What Is an ADP/ACP Test in a 401(k) Plan?
Two acronyms buried in a 401(k) plan’s annual compliance paperwork — ADP and ACP — rarely mean anything to the average participant. But when one of these tests fails, it can turn into a real check landing back in someone’s mailbox, with taxes attached.
The short answer
The ADP test (Actual Deferral Percentage) and the ACP test (Actual Contribution Percentage) are two related nondiscrimination checks that compare how much highly compensated employees defer or receive in matching contributions against how much everyone else does, relative to pay. ADP looks at employee deferrals; ACP looks at employer matching and after-tax contributions. A plan that fails either test generally has to correct the imbalance, often by refunding money to the higher-paid group.
What each test actually measures
The ADP test calculates the average percentage of pay that highly compensated employees defer into the plan and compares it to the average percentage deferred by everyone else. The ACP test does the same comparison, but for matching and after-tax contributions rather than employee deferrals. Both tests use a formula that allows the higher-paid group’s average to exceed the other group’s by only a limited margin, and that allowed gap is set by rule and adjusts based on how the broader group is participating.
Why the comparison matters
These tests exist because 401(k) plans receive favorable tax treatment partly on the condition that the benefit isn’t concentrated mainly among owners and top earners. If highly compensated employees are deferring heavily while participation among other employees is thin, the gap between the two groups’ averages can grow too wide, and the plan fails the test for that year. Broad, steady participation across pay levels is usually what keeps a plan comfortably within the allowed range.
What happens after a failed test
When a plan fails ADP or ACP testing, the most common fix is a “corrective distribution” — refunding a portion of the highly compensated group’s deferrals or matching contributions, generally within a set window after the plan year ends. Those refunded amounts typically become taxable income for the employees who receive them, which is why a surprise check or tax form sometimes shows up the following season for people who didn’t request any withdrawal. Some plans instead correct by making additional contributions for non-highly-compensated employees rather than refunding the higher-paid group.
How plans try to avoid this
Many employers sidestep ADP and ACP testing altogether by adopting a safe harbor plan design, which meets specific contribution or matching formulas set in advance and is deemed to satisfy these tests automatically. Others rely on strong participation encouraged through automatic enrollment, since a broader base of employees deferring at reasonable rates naturally narrows the gap the tests are measuring. Either approach reduces the odds of a corrective refund landing on higher-paid employees at year-end.
The takeaway
ADP and ACP testing exist to keep a 401(k) plan’s tax benefits spread across the workforce rather than concentrated at the top. For someone who defers a high percentage of pay, understanding these tests explains why an unexpected refund or a mid-year change to contribution limits sometimes happens, even when nothing about their own account went wrong.