Retirement Planning

401(k) vs. IRA: Traditional vs. Roth — The Complete Decision Guide

9 min read

The retirement account landscape can feel like alphabet soup: 401(k), Roth 401(k), Traditional IRA, Roth IRA, SEP IRA, SIMPLE IRA. Each has different rules, different tax treatments, and different income limitations. Making the wrong choice can cost tens of thousands of dollars over a career. This guide walks through every major decision point, in order, so you know exactly what to do.

The Decision Tree: Where Your Next Dollar Goes

Before we get into the details of each account type, here is the priority order that maximizes tax efficiency and employer benefits for most people:

Get the full employer 401(k) match. This is free money with a guaranteed 50% or 100% immediate return. No investment in the world beats it. If your employer matches 50% of contributions up to 6% of salary, contribute at least 6%. That match alone represents a 50% return before a single dollar is invested.
Max out a Health Savings Account (HSA), if eligible. The HSA is the only triple-tax-advantaged account: contributions are pre-tax, growth is tax-free, and withdrawals for medical expenses are tax-free. At 65, you can withdraw for any reason and pay only ordinary income tax — effectively turning it into a Traditional IRA with bonus medical benefits. For 2025, the HSA contribution limit is $4,300 for individuals and $8,550 for families.
Max out a Roth IRA (or use the Backdoor Roth if income exceeds limits). Roth dollars grow tax-free and withdraw tax-free. No required minimum distributions (RMDs) during your lifetime. Contributions (but not earnings) can be withdrawn at any time without penalty, giving the Roth IRA a secondary role as an emergency fund of last resort.
Go back and max out the 401(k) to the full $23,500 limit (2025). After the match is secured and the Roth IRA is filled, additional 401(k) contributions — Traditional or Roth depending on your tax situation — are the next priority.
If you're still saving, contribute to a taxable brokerage account. No tax benefits, but total flexibility and no contribution limits.

The Four Major Account Types

Traditional 401(k)

Contributions are made pre-tax through payroll deduction, reducing your taxable income in the year of contribution. Money grows tax-deferred. Withdrawals in retirement are taxed as ordinary income. For 2025, the employee contribution limit is $23,500, with a $7,500 catch-up contribution available for those age 50 and older (total: $31,000). The combined employee-plus-employer limit is $70,000.

Many employers now offer a Roth 401(k) option alongside the Traditional. A Roth 401(k) works like a Traditional 401(k) in structure but like a Roth IRA in tax treatment: contributions are after-tax, growth is tax-free, qualified withdrawals are tax-free. The contribution limits are the same. Importantly, Roth 401(k)s do have RMDs starting at age 75 (under current law), unlike Roth IRAs. However, you can roll a Roth 401(k) into a Roth IRA upon leaving your employer to eliminate that requirement.

Traditional IRA

Anyone with earned income can contribute to a Traditional IRA. For 2025, the contribution limit is $7,000 ($8,000 for those 50+). But the deductibility of those contributions depends on income and whether you (or your spouse) are covered by a workplace retirement plan.

If you are covered by a workplace retirement plan, the deductibility of Traditional IRA contributions phases out:

Single filers: Modified Adjusted Gross Income (MAGI) above $79,000 begins the phase-out; no deduction above $89,000 (2025, estimated).
Married filing jointly: MAGI above $126,000 begins the phase-out; no deduction above $146,000.

If you are not covered by a workplace plan but your spouse is, the phase-out for the non-covered spouse starts at a higher MAGI (around $236,000 for 2025).

A non-deductible Traditional IRA contribution still grows tax-deferred, but you'll pay ordinary income tax on the earnings when you withdraw. For high earners without a deduction, the non-deductible Traditional IRA is essentially only useful as the first step of a Backdoor Roth.

Roth IRA

Roth IRA contributions are made with after-tax dollars. Growth is tax-free, and qualified withdrawals (after age 59½, account open at least 5 years) are entirely tax-free. You can withdraw your contributions at any time without tax or penalty — but not earnings. No RMDs during your lifetime, making the Roth IRA a powerful estate planning tool.

The catch: income limits. For 2025, the ability to contribute directly to a Roth IRA phases out at:

Single filers: MAGI between $146,000 and $161,000. Above $161,000, no direct contribution is allowed.
Married filing jointly: MAGI between $230,000 and $240,000. Above $240,000, no direct contribution.

Note that modified adjusted gross income here is before the standard deduction and before 401(k) contributions. A single person earning $170,000 who contributes $23,500 to a Traditional 401(k) brings their MAGI to $146,500 — potentially under the Roth IRA phase-out threshold. This crossover is worth planning around.

Roth vs. Traditional: The Core Math

The Roth vs. Traditional decision boils down to one question: Is your marginal tax rate higher now, or will it be higher in retirement?

If your current marginal rate is higher than your expected retirement rate, Traditional wins. You take the deduction at your high current rate and pay tax at your lower future rate.

If your current marginal rate is lower than your expected retirement rate, Roth wins. You pay tax now at the low rate and enjoy tax-free withdrawals later.

If the rates are the same, the outcome is mathematically identical. $10,000 invested in a Traditional account at a 22% tax rate is equivalent to $7,800 invested in a Roth. At a 7% annual return over 30 years, the Traditional grows to $76,123 but is taxed at 22% on withdrawal, yielding $59,376 after tax. The Roth grows to $59,376 — the exact same number. The difference is entirely about rate arbitrage.

A Detailed Worked Example

Consider Sarah, age 35, earning $120,000 as a single filer. Her marginal federal rate is 24%. She has $10,000 of pre-tax money to invest this year and must decide between Traditional and Roth.

Scenario A: Traditional contribution. Sarah puts $10,000 into a Traditional 401(k). She saves $2,400 on her current tax bill (24% of $10,000). The $10,000 grows at 7% for 30 years to $76,123. If Sarah's retirement tax rate is 15% (typical for a retiree with moderate income), she owes $11,418 in tax. Net: $64,705.

Scenario B: Roth contribution. Sarah pays the $2,400 tax now and invests the remaining $7,600 in a Roth IRA. At 7% for 30 years, it grows to $57,853 — all tax-free. Net: $57,853.

Traditional wins by $6,852 because Sarah's current rate (24%) exceeded her retirement rate (15%).

But now imagine Sarah expects her income to rise sharply, and she plans to retire with substantial rental income, Social Security, and required minimum distributions pushing her into the 24% bracket — or if she believes tax rates will rise systemically. In that case, Roth would be the right call, locking in today's 24% rate against a future that could be higher.

When Roth Makes Sense

You're early in your career and your income (and tax rate) will almost certainly rise.
You're in a temporarily low-income year (sabbatical, job transition, graduate school).
You expect significant taxable retirement income from pensions, rental properties, or large Traditional IRA balances that will fill up lower tax brackets.
You want to leave tax-free assets to heirs.
You value the flexibility of being able to withdraw contributions without penalty before 59½.
You believe federal tax rates will increase in the future due to fiscal pressures.

When Traditional Makes Sense

You're in your peak earning years (high current marginal rate, lower expected retirement rate).
You need the current-year tax deduction to reduce your tax bill.
You expect to be in a lower tax bracket in retirement — which is true for most people, since retirement income typically replaces only 70-80% of pre-retirement income.
You're planning to retire early and will have years with low taxable income during which you can perform Roth conversions at favorable rates (the Roth conversion ladder strategy).
You live in a high-tax state now and plan to retire to a low- or no-tax state.

Tax Diversification: Why You Probably Want Both

The future of tax policy is unknowable. Congress can change rates. Your income in retirement may not look like what you project. Having assets in both Traditional and Roth accounts gives you flexibility to manage your taxable income year by year.

A common strategy: contribute enough to Traditional to fill the lower tax brackets in retirement (where you'll pay 10% or 12%), then use Roth withdrawals for spending above that threshold. This minimizes lifetime taxes without betting entirely on one rate environment.

In retirement, someone with a mix of account types can withdraw from Traditional accounts up to the top of the 12% bracket, then switch to Roth withdrawals for additional spending needs — avoiding the 22% bracket entirely. Someone with only Traditional accounts would be forced to pay the higher rate on every dollar above the threshold.

The Backdoor Roth IRA for High Earners

If your income exceeds the Roth IRA limits, you can still contribute via the Backdoor Roth strategy:

Contribute $7,000 to a Traditional IRA (non-deductible, since your income exceeds the deduction phase-out).
Immediately convert that Traditional IRA to a Roth IRA.
Because the contribution was non-deductible (after-tax), you owe no additional tax on the conversion — except on any earnings that accrued between contribution and conversion, which should be minimal if you convert promptly.
File IRS Form 8606 to report the non-deductible contribution and the conversion.

Critical warning: the pro-rata rule. If you have any existing pre-tax Traditional IRA balances (from previous deductible contributions or rollovers from old 401(k)s), the IRS treats the conversion as coming proportionally from pre-tax and after-tax dollars. You cannot selectively convert only the after-tax portion. This means a Backdoor Roth can trigger unexpected taxes if you have a large Traditional IRA. One way around this: roll your Traditional IRA into your current employer's 401(k) plan (if the plan accepts incoming rollovers), clearing the Traditional IRA balance before executing the Backdoor Roth.

The Mega Backdoor Roth

Some 401(k) plans allow after-tax contributions beyond the $23,500 pre-tax/Roth employee limit, up to the combined $70,000 limit (including employer match). If the plan also allows in-service withdrawals or in-plan Roth conversions of those after-tax contributions, you can funnel tens of thousands of additional dollars into Roth accounts each year.

The mechanics: contribute after-tax dollars to the 401(k) above the $23,500 limit, then immediately convert them to the Roth portion of the 401(k) or roll them to a Roth IRA. Since the contributions are after-tax, you owe tax only on any earnings that accrued before the conversion. Many plans automate this with daily conversions to minimize taxable earnings.

The Mega Backdoor Roth is not available in every plan. Check your plan documents for three things: after-tax contributions, in-service withdrawals, and Roth in-plan conversions. If all three are present, this is one of the most powerful wealth-building tools available.

The Complete Priority Flowchart

Does your employer match 401(k) contributions? Contribute enough to capture every dollar of the match.
Are you eligible for an HSA (high-deductible health plan)? Max it out ($4,300 individual / $8,550 family). Pay current medical expenses out of pocket and save receipts. Let the HSA grow.
Does your income allow direct Roth IRA contributions? If yes, max it out ($7,000). If no, execute a Backdoor Roth IRA (assuming no pro-rata complications from existing Traditional IRA balances).
Max out your 401(k) to the full $23,500 limit. Choose Traditional or Roth based on the rate comparison above. If your plan offers a Roth 401(k) and you expect a higher future rate, use it. Otherwise, stick with Traditional.
Does your 401(k) support the Mega Backdoor Roth? If yes, contribute after-tax dollars and convert.
Any remaining savings go to a taxable brokerage account. Use tax-efficient investments (index ETFs, municipal bonds if in a high bracket).

Common Mistakes

Contributing to a non-deductible Traditional IRA and leaving it there. Non-deductible Traditional IRA contributions grow tax-deferred, but earnings are taxed as ordinary income — worse than a taxable brokerage account, where long-term capital gains rates apply. Convert it to Roth or don't bother.
Choosing Roth over Traditional at peak earnings without thinking. If you're in the 32% or higher bracket, Traditional almost certainly wins unless you have a specific reason to believe your retirement rate will be even higher.
Neglecting the Roth IRA because you have a 401(k). The Roth IRA gives you an extra $7,000 of tax-advantaged space and the flexibility of withdrawing contributions. It complements the 401(k), not replaces it.
Forgetting to invest the cash. Contributions sitting in a money market settlement fund inside a Roth IRA do nothing. Contributions must actually be invested.

The retirement account puzzle isn't one-size-fits-all. The flowchart above covers roughly 90% of savers. If your situation is unusual — self-employed, expecting a large inheritance, planning to retire abroad — the principles remain the same even as the specific tactics need adjustment. Start with the match, capture the tax arbitrage, and let time do the rest.