Millions of Americans use 401(k) plans, and a small but significant fraction of them add as much money to their accounts each year as the 401(k) contribution limits allow. Those limits change over time, and it's important to know what they are so you can plan your retirement savings strategy accordingly. The general 2017 401(k) contribution limit remains at $18,000 for those younger than age 50 and $24,000 for those 50 or older, but there are additional details you should know. Below, we'll go into more detail about the 2017 401(k) contribution limits.

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How much can employees contribute to their 401(k)s in 2017?

The limit on 401(k) employee contributions for 2017 is the same as it was last year. Those under 50 can put in up to $18,000 for a 401(k) or similar employer-sponsored retirement plan. Those who are 50 or older are eligible to make a catch-up contribution, which remains at the same $6,000 level for 2017 as it was in 2016.

The 401(k) employee contribution limit hasn't changed for several years now, because inflation hasn't been high enough to kick the figures up to the next higher $500 mark. Those looking to contribute more will have to wait until at least 2018 before they'll get the chance at a higher employee contribution limit to their 401(k).

How much can employers contribute to their employees' 401(k)s in 2017?

Employer contributions to 401(k) accounts come in a couple of different forms. The one that most people are familiar with are matching contributions, where an employer agrees to supplement employee contributions with additional money up to a certain amount. However, some employers make profit-sharing contributions that aren't connected to how much the employee contributes.

The combined total of employer and employee contributions to a 401(k) for 2017 is $54,000 for a worker under age 50. That number is up by $1,000 from 2016's limits. For those 50 or older, the corresponding limit is $60,000.

What are the special limits for SIMPLE 401(k)s?

The limits above apply to standard 401(k) plans. However, some employers use SIMPLE 401(k) plans, which take advantage of special tax rules that simplify retirement plan administration for employers with 100 or fewer employees.

Although SIMPLE 401(k)s are easier for employers to administer, they come with lower contribution limits. The 2017 SIMPLE 401(k) employee contribution limit is $12,500 for those younger than 50, which is also unchanged from 2016's limit. Those 50 or older can add another $3,000 to the limit as a catch-up contribution.

The employer contribution limit for SIMPLE 401(k)s is either a 2% non-elective contribution or a dollar-for-dollar match up to 3% of pay. The 3% contribution is further subject to maximum compensation of $270,000 for 2017, making the absolute highest employer contribution allowed $8,100 in 2017.

Are there special contribution limits for highly paid employees?

Finally, special rules can apply with some employers to those who make more than certain income amounts. For 2017, the definition of a highly compensated employee is someone with earnings of more than $120,000, which is the same threshold as in 2016. These limits depend on the behavior of the other employees within the employer's plan, and anti-discrimination rules can in some instances reduce the amount that a highly compensated employee is allowed to contribute below what the general limitations would say.

The best thing to do for help in determining your 401(k) contribution options is to talk with your employer's human resources or payroll department. Because so many things about 401(k) plans are specific to the provisions that your particular employer chooses to put in, it's vital to know at the beginning whether some sort of special exceptions to the general rules described above will apply to your particular situation.