The 401(k) In-Plan Roth Conversion, Explained

A 401(k) in-plan Roth conversion is one of those retirement moves that sounds far more complicated than it is. Strip away the jargon and it comes down to a single choice: pay tax on some of your retirement savings now, so that it and its future growth can come out tax-free later. This guide walks through how it works, what it costs, and how to decide if it belongs in your plan.

What it actually is

Most employer retirement plans have two sides. The traditional side holds pre-tax money: you did not pay income tax on it when it went in, so you will pay tax when you take it out in retirement. The Roth side holds after-tax money: you already paid tax on it, so qualified withdrawals later are tax-free.

An in-plan Roth conversion simply moves money from the traditional side to the Roth side, inside the same plan. You do not withdraw anything, and you do not open a new account. You change how a chunk of your savings will be taxed down the road.

Two conditions have to be met. Your plan must offer a Roth option, and your plan must specifically permit in-plan conversions. Not every plan does, so the first practical step is always to confirm that yours allows it.

The trade you are making

When you convert pre-tax dollars, that amount is added to your taxable income for the year and taxed at your ordinary income rate. That is the cost, paid now. In return, once the money has met the plan's holding requirements, future qualified withdrawals of both the converted amount and everything it earned come out tax-free.

So the entire decision hinges on one question: do you expect your tax rate to be higher now, or higher in retirement? If you expect a higher rate later, paying tax now at today's lower rate tends to come out ahead. If you expect a lower rate later, waiting usually wins. Everything else is detail around that central bet.

Why you should usually pay the tax from outside money

You can cover the conversion tax bill in one of two ways: from savings outside the retirement account, or by having some of the converted money held back for taxes. Paying from outside money is generally the stronger move, for two reasons. First, it lets the full converted balance stay invested and keep compounding tax-free. Second, if you are under age 59 and a half, money held back to pay taxes can itself be treated as an early withdrawal and hit with a penalty. Keeping the whole balance intact avoids that.

In-plan conversion versus rolling to a Roth IRA

People often confuse the in-plan conversion with rolling money out to a Roth IRA. They both get you to Roth, but they are not the same path.

• In-plan conversion keeps the money inside your employer plan. You stay with the plan's investment menu and its rules, and you keep the strong creditor protections that workplace plans carry.
• Rolling to a Roth IRA moves the money out to an account you control, which usually opens up far more investment choices. The catch is that many plans only let you do this after you leave the employer, or under limited in-service withdrawal rules.

For federal employees and military members, the Thrift Savings Plan version of this is the TSP Roth in-plan conversion, which became available inside the plan in 2026. The mechanics and the tax math are the same idea covered here.

The holding rules before you withdraw

Roth money carries holding requirements before it can come out tax-free and penalty-free. In broad terms, converted amounts and their earnings need time in the account, and withdrawing too soon, especially before age 59 and a half, can trigger taxes or a 10 percent penalty. The precise age and timing rules have several moving parts, so treat a conversion as a long-term decision and confirm the withdrawal rules for your specific plan before you rely on the money in the near future.

One point worth knowing: under recent changes to the law, designated Roth accounts inside employer plans are no longer subject to required minimum distributions during the original owner's lifetime. For some savers, escaping those forced withdrawals is itself a reason to convert.

A related move: the mega backdoor Roth

Some plans also allow after-tax contributions on top of the standard pre-tax and Roth limits. Converting those after-tax dollars to Roth inside the plan is the strategy often nicknamed the "mega backdoor Roth." It is a cousin of the conversion described here, aimed at high savers who want to get more money into Roth than the normal limits allow. If your plan offers after-tax contributions and in-plan conversions together, it is worth asking a professional whether this applies to you.

Who tends to benefit

• Savers who expect to be in a higher tax bracket in retirement than they are today.
• People having a lower-income year, when the tax on the conversion is smaller.
• Those who can pay the conversion tax from outside savings rather than from the account.
• Anyone who values tax-free growth, tax diversification, or avoiding required withdrawals later.
• Savers with a long time horizon, so tax-free compounding has room to work.

Who should be cautious

• Anyone who expects a lower tax rate in retirement than they pay now.
• People who would have to pay the conversion tax out of the converted money itself.
• Savers who might need the money in the near term, before the holding rules are satisfied.
• Anyone for whom a large conversion would push a big chunk of income into a much higher bracket. Spreading conversions across several years often softens this.

Frequently asked questions

Does converting cost me anything beyond the tax?

Usually the tax is the main cost. Plans generally do not charge a fee to convert, but the converted amount is added to your taxable income for the year, which is the real expense to plan around.

Can I undo a conversion if I change my mind?

No. Unlike some older rules that allowed reversals, a Roth conversion today is generally permanent. That is exactly why it is worth running the numbers and, for larger amounts, talking to a professional first.

Will a conversion push me into a higher tax bracket?

It can, because the converted amount stacks on top of your other income for the year. Many people convert smaller amounts across several years to stay within a target bracket rather than converting a large sum all at once.

How do I estimate the impact?

Start with our conversion calculator, which compares the after-tax value of converting versus staying traditional using rates you enter. Then confirm the specifics with the official resources below and a qualified advisor.

What if my income is too high for a Roth IRA?

That is a different but related situation, solved by a separate strategy. See our guide to the backdoor Roth for federal employees, which explains how high earners get money into a Roth IRA and why TSP participants often have an easier time of it.

Sources

• Internal Revenue Service, rules on Roth accounts and rollovers: irs.gov rollover rules
• Internal Revenue Service, designated Roth accounts: irs.gov designated Roth account FAQs
• Thrift Savings Plan, Roth in-plan conversions: tsp.gov