Roth vs Traditional 401(k)

The Core Difference

Traditional 401(k): contributions reduce your taxable income today. The money grows tax-deferred. When you withdraw in retirement, every dollar is taxed as ordinary income.

Roth 401(k): contributions are made with money that was already taxed. The money grows tax-free. When you withdraw in retirement, neither principal nor growth is taxed.

Both have the same $23,500 employee contribution limit in 2026 ($31,000 with catch-up at 50+). Both grow inside the same plan with the same investment options. The only difference is the timing of the tax bite.

The Math (Side by Side)

When Roth Wins

• You are early-career and in a low tax bracket. A 25-year-old in the 12% federal bracket pays a small tax cost now to lock in tax-free growth. By retirement they may be in the 22% or 24% bracket, making the Roth payment look like a bargain.
• You expect higher taxes in retirement. If you believe federal tax rates will rise (deficit pressure, demographic shifts), or you plan to move to a higher-tax state, Roth hedges against that.
• You want maximum tax diversity. Roth provides a tax-free bucket alongside taxable accounts and Traditional 401(k). In retirement, you can blend withdrawals to manage your bracket year by year.
• You expect large account growth. $23,500 contributed to a Roth at age 25, growing at 7% for 40 years, becomes ~$352,000. None of it is taxed in retirement. The same contribution to Traditional becomes the same $352,000, but every withdrawal is taxed at ordinary income rates.
• You want to avoid Required Minimum Distributions (RMDs). Traditional 401(k) requires withdrawals starting at age 73, which can push you into a higher bracket and trigger Social Security taxation and Medicare IRMAA surcharges. Roth 401(k) (after the 2024 SECURE Act 2.0 changes) has no RMDs.

When Traditional Wins

• You are in a high earning year (peak career). A 50-year-old earning $200,000 in the 24% bracket pays $24,000 less in current federal tax by maxing Traditional. If they retire at 65 with a $4,000/month withdrawal rate, those withdrawals fall in the 12% bracket. Net tax savings: ~12 percentage points × the contribution amount.
• You expect lower taxes in retirement. Most retirees have lower income, especially after kids leave home, mortgage is paid, and they only need 70-80% of working income. Federal tax brackets move with you.
• You need the cash flow now. Traditional contributions reduce your current paycheck less than Roth, freeing up cash for emergency fund, debt payoff, or a home down payment.
• You plan to retire to a no-income-tax state. Working in California (13.3% state tax) and retiring to Texas (0% state) means Traditional pays state tax now at 13.3% and zero in retirement. Roth pays California's high state tax now for benefits that may not need state-tax shelter later.

The Split Strategy

If your plan allows both Roth and Traditional contributions, you can split. A common approach: contribute enough Traditional to drop into a lower federal bracket, then put the rest into Roth.

Example: a 35-year-old earning $130,000 single, currently in the 24% bracket. The 22% bracket cuts off at $103,350 of taxable income (after $15,000 standard deduction, gross of $118,350). Contributing $11,650 to Traditional pulls taxable income exactly to the 22% line. The remaining $11,850 goes to Roth. Result: 22% bracket savings on the Traditional portion, future tax-free growth on the Roth portion.

Tax planners call this bracket-filling. It is more nuanced than "all Traditional" or "all Roth" and matches the idea that future taxation is uncertain. Your marginal tax rate today versus your expected rate in retirement is the key input to this decision.

The Employer Match Question

Until 2023, employer matches always went into a Traditional sub-account, even if your contribution was Roth. The SECURE Act 2.0 changed this: employers can now offer Roth-match (you pay tax on the match in the year contributed, but it grows tax-free).

If your employer offers Roth-match, you can have a fully Roth account. If they only do Traditional-match (still common), you will have a mixed account: your Roth contributions plus the employer's Traditional match. The mixed account is fine; just track which portion is which for retirement planning.

The Highest-Income Loophole: Mega Backdoor Roth

High earners maxing $23,500 may want more Roth space. Some 401(k) plans allow after-tax (non-Roth) contributions up to the combined $70,000 limit, then in-service conversion to Roth. This is the mega backdoor Roth. It requires a plan that explicitly allows after-tax contributions and in-plan Roth conversions, which is more common at large tech and finance employers. If your plan supports it, you can effectively add up to $46,500 more per year to Roth.

What If You Cannot Decide?

The default safe choice is to capture the full employer match in Traditional (since most plans default that way), then do additional contributions in Roth if you are under 35 or in a 22% bracket or lower. Adjust as your career and bracket shift.

Or just do 50/50: half Traditional, half Roth. You give up some optimization but gain tax flexibility in retirement, and you stop overthinking it. For most people, "consistently saving 15% of income" matters far more than the precise Roth/Traditional split.