Are Part-Time Employees Eligible for a 401(k)?

For most of the time I have been running businesses, the answer to this question was effectively no. Part-time employees rarely got into a 401(k), not because anyone excluded them explicitly, but because plans required 1,000 hours of service and a person working twenty hours a week never got there. That was the design, and everybody understood it.

That changed, and a lot of small business owners have not caught up. The SECURE Act created a second route into the plan for long-term part-time employees, and SECURE 2.0 shortened it. If someone works 500 hours a year for two consecutive years, they must be allowed to contribute their own money to your 401(k), even if they never come close to 1,000 hours. This is not optional, it is already in effect, and the tracking obligation falls on you.

This guide covers what you must allow, what you can decline, exactly how the hours are counted, and the one plan design choice that makes the whole tracking problem disappear. The framing throughout is the small business without a benefits department. I build the hour tracking and employee records that make this administrable into FirstHR. Retirement plan rules are technical and this is general information rather than legal or tax advice, so confirm your specific plan design with your recordkeeper or plan advisor.

The Short Answer

Yes, part-time employees are eligible for a 401(k) in many cases, and increasingly they must be allowed in whether you want them or not. The rule that changed this is the long-term part-time employee rule, and its core requirement is short enough to state in one sentence: an employee who is at least 21 and works at least 500 hours in each of two consecutive 12-month periods must be permitted to make elective deferrals into your plan.

The gap this rule was written to close is real and measurable. Part-time workers have historically been largely locked out of workplace retirement savings, and the numbers show it plainly.

That framing is worth holding onto, because it explains why the obligation is written the way it is: you must let them save, but nobody is forcing you to fund it. The distinction between those two things is where almost all of the confusion in this topic lives, and it is the next thing to get straight.

The Two Routes to Eligibility

A part-time employee can get into your 401(k) by two entirely different paths, and they carry different consequences for you. Understanding which route a given person took is what tells you whether you owe them a match.

The standard rules set the outer limits of what a plan may require. Per IRS plan qualification requirements, a plan cannot require an employee to be older than 21, and it must allow an employee to make elective deferrals after no more than one year of service. A plan may require up to two years of service for eligibility to receive an employer contribution, but only if the participant is 100 percent vested in all plan account balances after no more than two years.

You can always be more generous than the maximums. A plan may allow immediate participation, or set the age at 18, or require no hours at all. What you cannot do is be stricter: no age condition above 21, no service condition above one year for deferrals. That ceiling is what makes the 1,000-hour condition the standard design, and it is exactly the condition the long-term part-time rule works around.

The Long-Term Part-Time Rule, in Detail

The rule arrived in two pieces and its history matters because the effective dates and the number of years both changed. Getting this timeline right is where most articles on the subject slip.

Three details in that table do the heavy lifting. First, service in 12-month periods beginning before January 1, 2021 is disregarded entirely for eligibility purposes. An employee who worked part time for you throughout the 2010s starts from a clean slate; only 2021 onward counts. Second, the requirement dropped from three consecutive years to two for plan years beginning after December 31, 2024. Third, and most importantly, what is required is the opportunity to defer, not an employer contribution.

That distinction between operational compliance and document amendment is genuinely important and routinely misunderstood. You do not get to wait. If an employee qualified on January 1 and you did not offer them the chance to defer, that is an operational failure requiring correction, regardless of whether your plan document has been formally updated yet.

A Worked Example

The hours arithmetic is easier to see than to describe, so here is a concrete case of the kind that actually shows up at a fifteen-person business.

Two things about Maria are worth drawing out. Her long tenure before 2021 counts for nothing on the eligibility clock, which is a relief for employers worried about a sudden wave of newly eligible veterans. And once she is in, she is in on a limited basis: she can put her own money in, and you owe her nothing beyond administering that.

Note also that if Maria ever crosses 1,000 hours in a single year, everything changes. She becomes a regular participant under standard eligibility rather than a long-term part-time employee, and as a regular participant she is entitled to whatever employer contributions the plan provides to everyone else. The 1,000-hour route always wins when both are satisfied.

Do You Have to Match Their Contributions?

No, and this is the answer most employers are actually looking for. An employee who becomes eligible solely through the long-term part-time rule must be allowed to make elective deferrals, but you are not required to provide matching, non-elective, profit-sharing, or safe-harbor contributions on their behalf. That holds even if you make those contributions for your full-time employees.

If you do choose to make employer contributions for long-term part-time employees, one mechanical consequence follows immediately: they earn a year of vesting credit for each 12-month period with at least 500 hours of service, rather than the usual 1,000-hour threshold. That is a more generous vesting rule than applies to regular participants, and it is the tradeoff attached to contributing.

You also retain a set of testing choices. Long-term part-time employees who entered solely under the rule may generally be excluded from nondiscrimination and coverage testing, and from top-heavy minimum contribution requirements, which is a meaningful administrative relief. Whether to take those elections is a conversation to have with your recordkeeper, because the answer depends on your specific plan design.

What You Cannot Do

The instinct of a small business owner facing a new tracking obligation is to look for a way around it, and there are a few obvious-seeming moves here that do not work. Knowing which doors are closed saves you from an expensive correction later.

The seasonal-worker trap deserves particular attention because it is the one people try. A business with heavy seasonal staffing looks at the long-term part-time rule, notices that its seasonal people work 600 hours every summer, and reaches for a plan amendment excluding seasonal employees. That does not work, because the exclusion is functionally service-based and the rule was designed precisely to defeat it.

What does work is a genuine, non-service-based classification. Collectively bargained employees are excluded by statute. Nonresident aliens with no US-source income are excluded. Independent contractors are not employees at all. Those are real distinctions. Part-time and seasonal are not, in this context, and the correct framing on what counts as part-time is worth being clear about before you design around it.

How to Count the Hours

All of this hinges on hours of service, so the counting method matters. There are three approaches, and one of them makes the long-term part-time problem disappear entirely.

The measurement period is the other mechanical detail. An employee's initial 12-month period runs from their date of hire. After that, the plan may either continue using anniversary years or switch to the plan year, and most plans switch for administrative sanity. The switch can cause some months to be counted in two overlapping periods during the transition, which is permitted and is a known quirk rather than an error.

Notice the middle row of that table, because it is the escape hatch nobody mentions. A plan using the elapsed-time method does not count hours at all, and therefore can never produce a long-term part-time employee. That is a legitimate plan design choice, though it comes with its own consequences and is a conversation for your plan advisor rather than a decision to make on your own.

The Simplest Way Out of the Tracking Problem

Here is the thing almost every guide on this topic buries or omits, and it is the most useful single fact for a small business: you can make the entire tracking obligation vanish by letting everyone in.

The long-term part-time rule only applies to employees who become eligible solely because of it. If your plan makes all employees eligible to defer immediately on hire, or after a very short waiting period, then nobody ever becomes eligible solely under the long-term part-time route, because they were already eligible under your ordinary plan terms. No long-term part-time employees means no separate hour tracking, no separate vesting computation, and no two-class participant administration.

The tradeoff is real but usually smaller than it sounds. More people eligible means more participants, which means more accounts for your recordkeeper to administer and potentially higher fees. At scale, a rising participant count can eventually cross the threshold that triggers a plan audit requirement, though a business in the five-to-fifty range is generally far from that line. Weigh those against the ongoing cost of tracking hours for every part-timer, forever, with correction exposure if you get it wrong.

What a Small Business Should Actually Do

Stripping away the regulatory detail, here is the sequence that gets a business with five to fifty employees into compliance without a benefits department.

Two of those steps carry most of the risk. The recordkeeper conversation is the one nobody has and everybody assumes: verify, explicitly, who is counting hours, because a plan where both parties assume the other is tracking is a plan that is out of compliance and does not know it. And the employee communication step matters legally, not just as a courtesy; the IRS plan fix-it guide treats failing to give an eligible employee the opportunity to make a deferral election as an operational failure with a correction procedure attached, and it specifically flags the assumption that a plan does not cover part-time employees as a common cause.

The broader point is that this rule is not really about retirement policy for a small business. It is an administrative obligation attached to a plan you already sponsor, and the winning move is usually to design the obligation out of existence rather than to build machinery to service it. How this fits into your wider benefits picture is covered in the employee benefits guide, and the tax treatment of retirement contributions sits in the fringe benefits guide.

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