The Actual Contribution Percentage (ACP) Test is a rule that applies to certain types of workplace retirement plans to make sure that benefits are fair for all employees—not just for owners or highly paid workers. It looks at how much employees are putting into the plan through things like employer matching and after-tax contributions, and compares the percentages across different groups. If the plan gives too much to high earners compared to others, it may fail the test and require corrections. Most Solo 401k plans don’t have to worry about this rule, because they typically don’t have employees other than the business owner and possibly their spouse.