The maximum contribution an employee can make to a 401(k) retirement plan is $18,000 in 2015, assuming its his only retirement plan. An individual age 50 or older by year-end can contribute an additional $6,000 in 2015, according to the Internal Revenue Service.
The additional contribution, called a catch-up contribution, increases the limit to $24,000. Employees should make sure they do not go over their annual individual limits. If they do, they need to return the overage by April 15 of the following year, notes the IRS. Otherwise, the IRS taxes the excess funds in the year the salary is deferred and a second time when the employee receives his distribution.