elective deferral limit, which is the one most people know. Hitting the deferral limit does not fill the plan. What is left between the two is the room a mega backdoor Roth uses, and it is usually the larger number.

Two rules decide how much of it you actually have, and they pull in opposite directions:

  • Employer money consumes the ceiling. Match and profit sharing count against the same limit you do. A better match leaves less after-tax room, not more, and the room shrinks through the year as the match accrues.
  • Catch-up sits outside it. §414(v)(3) excludes catch-up contributions from annual additions entirely. From 50 you have the whole ceiling and the catch-up, and for the four years you turn 60 through 63 the catch-up itself is larger.

Worked example

Someone earning $250,000.00 defers the full $24,500.00 and gets no employer contribution at all. The ceiling is $72,000.00, so after the deferrals there is $47,500.00 of after-tax room.

Give the same person a $15,000.00 match and nothing else changes except the arithmetic: the ceiling is the same, the deferrals are the same, and the after-tax room falls to $32,500.00. The match has taken $15,000.00 of it.

That is not a reason to want a worse match. The second person has $15,000.00 they did not contribute. It is a reason to work the room out net of the match rather than against the headline ceiling, because contributing after-tax against the first figure and then receiving the match is how people end up with a refund they did not plan for.

What this does not tell you

Whether your plan permits any of it. After-tax contributions and in-plan Roth conversion are two separate permissions, most plans grant neither, and a plan may cap after-tax contributions well below the statutory room. The plan document or the administrator is the only answer, and this page cannot see it.

It also does not model non-discrimination testing, which can force a refund of after-tax contributions in a plan where few people make them, and it does not tell you whether the strategy is worth doing. How we compute things sets out what these tools do and do not do.

Sources

References used to explain this page. Listing a publisher is not a claim that they endorse it.

Figures are transcribed from this document directly. Where a value has not been verified against its source, this page shows no number rather than an estimate.

Questions

Why does my employer's match reduce my after-tax room?
Because it comes out of the same ceiling. Section 415(c) caps everything going into the plan in a year from every source at once, so match and profit sharing consume the same limit your own contributions do. A better match leaves less after-tax room, which runs the opposite way to most people's intuition. It is still better to have the match: it is money you did not contribute.
Does catch-up count against the limit?
No. Section 414(v)(3) puts catch-up contributions outside the annual additions limit. Someone 50 or over has the whole ceiling available and the catch-up, which is why this calculator reports them separately rather than adding them together. The catch-up is extra elective deferral room, though, not extra after-tax room.
Is the room the same all year?
No, and this is where people get caught. Employer contributions accrue through the year, so capacity calculated in January against no match is larger than the capacity you actually have in December. Contributing after-tax against the January figure can push the total over the ceiling, and plans deal with that by refusing or refunding. Work it out net of the match you expect for the whole year.
My plan does not allow after-tax contributions. Why does this still show a number?
Because the number is the ceiling if the plan says yes, and that is the thing worth asking about. Most plans do not permit after-tax contributions and the ones that do often do not permit in-plan conversion, which is a separate permission. The statute sets the room; the plan document decides whether any of it is reachable.
What happens if I contribute after-tax and cannot convert it?
The money sits in the plan as after-tax basis and its earnings grow taxable rather than tax-free. For most people that is a worse outcome than not contributing, because the conversion is the entire point. In-plan Roth conversion or in-service withdrawal is what makes the strategy work, and both are plan permissions rather than statutory rights.
Which compensation figure should I use?
Plan compensation, which is defined in the plan document and is often not the same as your gross pay. Section 401(a)(17) also caps the compensation any allocation can be based on. If your pay is below the annual additions dollar limit, compensation is what binds, and the calculator says which of the two is in play.
What does this not cover?
After-tax limits a plan sets below the statutory one, which are common and invisible from outside. Non-discrimination testing, which can force a refund in a plan where few people contribute after-tax. And whether doing this at all is the right call, which depends on what else the money would do.

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