There are two different rules, both of which would allow you to contribute, although figuring the maximum amount is complicated.
First, you can contribute to an HSA if you are not someone else’s dependent and you are covered by a qualified HDHP. This applies to individuals who are still eligible to be covered by their parent‘s insurance because they are under age 26, but not eligible to be claimed as a dependent because they are over age 23, or they are over age 18 while not being a full-time student. In this case, since you are covered by someone else’s family HDHP, your maximum contribution is $7300 per year or $608 per month. This is an interesting situation in the tax law. Your parents share a family maximum contribution of $7300, but you are allowed a separate maximum of $7300 because you are covered by a family plan, and your parents‘ contributions don’t limit your contributions. You might consider this a “loophole“ or oversight when the tax laws were written, but Congress has not corrected it yet so you are allowed to do this.
Once you are covered by your own single HDHP, you are allowed to contribute up to the single maximum of $3650 per year or $304 per month. You can use the “last month rule“ to contribute the full annual amount even if you are only covered at the end of the year, as long as you maintain full coverage of the following year.
However, because you were covered by a family plan, you don’t have to rely on the last month rule. In your situation, your maximum contribution for 2021 is $6384, which represents 9 months of contributions for a family HDHP and 3 months of contributions for the single HDHP coverage. When you prepare your tax return, make sure that you indicate you had qualifying HDHP coverage for the entire year (because you were covered even if you weren’t the owner of the plan), And then go down the list of months and tick family or single depending on how you were covered in each month.
