Mega Backdoor Roth Calculator
This year's 401(k), and the ceiling above it
Your result will appear here
Fill in the fields on the left and this updates as you type.
Know what this estimate is based on
- Jurisdiction
- General mathematical model
- Scope and limitations
- Long-range scenario, not a guarantee. Small changes in returns, inflation, fees, taxes, and withdrawal timing can materially change the result.
- Source links checked
- Jul 30, 2026
Built and regression-tested by Smart Tools Lab. It has not been individually reviewed by a licensed financial, tax, or legal professional.
How to use
- 01
Enter your own 401(k) deferrals for the year, pre-tax and Roth added together — they share one $24,500 limit.
- 02
Enter everything the employer is putting in: match, true-up and profit sharing. Every employer dollar takes a dollar of the room.
- 03
Add your W-2 compensation from this employer. Annual additions can never exceed 100% of your pay, whatever the dollar limit says.
- 04
Enter your age. From 50 a catch-up is available, and it sits outside the $72,000 ceiling rather than inside it.
- 05
Read the room, then check the gate: your plan must allow after-tax (not Roth) contributions AND either in-plan Roth conversion or in-service withdrawal. Without both, the number is arithmetic rather than a plan.
Formula
Ceiling = the lesser of $72,000 and 100% of your compensation from this employer. Used = your deferrals (capped at the $24,500 elective limit) + employer match and profit sharing + any after-tax contributions already made. Room = ceiling − used, floored at zero. The age-50 catch-up is added on top of the ceiling, not inside it. Tax at conversion = earnings between contribution and conversion × your marginal rate; the contributions themselves have already been taxed.
Example
$24,500 of deferrals, $12,600 of employer match and profit sharing, $235,000 of W-2 pay. The ceiling is $72,000 (well under 100% of pay), $37,100 is used, so $34,900 of after-tax room is left. At 54 an $8,000 catch-up sits outside that ceiling, taking the real maximum for the year to $80,000. Converted the day it lands there is no tax at all. Left to grow at 6.5% for 15 years, $34,900 becomes $89,757 — every dollar tax-free — against $81,529 for the same money in a brokerage account after 15% on its $54,857 gain, a difference of $8,229 on one year's contribution.
Definitions
- Annual additions
- Everything credited to your 401(k) account for the year — your deferrals, employer contributions, after-tax contributions and forfeiture allocations. Capped at $72,000 for 2026 under IRC 415(c).
- After-tax contributions
- A third employee bucket, distinct from pre-tax and Roth deferrals. Taxed going in, they do not touch the $24,500 elective limit and count only against the $72,000 ceiling.
- In-plan Roth conversion
- Moving after-tax money into the plan's Roth account without leaving the plan. The alternative is an in-service withdrawal to a Roth IRA.
- ACP test
- A nondiscrimination test on employee after-tax and matching contributions. Failing it forces refunds to highly compensated employees, and safe-harbor status does not exempt it.
Good to know
Two limits, and the gap between them
Almost everyone knows the first number: $24,500 in 2026 is what an employee may defer into a 401(k), pre-tax and Roth added together, and it is one limit per person across every employer. Far fewer know the second: $72,000 is what may be credited to the account in total for the year — your deferrals, everything the employer puts in, and any after-tax contributions, added together. That figure is per plan rather than per person, and it is a very large number sitting quietly above a very well-known one. The gap between them is the whole subject. On $24,500 of deferrals and $12,600 of employer match and profit sharing, $34,900 of that ceiling is untouched. Filling it with after-tax contributions and converting them to Roth moves five figures a year into a tax-free account, which is several times what any IRA route can reach. One more figure that surprises people: the age-50 catch-up sits OUTSIDE the $72,000 rather than inside it, so a 54-year-old's genuine maximum for the year is $80,000.
After-tax is a third bucket, not a synonym for Roth
This is the confusion that ends most attempts before they start. A 401(k) can hold three kinds of employee money. Pre-tax deferrals reduce this year's income. Roth deferrals do not, but grow tax-free — and, crucially, they count against the same $24,500 limit as pre-tax deferrals do. After-tax contributions are the third bucket: taxed going in like Roth, but they do not touch the $24,500 limit at all and count only against the $72,000 ceiling. On their own they are mediocre — the contributions come out tax-free but the earnings are taxed as ordinary income — which is why the conversion step is not optional. Converted to Roth, the whole balance becomes tax-free. When you ring the recordkeeper, ask about "after-tax, non-Roth contributions": ask about "Roth" and you will be told confidently that the plan offers it, meaning the Roth deferrals that share the limit you have already filled.
Three gates, all outside your control
The arithmetic is easy and the availability is not. The plan must permit after-tax contributions at all — a minority of plans do, concentrated in large employers and the technology sector. It must then let the money out, either by in-plan Roth conversion or by in-service withdrawal to a Roth IRA; a plan that takes after-tax contributions but will not release them leaves you with the mediocre version and no way to improve it. And even where both exist, the plan often caps after-tax contributions at a percentage of pay well below what the ceiling would allow. There is a fourth gate that operates after the fact: after-tax contributions are tested under the ACP nondiscrimination test, and safe-harbor status does not exempt them. In a plan where mostly highly paid employees use the feature, the test can fail and part of your contribution is refunded to you months later as taxable income. Read the summary plan description before building a year's saving around the number on this page.
Convert immediately, and what it is worth
After-tax contributions have already been taxed, so at conversion only what they EARNED in the meantime is taxable. A plan with automatic same-day conversion makes that zero. A plan that sweeps quarterly leaves three months of growth taxed at ordinary rates on every sweep, which on a large contribution is a real and entirely avoidable cost. Set the automatic conversion if the plan offers one; if it does not, convert manually and often. What the finished result is worth is best measured against the honest alternative, which is not doing nothing but holding the same money in a taxable brokerage account: $34,900 growing at 6.5% for fifteen years becomes $89,757 either way, but the Roth version keeps all of it while the taxable one gives up 15% of a $54,857 gain — $8,229 on a single year's contribution, and that comparison flatters the brokerage account by ignoring the dividends it is taxed on every year along the way. The one thing that can reverse the verdict is the plan's fund menu: a mega backdoor Roth into an expensive plan can hand the advantage straight back in fees.
Frequently asked questions
What is the mega backdoor Roth?
Filling the gap between the $24,500 you may defer and the $72,000 total that may go into your 401(k) in a year, with after-tax contributions, then converting them to Roth. It moves five figures a year into a tax-free account that no IRA route can reach.
How much room do I actually have?
$72,000, less your own deferrals, less every employer dollar, less any after-tax contributions already made. On $24,500 of deferrals and $12,600 of employer money that is $34,900. Watch the employer side: a profit-sharing contribution or a match true-up paid in January shrinks the figure after the fact.
Is after-tax the same as a Roth 401(k)?
No, and this is the confusion that ends most attempts at it. Roth 401(k) deferrals count against the $24,500 elective limit exactly as pre-tax deferrals do. After-tax contributions are a third bucket that does not touch that limit at all and counts only against the $72,000 ceiling. Ask your recordkeeper about "after-tax, non-Roth contributions" — asking about "Roth" gets you the wrong answer.
Why does my plan not offer it?
Two features are needed and most plans have neither. The plan has to accept after-tax employee contributions, and it has to let them out — by in-plan Roth conversion, or by in-service withdrawal to a Roth IRA. Check the summary plan description before building around it, and check for a plan-level cap on after-tax contributions, which is common and often well below what the ceiling allows.
When should I convert?
Immediately. After-tax money has already been taxed, so only what it EARNS between contribution and conversion is taxable at conversion. A plan with automatic same-day conversion makes that zero; one that converts quarterly leaves three months of growth taxed at ordinary rates every time.
Does the catch-up eat into the room?
No — that is the point of it. Catch-up contributions sit outside the annual-additions ceiling, so a 54-year-old's real maximum is $80,000 rather than $72,000. Whether the catch-up has to be made as Roth, and how the enhanced 60-to-63 figure works, are separate questions.
Can my contribution be refunded?
Yes, and it catches people out. After-tax contributions are tested under the ACP test, and safe-harbor status does not exempt them. In a plan where mostly highly paid employees use the feature the test can fail and part of your contribution comes back to you months later.