The number of months you are covered by an HSA-eligible medical plan may affect the amount you can contribute to an HSA for the year. The specifics will depend on whether you are starting or ending your enrollment in the HSA-eligible medical plan.
Starting Your Enrollment: The first year you enroll in the HSA-eligible medical plan, you can make the full HSA contribution allowed for the year, subject to the following conditions:
You must be enrolled in the HSA-eligible medical plan by the first day of the last month of the year (for most enrollees, this means December 1; if you use something other than the calendar year when filing tax returns, it would be the first day of the last month of your tax year) and You must remain enrolled in the HSA-eligible medical plan for a 13-month period starting that December 1 (or your tax year’s equivalent of that date).
Example: Your coverage under the HSA-eligible medical plan starts December 1, 2016. You may make the full HSA contribution allowed for 2016, provided you remain enrolled in the HSA-eligible medical plan from December 1, 2016, through December 31, 2017.
Your coverage level and age on the December 1 (or your tax year’s equivalent) will determine which maximum contribution amount will serve as your “full HSA contribution” for the year you first enroll:
If you have self-only coverage then, you may make the self-only maximum contribution (plus the catch-up contribution, if you are age 55 or older).
If you have family coverage then, you may make the family maximum contribution (plus the catch-up contribution, if you are age 55 or older).
A change to a different coverage level before the 13-month period is up will not affect the contribution amount allowed for the initial year you enroll (but does affect your contribution limit for the year in which your coverage changes).
If you do not meet these conditions, your maximum contribution will be calculated on a month-by-month basis for your months of enrollment in the HSA-eligible medical plan and your coverage level(s) during those months. Any contribution amount exceeding the resulting maximum will be taxable income and will be subject to a 10% penalty. Exceptions are made in cases of death or disability.