Rollover IRA vs New 401(k): The Math to Keep More of Your Job-Change Money

The job-change decision that can quietly cost (or save) you five figures

When you leave a job, your old 401(k) doesn’t just “sit there.” It becomes a choice with real math behind it: roll it to an IRA, move it to your new employer’s 401(k), or (sometimes) leave it where it is. People treat this as paperwork. It’s not. It’s portfolio access, fee drag, creditor protection, and—often missed—the ability to do a backdoor Roth IRA later.

Here’s what the numbers tell us: a 0.50% difference in all-in fees on a $150,000 balance over 20 years can easily add up to tens of thousands of dollars. And the wrong account structure can trigger the pro-rata rule and make a backdoor Roth far less effective.

I’m going to crunch this the way I’d do it for my own money: cost first, then rules, then convenience.

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Data: the 5 variables that usually decide the rollover

Below is a decision table you can use without guessing. “Best” depends on your situation, but the trade-offs are consistent.

Two key reference points:

• IRA and 401(k) contribution rules and rollover rules live at the IRS. If you want the source material, start at IRS retirement plan rollovers on irs.gov.
• If you’re using labor market data to time a job hop (and you should), the BLS is the cleanest public source for job openings, quits, and wage data at bls.gov.

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Analysis: fee drag is the silent killer (and it’s easy to underestimate)

Fees don’t feel like a bill. They feel like “slightly lower returns.” That’s why they’re dangerous.

Example: 0.50% extra fees on a mid-career balance

Assume:

• Starting balance: $150,000
• Time horizon: 20 years
• Gross return assumption: 7.0%
• Scenario A all-in cost: 0.10% (low-cost IRA portfolio)
• Scenario B all-in cost: 0.60% (expensive 401(k) plan or high-fee funds)
• Net return: 6.90% vs 6.40%

Approximate outcome:

• At 6.90% for 20 years: ~$150,000 × (1.069^20) ≈ $570k
• At 6.40% for 20 years: ~$150,000 × (1.064^20) ≈ $520k
• Difference: ~$50,000

That’s not theoretical. That’s a used car, a year of college tuition in-state, or a major chunk of your retirement “buffer.”

What to do with this info (practically)

Before you move a dollar, ask for:

• Your old plan’s expense ratios (fund fees)
• Any plan administrative fees (sometimes listed as a % or per-quarter charge)
• Your new plan’s fee disclosure (it exists; HR can provide it)
• Your IRA provider’s trading/maintenance fees (often $0, but verify)

If you need a refresher on which account type fits your broader retirement setup, see 401(k) vs IRA: Which Retirement Account Is Right for You?. The rollover decision is basically that comparison—but with one extra landmine: backdoor Roth math.

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Analysis: the backdoor Roth “pro-rata” rule is the most common rollover mistake

If your income is high enough that you’re thinking about a backdoor Roth IRA, you need to understand one thing: pre-tax money in Traditional IRAs can make the backdoor Roth partially taxable due to the pro-rata rule.

Here’s the plain-English version:

• The IRS doesn’t let you cherry-pick “just the after-tax dollars” to convert.
• It looks at all your Traditional IRA balances (including rollover IRAs, SEP IRAs, and SIMPLE IRAs) as one big bucket.

Example: why a rollover IRA can mess up a clean backdoor Roth

Assume you do a $7,000 non-deductible IRA contribution and then convert it.

• You also have a rollover IRA worth $93,000 (all pre-tax).
• Total IRA balance: $93,000 + $7,000 = $100,000
• Non-deductible portion: $7,000 / $100,000 = 7%
• That means about 93% of your conversion is taxable.

So instead of a mostly tax-free conversion, you just created taxable income you didn’t plan for.

In many real households (especially dual-income), this is the deciding factor: rolling old 401(k) money into a new 401(k) can keep your IRA balance at (or near) $0 and preserve backdoor Roth flexibility.

For the deeper tax-break math on contributions, see Roth IRA vs Traditional IRA: Tax Break Math for 2026 Contributions.

Practical workaround that often wins

If your new employer’s 401(k) accepts roll-ins and has decent low-cost index options, moving pre-tax dollars into the 401(k) can:

• consolidate accounts, and
• keep IRAs clear for clean backdoor Roth conversions.

Not every plan allows this. Some do, some don’t. You have to ask.

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Analysis: protection, access, and the real-life “messy” factors

Money decisions aren’t made in a vacuum. Here are the factors that aren’t pure return math—but still matter.

1) Creditor protection (state rules matter)

401(k)s governed by ERISA generally have strong creditor protections. IRAs also have protections, but the details can vary by state and situation. If you’re in a higher-liability profession (medical, real estate, business owner), the “safer wrapper” can be a real consideration.

My take: if you’re choosing between two similar-cost options, I give extra weight to the employer plan’s legal protections—especially if you live in a state where IRA protections are less straightforward.

2) Loans and “I might need the cash” thinking

A 401(k) may allow loans; IRAs generally don’t. That sounds like a feature—until it becomes a habit.

Let me show you

• You roll into a new 401(k).
• Six months later, you’re tempted to borrow $10,000 for a kitchen remodel.
• If you lose your job again, that loan can become due fast, and a default can become taxes + penalties.

If you’re paycheck to paycheck, the best solution is usually not “retirement account flexibility.” It’s cash reserves and a spending system. A good companion read is Budgeting Basics: The 50/30/20 Rule because many loan temptations are really cash-flow problems in disguise.

3) Investment quality: some 401(k)s are excellent, some are trash

I’ve seen 401(k) lineups with:

• a 0.03% total market index fund,
• a 0.08% bond index fund,
• and a 0.12% target-date series.

I’ve also seen plans where the cheapest option was a 0.75% actively managed fund with a salesy pitch in the description.

Your job-change rollover decision is, in part, a judgment on your new plan’s lineup.

Specific local example with real numbers (New York City)

Let’s say you’re a NYC employee moving from a mid-size firm to a larger employer, and you’re sitting on $220,000 in your old 401(k). You’re in a combined marginal bracket that can easily be 30%+ when you stack federal, New York State, and New York City income taxes. (NYC adds its own local income tax—most Americans don’t deal with that.)

A sloppy rollover that accidentally triggers withholding or a taxable distribution isn’t just “oops.” It can create a real tax bill. For higher-tax locations like NYC, the margin for error is thinner—so clean, direct rollovers matter more.

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Checklist: how to decide in under an hour (without missing landmines)

Step 1: Identify what you actually have

• Old account is Traditional 401(k), Roth 401(k), or both
• Any after-tax (non-Roth) contributions in the plan?
• Any existing Traditional/SEP/SIMPLE IRA balances?

Step 2: Compare the plan costs and fund quality (write down numbers)

• Old 401(k) lowest-cost broad index fund expense ratio: ____%
• New 401(k) lowest-cost broad index fund expense ratio: ____%
• Any admin fee (percent or $/quarter): ____%
• IRA option: can you build a simple 3-fund ETF portfolio under ~0.10%? yes/no

Step 3: Backdoor Roth check (this is the fork)

• Do you expect to need backdoor Roth contributions in the next 1–5 years? yes/no
• If yes:
  • Avoid creating/keeping big pre-tax IRA balances (pro-rata risk)
  • Prefer rolling pre-tax 401(k) into a 401(k), if the plan allows and is reasonably priced

Step 4: Execute cleanly (avoid taxable mistakes)

• Use a direct rollover (trustee-to-trustee), not a check made out to you
• Confirm how Roth dollars will be handled (Roth IRA vs Roth 401(k) roll-in rules)
• Keep rollover confirmation letters for tax time

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The quick summary:

Here’s what the numbers tell us: for most job-changers, the best rollover choice is the one that (1) keeps ongoing fees low, (2) preserves tax flexibility (especially backdoor Roth options), and (3) reduces the chance of a costly paperwork mistake.

• If your new 401(k) is low-cost and accepts roll-ins, it’s often the best “clean” move for pre-tax money—especially if you’re a backdoor Roth candidate.
• If your new 401(k) is expensive or has weak fund options, a rollover IRA can be a better bang for your buck—unless it blocks your future backdoor Roth strategy.
• Leaving money in the old 401(k) can be fine temporarily, but long-term it tends to create account sprawl and missed optimization.

If you’re going to be picky about one thing, be picky about fee drag and pro-rata risk. Those are the two quiet leaks that turn a routine job change into a five-figure mistake.