Mega Backdoor Roth Capacity Calculator — Rex's Toolbox

Rex says

Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.

Try a scenario

Click to load — tweak from there.

Inputs

Your age

Employee deferrals made this year (pretax + Roth)($)

Employer match/profit sharing this year($)

Total 415(c) contribution limit this year($)

Result

Available after-tax (mega backdoor) room

$38,000

% of total limit already used

44.9%

Employee deferrals so far

$23,000

Employer contributions so far

$8,000

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How to use this

1Enter your age.
2Enter employee deferrals made this year (pretax + roth) ($).
3Enter employer match/profit sharing this year ($).
4Enter total 415(c) contribution limit this year ($).
5Read your available after-tax (mega backdoor) room on the right — it updates as you type.
6Hit Share to keep the scenario or send it to someone.

About this calculator

The mega backdoor Roth uses the gap between the employee deferral limit ($23,000 in 2024, plus $7,500 catch-up at 50+) and the total combined contribution limit across employee, employer match, and after-tax contributions ($69,000, or $76,500 with catch-up). If your plan allows after-tax (non-Roth) contributions and in-plan Roth conversions or in-service withdrawals, you can fill that gap with after-tax dollars and immediately convert them to Roth, getting far more into Roth space than the standard $7,000 IRA limit allows. This calculator figures out your remaining after-tax contribution room given your pretax/Roth deferrals already made and your employer match, so you know the maximum you can push through the mega backdoor this year — a feature only available if your specific 401(k) plan document permits both after-tax contributions and timely conversions.

FormulaAfter-tax room = total 415(c) limit − employee deferrals − employer contributions. Total 415(c) limit for 2024 = $69,000 (under 50) or $76,500 (50+, includes catch-up in the combined limit calculation per IRS rules).

Worked example

Using the values the calculator loads with:

Inputs

Your age: 40
Employee deferrals made this year (pretax + Roth): 23000 $
Employer match/profit sharing this year: 8000 $
Total 415(c) contribution limit this year: 69000 $

Results

Available after-tax (mega backdoor) room: $38,000
% of total limit already used: 44.9%
Employee deferrals so far: $23,000
Employer contributions so far: $8,000

What each field means

Inputs

Your age: The your age used in the calculation. Starts at 40 so you have a working example on load. Accepted range: 18–75.
Employee deferrals made this year (pretax + Roth) ($): The employee deferrals made this year (pretax + roth) used in the calculation, measured in $. Starts at 23000 $ so you have a working example on load.
Employer match/profit sharing this year ($): The employer match/profit sharing this year used in the calculation, measured in $. Starts at 8000 $ so you have a working example on load.
Total 415(c) contribution limit this year ($): The total 415(c) contribution limit this year used in the calculation, measured in $. Starts at 69000 $ so you have a working example on load.

Results

Available after-tax (mega backdoor) room: Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
% of total limit already used: Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Employee deferrals so far: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Employer contributions so far: Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.

FAQ

Does my 401(k) plan automatically support the mega backdoor Roth?

No, and most don't. You need two specific plan features: the ability to make after-tax (non-Roth) contributions beyond the standard deferral limit, and either in-plan Roth conversions or in-service withdrawals that let you move that after-tax money to Roth promptly. Check your plan's summary plan description or ask HR/the plan administrator directly.

Why does the conversion need to happen quickly?

After-tax contributions grow with taxable earnings until converted. If you let the money sit and grow before converting, the growth is taxable at conversion, undermining the strategy. Ideally your plan supports automatic or same-day conversion of each after-tax contribution to minimize taxable growth.

How is this different from a regular backdoor Roth IRA?

A regular backdoor Roth uses IRA contribution limits (only $7,000 for 2024, $8,000 at 50+) and is vulnerable to the pro-rata rule if you hold other pretax IRA money. The mega backdoor operates entirely inside your 401(k) plan, can move tens of thousands more per year, and isn't affected by the IRA pro-rata rule since it uses separate plan rules under IRC 415(c).

Does employer match reduce my mega backdoor room dollar for dollar?

Yes. The $69,000 (or $76,500 with catch-up) ceiling for 2024 is a combined total across employee deferrals, employer contributions, and after-tax contributions. Every dollar of match or profit sharing your employer puts in directly reduces the after-tax room available to you.

Accuracy and limitations

Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.

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Cite this calculator

Writing about this topic? Grab a citation — every link helps keep these tools free.

APA

RevenueLab. (2026). Mega Backdoor Roth 401(k) Capacity Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/mega-backdoor-401k-capacity

HTML

<p>Source: <a href="https://www.revenuelab.fyi/toolbox/mega-backdoor-401k-capacity" target="_blank" rel="noopener">Mega Backdoor Roth 401(k) Capacity Calculator — RevenueLab</a> (2026).</p>

Markdown

Source: [Mega Backdoor Roth 401(k) Capacity Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/mega-backdoor-401k-capacity) (2026).