Under federal law, an employer-sponsored retirement plan generally cannot make you wait longer than one year of service and your 21st birthday before letting you participate. These minimum service rules sit in both the Internal Revenue Code and ERISA, and they apply to 401(k)s, pensions, and most other qualified plans.1Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards2Office of the Law Revision Counsel. 29 US Code 1052 – Minimum Participation Standards A plan is free to be more generous. Many 401(k)s let you contribute on day one. What a plan cannot do is set the bar higher.
Eligibility turns on whichever milestone comes later. If you reach a year of service before turning 21, you wait until your birthday. If you turn 21 first, you wait until the year of service is complete.
What a Year of Service Actually Means
A year of service is not twelve months on the payroll. Under federal regulations, you complete a year of service when you log at least 1,000 hours of service during a designated twelve-month measurement window called a computation period.3eCFR. 29 CFR 2530.200b-1 – Computation Periods For a 40-hour-a-week schedule, that works out to roughly 25 weeks of full-time work. Part-time employees can still clear 1,000 hours if their time adds up over the full year.
Hours of service covers more than active work. It includes every hour for which you are paid or entitled to payment, so vacation, holidays, sick leave, and back-pay awards count. The plan must apply the same counting rules to everyone.
How the Computation Period Works
Your first computation period starts on your date of hire and runs twelve consecutive months. Hit 1,000 hours in that window and you’ve satisfied the service test. Fall short, and the plan shifts to a second computation period. Most plans line this second window up with the plan year, often January through December, though the plan document can instead use the anniversary of your hire date.
The shift can help you. If you were hired October 1 and did not reach 1,000 hours by the following September 30, a plan-year computation period starting January 1 gives you a fresh twelve-month window that overlaps with part of your first one. The plan document will spell out which method is in use.
The Elapsed Time Method
Some plans skip hour-counting and use the elapsed time method. Service is measured from hire date to separation date, and a year of service is simply twelve continuous months of employment.4eCFR. 26 CFR 1.410(a)-7 – Elapsed Time It is common for salaried workers whose hours are not tracked closely. Once a plan picks a method, it has to apply it uniformly to everyone in the same classification.
The Two-Year Service Exception
A plan may require two full years of service instead of one, but only if it gives you 100% immediate vesting in all employer contributions the moment you become a participant.1Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards No graded vesting schedule is permitted. Every employer dollar is yours from day one.
For 401(k) plans, this exception has a critical limit. It only reaches employer contributions. Even where a plan uses a two-year rule for the employer side, it still has to let you make your own elective deferrals after no more than one year of service.5Internal Revenue Service. 401(k) Plan Qualification Requirements
Long-Term Part-Time Workers
Before 2024, a part-time worker who never hit 1,000 hours in a single year could be shut out of a 401(k) indefinitely. The SECURE Act and SECURE 2.0 Act added a separate route in. If you work at least 500 hours in each of two consecutive twelve-month periods, the plan must let you make elective deferrals, provided you have reached age 21 by the end of the second period.6Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans
SECURE 2.0 shortened what had been a three-year rule to two consecutive years. The two-year version applies to plan years beginning after December 31, 2024, so it is fully in force for 2026 plans.7Internal Revenue Service. Additional Guidance with Respect to Long-Term, Part-Time Employees Twelve-month periods before January 1, 2021 do not count toward the total.8Federal Register. Long-Term, Part-Time Employee Rules for Cash or Deferred Arrangements Under Section 401(k)
The pathway covers 401(k) elective deferrals and, for plan years after December 31, 2024, extends to ERISA-covered 403(b) plans.7Internal Revenue Service. Additional Guidance with Respect to Long-Term, Part-Time Employees It does not entitle long-term part-time workers to employer contributions. Collectively bargained employees and nonresident aliens with no U.S.-source income may be excluded from these rules.8Federal Register. Long-Term, Part-Time Employee Rules for Cash or Deferred Arrangements Under Section 401(k)
When You Actually Enter the Plan
Meeting the age and service tests does not mean you participate that same day. The plan gets a short grace period, but it is capped. You must be admitted no later than the earlier of:
- The first day of the next plan year after you satisfied the requirements, or
- Six months after the date you satisfied them.1Office of the Law Revision Counsel. 26 USC 410 – Minimum Participation Standards
Whichever date lands first is the deadline. Qualify on March 1 under a calendar-year plan and six months out is September 1, before the next January 1, so September 1 is your entry date. Qualify on October 1 and the next January 1 comes before the six-month mark, making January 1 the entry date. The maximum gap between satisfying eligibility and entering the plan is six months.
The same entry-date rules apply to long-term part-time workers who qualify through the 500-hour pathway.8Federal Register. Long-Term, Part-Time Employee Rules for Cash or Deferred Arrangements Under Section 401(k)
Breaks in Service and Rehires
Leaving a job and returning raises the question of whether prior service still counts. A one-year break in service happens when you complete 500 or fewer hours during a computation period.9eCFR. 29 CFR 2530.200b-4 – One-Year Break in Service Cutting hours sharply without leaving can trigger one. When you come back, the plan measures eligibility from your reemployment commencement date with a fresh twelve-month computation period. Prior service may still have to be credited, depending on the plan’s rules and how many consecutive breaks you accumulated.
Federal law also protects absences for pregnancy, childbirth, adoption, or caring for a child immediately after birth or placement. The plan must credit up to 501 hours during such an absence solely to prevent a one-year break in service.2Office of the Law Revision Counsel. 29 US Code 1052 – Minimum Participation Standards Those hours are credited either in the year the absence begins or the following year, whichever prevents the break.
Service With Related Employers
If your employer is part of a controlled group or an affiliated service group, service with any member of that group counts toward eligibility in a plan maintained by any other member. Federal law treats all employees across the group as working for a single employer when testing participation, vesting, and contributions.10Internal Revenue Service. Controlled and Affiliated Service Groups If you transfer from one entity to another within the same group, your eligibility clock should not restart. The practical catch is that employees often do not know their employer is part of such a group, and administrators sometimes miss the cross-entity service when records are not flagged.
Employees a Plan Can Still Exclude
The minimum rules set a floor, not a ceiling. Even after you meet the age and service tests, some categories of workers can be lawfully excluded. Union employees covered by a collective bargaining agreement can be left out of a plan covering non-union employees, as long as retirement benefits were actually part of the bargaining. Nonresident aliens with no U.S.-source earned income from the employer are also treated as excludable.11eCFR. 26 CFR 1.410(b)-6 – Excludable Employees These exclusions are optional. A plan can cover these groups if it chooses, and it still has to pass nondiscrimination testing on the employees it does cover.
Different Rules for SIMPLE IRAs and 403(b) Plans
Not every retirement plan uses the one-year, 1,000-hour framework.
SIMPLE IRA Plans
SIMPLE IRAs, built for small employers, replace hour-counting with a compensation test. The employer must let any employee participate who received at least $5,000 in compensation during any two preceding calendar years and is reasonably expected to earn at least $5,000 in the current year. There is no age 21 requirement and no 1,000-hour test. The employer may set a lower compensation threshold but cannot add extra service or age conditions. For taxable years after December 31, 2024, the $5,000 figure is indexed for inflation and rounded to the nearest $100.12Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts Check the IRS’s annual cost-of-living adjustments for the current threshold.
403(b) Plans
The 403(b) plan, used by public schools, universities, and nonprofits, carries a universal availability rule. If the plan allows any employee to make elective deferrals, it generally must offer the same opportunity to essentially all employees.13eCFR. 26 CFR 1.403(b)-5 – Nondiscrimination Rules Certain groups can be excluded, including employees already eligible under a 401(k) or 457(b) plan of the same employer, nonresident aliens, and students covered by the student exception to FICA.
For employer contributions to an ERISA-covered 403(b), the standard age 21 and one-year service rules apply much as they would for a 401(k). Governmental 403(b) plans and non-electing church plans have more flexibility, since they are exempt from most ERISA participation and vesting rules; their eligibility terms are set by the plan document, subject to basic nondiscrimination principles.
If an Employer Misses Your Enrollment Date
Failing to enroll an eligible employee on time is a common administration mistake, and the IRS treats it as a qualification defect that, left uncorrected, can threaten the plan’s tax-exempt status.
The standard IRS fix requires the employer to make a qualified nonelective contribution equal to 50% of the deferrals you missed, calculated either from your actual election if known or from the average deferral percentage of your group for that year.14Internal Revenue Service. 401(k) Plan Fix-It Guide – Eligible Employees Weren’t Given the Opportunity to Make an Elective Deferral Election The corrective contribution has to be adjusted for earnings from when the deferrals should have gone in through the date of the correction, and you are immediately 100% vested in it.15Internal Revenue Service. Fixing Common Plan Mistakes – Correcting a Failure to Effect Employee Deferral Elections If the plan catches the error quickly, reduced corrective contributions are available in some circumstances. The cost of the fix falls on the employer, not on the employee who was wrongly excluded.