To figure out what type of 401(k) you have, start with three documents you already have: your most recent Form W-2, your Summary Plan Description, and your latest account statement. Box 12 of the W-2 carries a single letter code that answers the first question most people are asking. Code D means your contributions were pre-tax (a traditional 401(k)); code AA means designated Roth. If both codes appear, you contributed to both buckets during the year.1Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3 The SPD and your account statement fill in the rest: the plan’s full name, any specialized designation like Safe Harbor or SIMPLE, and your vesting schedule.
Reading Box 12 of Your W-2
Box 12 uses letter codes to categorize how each contribution was treated for tax purposes. For 401(k) purposes, only two matter.
Code D next to a dollar amount means standard pre-tax elective deferrals. The money came out of your paycheck before income tax was calculated, and you’ll pay tax on it when you withdraw in retirement. On your pay stub, the same contributions are usually labeled “pre-tax” or “elective deferrals.” If you were automatically enrolled in your plan and never picked a contribution type, your contributions are almost certainly pre-tax, since that is the default in most plans.2Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans
Code AA means designated Roth 401(k) contributions. You paid income tax on the money before it went in, and qualified withdrawals in retirement come out tax-free. The plan tracks Roth money in a separate account from any pre-tax balance so each bucket keeps its tax treatment.3Internal Revenue Service. Roth Comparison Chart
Seeing both D and AA in Box 12 means you split your deferrals during the year. The combined total still can’t exceed the annual deferral cap of $24,500 for 2026; splitting doesn’t double the limit.3Internal Revenue Service. Roth Comparison Chart
What Your Plan Documents Add
The W-2 tells you about tax treatment. It doesn’t tell you what your plan is called or how the employer side works. That’s what the Summary Plan Description is for. Federal law requires your employer to give you an SPD written in language an average participant can understand, and it must lay out eligibility, contribution types, vesting, and claims procedures.4Office of the Law Revision Counsel. 29 USC 1022 – Summary Plan Description Most employers post it in an HR portal or hand it over on request from the benefits coordinator.
Your annual account statement from the financial institution holding the plan will label each account: “Pre-Tax 401(k),” “Roth 401(k),” and sometimes a third line item for after-tax contributions. If your plan files a Form 5500, you can also look it up through the Department of Labor’s public search tool at 5500search.dol.gov, which shows the plan’s total assets, participant count, and plan type.5United States Department of Labor. EBSA Website Update – New Online Search Tool for Form 5500s Filed With the U.S. Department of Labor
An After-Tax Bucket That Isn’t Roth
Some plans allow a third contribution type: voluntary after-tax contributions that are not designated as Roth. You pay tax on the money going in like Roth, but earnings are taxed as ordinary income when withdrawn. This type doesn’t get its own letter code in Box 12; the plan tracks it internally. If your account statement shows a separate balance labeled “after-tax” or “voluntary after-tax” alongside your pre-tax or Roth balance, your plan permits these contributions. They count toward the total annual additions limit under Section 415(c), which is $72,000 for 2026.6IRS.gov. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living
Signs You Have a Specialized Plan Type
Beyond pre-tax versus Roth, some 401(k) plans carry a designation that changes employer contributions, vesting, or contribution limits. The SPD and account statements normally name the plan, but the tells below are quick sanity checks.
Safe Harbor 401(k)
A Safe Harbor 401(k) requires your employer to make a minimum contribution each year, either a match or a flat contribution regardless of whether you defer. The clearest indicator is that Safe Harbor employer contributions are fully vested immediately: you own 100% of them from day one. If your SPD or online portal shows employer contributions as 100% vested with no waiting period, you likely have a Safe Harbor plan. Plans that use a Qualified Automatic Contribution Arrangement (QACA) are a variation and may apply a two-year cliff vesting schedule instead.
Safe Harbor plans also send an annual notice, typically 30 to 90 days before the start of each plan year, describing the matching formula and your right to change your deferral percentage.7Internal Revenue Service. Retirement Topics – Notices Receiving that notice each fall is another strong sign.
SIMPLE 401(k)
A SIMPLE 401(k) is used by small businesses and has a lower employee deferral limit: $17,000 for 2026, versus $24,500 for a standard 401(k).6IRS.gov. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living If the contribution cap shown on your account statement or in your SPD is well below the standard amount, you’re in a SIMPLE plan. SIMPLE catch-up contributions are also smaller: $4,000 for participants aged 50 and older, or $5,250 for those aged 60 through 63, in 2026.
Solo 401(k)
A Solo 401(k), also called a one-participant 401(k), Solo-k, or Uni-k, is available only to business owners with no employees other than a spouse.8Internal Revenue Service. One-Participant 401(k) Plans If you’re self-employed and opened the plan yourself, this is your plan type. Documents from the brokerage typically include “Individual” or “One-Participant” in the plan name. Solo plans follow the same $24,500 deferral limit as standard plans but also permit employer-side profit-sharing contributions, up to the $72,000 total annual additions cap for 2026.6IRS.gov. 2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living
Using 2026 Contribution Limits as a Tell
The maximum deferral your plan allows is often the fastest way to confirm which specialized type you have. The 2026 limits:9Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
- Standard 401(k) deferral, pre-tax or Roth: $24,500
- Catch-up for age 50 and older: $8,000, for a total of $32,500
- Enhanced catch-up for ages 60 through 63 under SECURE 2.0: $11,250, for a total of $35,750
- Total annual additions (employee + employer + after-tax): $72,000
- SIMPLE 401(k) deferral: $17,000
- SIMPLE 401(k) catch-up: $4,000 at age 50+, $5,250 at ages 60 through 63
A plan capped at $17,000 is SIMPLE. A plan capped at $24,500 with immediate vesting on employer money is a strong Safe Harbor candidate. A plan capped at $24,500 with room for large employer profit-sharing contributions and a single participant is a Solo.
If You Still Can’t Tell
When the documents don’t settle the question, a few direct steps will:
- Log into your plan’s online portal and open “Account Overview” or “Plan Profile.” The classification of each account, pre-tax, Roth, or after-tax, usually appears on the main dashboard.
- Ask your plan administrator or benefits coordinator for a written confirmation of your plan type, contribution structure, and vesting schedule.
- Call the customer service number on your account statement. A representative can confirm the account classification and whether contributions are pre-tax, Roth, or both.
Keep any written confirmation with your SPD and annual statements so the answer is in one place next tax season.