Mega Backdoor Roth Calculator
How to plan mega backdoor Roth contribution space
This mega backdoor Roth calculator estimates how much after-tax 401(k) room is still available under the annual additions limit after your employee deferrals, employer contributions, and any catch-up amount are counted. It is designed for mid-year checks, end-of-year true-ups, and any time you want a quick read on whether the plan still has room for the after-tax step that later gets moved into Roth treatment.
Use numbers from your paystub, payroll portal, or retirement plan record, then treat the result as a year-specific planning estimate. That makes it easier to see whether you can still direct new money into after-tax contributions, but it also means you should verify the plan rules that apply to your own employer before changing elections.
How to use this mega backdoor Roth calculator
This mega backdoor Roth calculation begins with the contribution totals already recorded for the plan year. The calculator asks how much of the annual additions cap has been used so far, then subtracts those amounts from the limit to show the remaining after-tax space.
Start with employee deferrals, whether you send them in as pre-tax or Roth elective contributions, because both types consume the same deferral bucket. Then add any employer deposits, such as matching contributions or profit sharing, and include catch-up contributions only if you want them counted in the space calculation.
Because the inputs are dollar totals, enter year-to-date or expected-year amounts from payroll rather than percentages or per-paycheck rates. If your plan records a match in separate installments, add those installments together before entering the employer figure so the calculation lines up with what the plan administrator sees.
If you are checking this number before a bonus, a true-up, or a final payroll posting, be conservative with estimates. A large employer deposit can reduce the room available for after-tax contributions much faster than most people expect, which is why the calculator is most useful when you rerun it after each meaningful change.
The big picture is that the mega backdoor Roth strategy depends on what has already been counted inside the plan ledger, not on how much you hope to save by year end. If the employee deferral election rises, the remaining room falls; if the employer contribution grows, the remaining room falls again; and if you decide to count catch-up contributions in the same estimate, the room drops by that amount as well.
That is why the calculator works best when you keep the input values aligned with actual records instead of rough percentages. A percentage election may be convenient for payroll, but the contribution limit is measured in dollars, so the calculator needs the running totals that payroll will ultimately post. When those totals change, the answer changes with them.
For many savers, the most helpful use of this page is not a one-time year-end check but a repeat check during the year. A plan that looked open in spring may be much tighter after a midyear bonus, a quarterly employer contribution, or a true-up match. Re-entering the figures only takes a moment, and it shows which bucket is consuming the available room.
It also helps to think about the after-tax contribution as only one step in the larger Roth move. The calculator tells you whether the after-tax deposit can fit inside the plan year limit; it does not decide whether the money will later be converted inside the plan or rolled out and converted elsewhere. Those choices depend on your employer's plan rules and the conversion path the plan supports.
If you have more than one source of employer money, combine them before using the form. Match deposits, non-elective contributions, and profit sharing can all shrink the available space, and the calculator is only as accurate as the total you enter. The same idea applies to employee deferrals: if you split them across pre-tax and Roth, add both together because the plan counts them against the same overall bucket.
In practice, the result is most useful when it tells you whether you are still comfortably below the limit or already close enough that the next payroll posting might change the answer. A wide margin means you may have flexibility to increase the after-tax election, while a narrow margin means the next deposit could consume the remaining room. Either way, the page gives you a fast way to gauge the contribution space before you act.
This calculator is intentionally focused on contribution space rather than long-term retirement forecasting. It does not project investment returns, tax brackets, or the future value of the Roth money after conversion. Instead, it helps you answer one very practical question: given the money already recorded for the year, how much after-tax 401(k) room is left for the mega backdoor Roth route?
Formula: how mega backdoor Roth space is calculated
For this mega backdoor Roth calculator, the remaining after-tax space is the annual additions limit minus the employee deferrals, employer contributions, and any catch-up amount you choose to include. The calculator does not try to forecast investment growth or the tax treatment of a later conversion; it only measures how much of the plan's yearly ceiling has not yet been used.
Formula: R = L - E - M - C
Formula: E = E_pre + E_Roth
Formula: M = M_match + M_profit
Formula: T = E + M + C
Formula: R > 0
Formula: R ≤ 0
Those MathML expressions mirror the arithmetic in the form. R is the remaining after-tax room, L is the annual additions limit, E is employee deferrals, M is employer contributions, and C is the catch-up amount you want counted in the estimate. T is simply the total of the amounts that already consume space inside the plan.
If your employer contribution arrives in pieces, combine those pieces before entering the number here. If your employee deferrals are split between pre-tax and Roth payroll elections, add them together as well, because the calculator only needs the yearly total that has already been applied against the limit. When catch-up contributions are not part of your current estimate, leave that field at zero and the formula automatically reduces to the simpler limit-minus-deferrals-minus-employer version.
The sign of the result matters just as much as the size of the number. A positive value means the plan still has room for more after-tax money under the assumptions you entered. A zero or negative value means the planned contributions have already used up the limit, so the after-tax contribution would need to be reduced, delayed, or skipped for that plan year.
Worked example: a mid-year mega backdoor Roth check
A practical mega backdoor Roth example makes the subtraction easier to picture. Suppose the plan-year limit is $69,000, the employee has already deferred $23,000, the employer has contributed $11,500, and the catch-up amount is $7,500.
The calculation is 69,000 - 23,000 - 11,500 - 7,500 = 27,000. Under those assumptions, the calculator shows $27,000 of remaining after-tax space. If the employee changes payroll elections later, or if the employer posts a larger true-up, that remaining amount would shrink.
The same method works in less tidy situations. If the employer contribution is still pending, enter a conservative estimate and rerun the calculator after the deposit lands. If your plan does not count catch-up contributions toward the same limit you are monitoring, you can leave that field at zero and the result will reflect only the amounts you want to reserve.
You can also use the example logic to sanity-check your own year-to-date totals. If your employee deferrals are already close to the limit, even a modest employer contribution can push the result down quickly. If the employer side is small, the calculator will usually leave more room open for the after-tax deposit, which is exactly the circumstance many savers are looking for when they use the mega backdoor Roth route.
When the result looks surprisingly low, the most common reason is that one of the inputs is larger than expected. A bonus match, a final true-up deposit, or an overlooked catch-up contribution can consume the remaining space faster than a simple percentage election suggests. Rechecking the numbers with the latest payroll records is usually the quickest way to find the source of the change.
That is also why a concrete example is useful for planning a payroll election. It lets you see how the pieces interact before the next contribution posts. Once you understand which bucket is largest, you can decide whether to increase the after-tax percentage, keep it steady, or wait until later in the year when you have a clearer view of the remaining space.
Limitations: when a mega backdoor Roth estimate can mislead
The biggest limitation of a mega backdoor Roth estimate is that the calculator cannot verify whether your plan actually permits after-tax contributions, in-service withdrawals, or in-plan Roth conversions. Those features are what make the strategy workable in practice, so the arithmetic alone is never the whole story.
Results depend entirely on the figures you enter and on whether those figures match the records your payroll system and plan administrator are using. A stale match estimate, a missing profit-sharing deposit, or an outdated annual limit can make the result look better than the actual plan ledger will support.
The calculator also cannot account for posting delays, conversion timing, or earnings that accrue before after-tax money is moved to Roth status. Those operational details do not change the subtraction itself, but they can affect how smoothly the strategy works and how much money you ultimately move.
Use the result as a planning estimate rather than a compliance opinion. Your plan document, payroll records, and administrator guidance are the sources that determine whether a contribution can be made, when it can be moved, and how each deposit is recorded.
If your employer uses a true-up match or posts profit sharing near the end of the year, check the calculator again after those deposits appear. The remaining room for after-tax contributions can tighten quickly, and a number that looked generous in spring can become much smaller by year-end.
In short, the tool is most useful when you want a fast read on remaining mega backdoor Roth space and a reminder of which contribution bucket is doing the most to absorb it. It is not a substitute for plan documents, payroll confirmation, or professional tax advice.
Arcade Mini-Game: Mega Backdoor Roth Planning Run
Use this quick arcade run to practice spotting the contribution inputs that shrink or expand remaining after-tax 401(k) space before you rely on the calculator output.
Start the game, then use your pointer or arrow keys to catch useful contribution inputs and avoid bad assumptions about plan limits.