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Generally 401(k) contributions will be automatically deducted from payroll. Contributions to a 401(k) account are generally taken out of compensation during payroll, before taxes are withheld. For example, if you receive monthly paychecks, the contributions into your 401(k) occur monthly.

Some employers may be more flexible and allow employees to make deposits when it is convenient, or to adjust their contributions at year-end, but larger employers will probably not have time for that, unless it is built into the plan interface in a way that makes it convenient for the payroll department at the sponsoring place of employment. Remember, any contributions must be within the limits allowed by the plan. Excess contributions must be corrected promptly.

For more, see “What are the 401(k) Contribution Limits?”