How do you set up a 401(k) in the United States in 2026?

After you complete Form I-9 and start work, ask Human Resources or payroll for the 401(k) summary plan description and the online enrollment link. Many plans use vendors such as Fidelity, Vanguard, or Empower. You need a Social Security number (SSN) for wage reporting and plan eligibility in almost every case.

Enrollment may be automatic or voluntary. Some employers auto-enroll at a default percentage and a default fund. Others require you to opt in. Either way, log in, set your deferral percentage, and choose investments from the plan menu. Contributions are subject to annual Internal Revenue Service (IRS) limits. Those dollar caps change over time, so treat them as a moving rule: check current IRS figures for employee deferrals and catch-up contributions rather than memorizing one year's number.

Employer matching contributions are optional for the company. If a match exists, contribute at least enough to capture the full match before you chase other savings goals. Match money often follows a vesting schedule. Vesting means you keep your own deferrals immediately, but you may keep the employer match only after a set period of service. Leaving early can forfeit unvested match dollars.

A traditional 401(k) deferral usually lowers your taxable wages for the year. You pay income tax later when you withdraw in retirement (with IRS rules on early withdrawals). A Roth 401(k) deferral is after-tax: you do not get the upfront income-tax break, but qualified withdrawals can be tax-free later if the plan and IRS rules are met.

Not every plan offers Roth. If both exist, the choice depends on whether you expect higher tax rates later and on your cash flow now. You can often split deferrals between traditional and Roth inside the same plan, subject to the combined annual IRS limit.

Payroll also withholds Federal Insurance Contributions Act (FICA) taxes for Social Security and Medicare separately from 401(k) elections. Deferring into a 401(k) does not remove FICA in the usual case.

Many small US employers do not sponsor a plan. In that case you cannot invent a 401(k) yourself. Open an IRA at a bank or brokerage after you have a US bank account and tax ID. Traditional and Roth IRA rules differ from 401(k) rules, including income limits for Roth IRA contributions. IRS pages list current IRA limits each year.

If you later join a company with a 401(k), you can usually keep the IRA and also enroll in the workplace plan. Do not roll money between accounts until you understand plan fees and tax treatment.