How a Mega Backdoor Roth Works and Who Can Use It

A mega backdoor Roth lets you move after-tax money into a Roth account in much larger amounts than a regular backdoor Roth

A mega backdoor Roth is a strategy where you contribute after-tax money to your employer's 401(k) plan, then move that money into a Roth IRA or Roth 401(k). The key difference from a regular backdoor Roth is the amount: a mega backdoor Roth uses your employer plan's after-tax contribution space, which is separate from the regular contribution limit. In 2024, the total you can put into a 401(k) is $69,000 (or $76,500 if you're 50 or older). A regular backdoor Roth uses only the $7,000 annual IRA contribution limit. A mega backdoor Roth can use the gap between what you've already contributed to your 401(k) and that $69,000 ceiling.

Not every employer plan allows this. Your plan must permit after-tax contributions and in-service distributions (or in-service conversions). You'll need to check with your plan administrator to see if both features are available. If they are, you can contribute thousands of dollars more per year into a Roth account than you could through a regular backdoor Roth alone.

Key Takeaways

A mega backdoor Roth uses after-tax contributions to your 401(k), not your IRA, so the contribution room is much larger than a regular backdoor Roth.
Your employer's 401(k) plan must allow after-tax contributions and either in-service distributions or in-service conversions for this strategy to work.
The total amount you can contribute to a 401(k) in 2024 is $69,000 (or $76,500 if age 50+), and a mega backdoor Roth uses the after-tax portion of that limit.
You move the after-tax money out of the 401(k) into a Roth account as soon as possible to avoid taxes on investment growth.
The pro-rata rule applies if you have other pre-tax IRA balances, which can create a tax bill on part of the conversion.

How the contribution limits work for a mega backdoor Roth

The IRS sets an annual limit on total contributions to a 401(k): $69,000 in 2024 for people under 50, and $76,500 for people 50 and older. This limit includes three types of money: your own salary deferrals (what you choose to have withheld from your paycheck), your employer's matching contribution, and your employer's profit-sharing contribution. After you add up all three, whatever remains below the $69,000 ceiling is your after-tax contribution space.

For example, if you earn $150,000 and contribute $23,500 of your own money (the 2024 salary deferral limit), and your employer contributes $5,000 in matching funds, you have used $28,500 of the $69,000 limit. You could then contribute up to $40,500 more as after-tax money. That $40,500 is what you would move into a Roth account through the mega backdoor strategy.

The exact amount available varies by person and employer. Your plan administrator or payroll department can tell you how much after-tax contribution room you have left in any given year.

The difference between in-service distributions and in-service conversions

After you contribute after-tax money to your 401(k), you need a way to move it into a Roth account. Your plan must allow one of two methods: in-service distributions or in-service conversions.

An in-service distribution means you withdraw the after-tax money from your 401(k) while you're still employed and still contributing to the plan. You then roll that money into a Roth IRA outside the plan. An in-service conversion means the plan itself converts the after-tax money directly into a Roth 401(k) without you taking a distribution first. Both routes end with after-tax money in a Roth account, but the paperwork and timing differ. Ask your plan administrator which option your plan supports.

The reason timing matters is investment growth. Money sitting in a regular 401(k) as after-tax contributions will grow, and that growth is taxable when you convert it. The sooner you move the money into a Roth account, the less time it has to grow and create a tax bill. Many people do this conversion within days or weeks of making the after-tax contribution.

Tax treatment when you convert after-tax money to a Roth

The after-tax money itself is not taxed again when you convert it to a Roth—you already paid income tax on it when you contributed it. However, any earnings (investment growth) that happened while the money sat in your 401(k) is taxable in the year you convert. If you move the money quickly, there is little to no earnings, so the tax bill is small or zero.

The pro-rata rule complicates this if you have other pre-tax IRA balances. The IRS treats all your IRAs as one pool for tax purposes. If you have $50,000 in a traditional IRA and you convert $10,000 of after-tax money from your 401(k) to a Roth IRA, the IRS calculates what portion of that $10,000 comes from pre-tax money (based on the ratio of pre-tax to after-tax across all your IRAs). That pre-tax portion is taxable; the after-tax portion is not. This can create an unexpected tax bill if you have significant pre-tax IRA balances.

You do not face the pro-rata rule if you have no traditional, SEP, or straightforward IRAs. The rule applies only to IRAs, not to money still sitting in your 401(k).

When you can access the money after conversion

Once money is in a Roth IRA through a mega backdoor Roth conversion, the same Roth IRA rules explore. You can withdraw your contributions (the after-tax money you put in) at any time without penalty or tax. You cannot withdraw the earnings until you are 59½ and the account has been open for at least five tax years, with limited exceptions for disability, death, or first-time home purchase.

If you converted the money into a Roth 401(k) instead of a Roth IRA, the rules are stricter. You generally cannot withdraw anything until you leave your job, reach 59½, become disabled, or die. Some plans allow in-service distributions from a Roth 401(k), but this is less common than allowing distributions from a regular 401(k).

Employer plans that allow mega backdoor Roths

Large employers and plans designed for self-employed people or small business owners are most likely to allow after-tax contributions and in-service distributions. Fortune 500 companies often have this feature. Small employers sometimes do not. Public sector plans (government employee plans) rarely allow it.

The only way to know is to ask your plan administrator directly. You can also check your plan's summary plan description (SPD), a document your employer is required to provide. Look for language about "after-tax contributions" and "in-service distributions" or "in-service conversions." If you cannot find it in the SPD, contact your HR or benefits department and ask whether your plan permits both features.

Frequently Asked Questions

Do I have to do a mega backdoor Roth if my plan allows it?

No. It is entirely optional. You can contribute after-tax money to your 401(k) and leave it there, or you can convert it to a Roth. The strategy makes sense only if you have extra money to save and want the tax-free growth that a Roth provides. There is no penalty for not using it.

What happens if I leave my job after making an after-tax contribution?

You can still convert the after-tax money to a Roth IRA after you leave, though the timing and rules depend on your plan. Some plans require you to convert before you leave; others allow it after. Check with your plan administrator before you separate from your employer.

Can I do a mega backdoor Roth and a regular backdoor Roth in the same year?

Yes. They use different contribution limits—the mega backdoor uses your 401(k) plan's after-tax space, and the regular backdoor uses your IRA contribution limit. You can do both in the same year, though the pro-rata rule still applies if you have pre-tax IRA balances.

What if my plan allows after-tax contributions but not in-service distributions?

You cannot use the mega backdoor Roth strategy. After-tax contributions alone do not create a Roth conversion opportunity. Your plan must allow you to move the money out or convert it while you are still employed. If your plan does not, you are limited to a regular backdoor Roth through your IRA.

Is there a limit to how much I can contribute as after-tax money?

Yes. The total of all contributions to your 401(k)—salary deferrals, employer match, employer profit-sharing, and after-tax contributions—cannot exceed $69,000 in 2024 (or $76,500 if age 50+). After-tax contributions fill only the space left after the other three types are accounted for.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.