Mega Backdoor Roth Calculator
Calculate how much you can contribute via the mega backdoor Roth — the after-tax 401(k) space above your deferral and employer match, using 2025 IRS limits. Runs in your browser.
Mega backdoor room (2025)
- Total 401(k) additions limit
- $70,000.00
- − Your elective deferral
- −$23,500.00
- − Employer match/contribution
- −$8,000.00
- = After-tax space for mega backdoor
- $38,500.00
You could contribute up to $38,500.00 in after-tax 401(k) money and convert it to Roth.
The mega backdoor uses the gap between your elective deferral + employer contributions and the total 401(k) additions limit ($70,000.00 for 2025, +$7,500.00 catch-up at 50+). You make after-tax (not Roth, not pre-tax) contributions, then do an in-plan Roth conversion or in-service rollover to a Roth IRA. Only works if your plan allows both after-tax contributions and in-service conversions — many don't. 2025 IRS limits. Estimate only — not tax advice; confirm with your plan and a CPA.
About this tool
The mega backdoor Roth lets high savers move a large sum into Roth accounts each year by exploiting the gap between two IRS limits. Your own pre-tax/Roth 401(k) deferral is capped ($23,500 in 2025), but the total of all contributions to your 401(k) — your deferral plus employer match plus after-tax contributions — can go up to a much higher overall limit ($70,000 in 2025, or $77,500 with the age-50 catch-up). This calculator computes the leftover after-tax room: total limit minus your deferral minus the employer contribution. You contribute that amount as after-tax (a distinct bucket, not Roth and not pre-tax) and then convert it to Roth via an in-plan Roth conversion or in-service rollover to a Roth IRA, so future growth is tax-free. The essential caveat it states: this only works if your specific 401(k) plan permits both after-tax contributions and in-service conversions/withdrawals — many plans do not, so check with your plan administrator first. It uses 2025 IRS figures. It is educational, not tax advice. Everything runs in your browser.
How to use it
- Enter your 401(k) elective deferral for the year.
- Enter your employer's match/contribution.
- Check the box if you're 50+ (higher limit).
- Read the after-tax space available to contribute and convert to Roth — then confirm your plan allows it.
Frequently asked questions
- What is the mega backdoor Roth?
- A strategy to get far more than the normal limit into Roth: contribute after-tax dollars to your 401(k) up to the overall additions limit, then convert them to Roth. It can move tens of thousands into Roth annually, well beyond the IRA or elective-deferral caps.
- How much can I contribute in 2025?
- The after-tax room is the total 401(k) additions limit ($70,000, or $77,500 with the 50+ catch-up) minus your elective deferral and any employer contributions. So if you defer $23,500 and your employer adds $8,000, up to about $38,500 of after-tax space remains.
- Why does my plan have to allow it?
- Two features are required and not universal: the plan must accept after-tax (non-Roth, non-pre-tax) contributions, and it must permit in-plan Roth conversions or in-service rollovers so you can move that money to Roth. Without both, the mega backdoor is not available to you.
- How is this different from the regular backdoor Roth?
- The regular backdoor uses a Traditional IRA → Roth IRA conversion, limited to the IRA contribution cap ($7,000–$8,000). The mega backdoor works inside a 401(k) and allows far larger amounts (often $30k–$40k+), but only if your plan supports it.
- Should I convert the after-tax money quickly?
- Generally yes — converting soon after contributing minimizes taxable earnings on the after-tax balance before conversion. Earnings that accrue before conversion are taxable when converted, so frequent or automatic conversions keep it clean.
- Is this tax advice?
- No. It is educational. Confirm your plan's features and the tax mechanics with your plan administrator and a CPA before using the strategy.