Monthly Contribution Rule
Regardless of when you enroll in the HSA, if you are not HSA eligible as of December 1 of the applicable tax year, your contribution limit will be the sum of the indexed contribution limits determined separately for each month you are HSA eligible. The monthly limit is determined by calculating 1/12 of the annual statutory limit. If you are HSA eligible for all 12 months of 2026, you are eligible for the full annual HSA contribution.
For example, in 2026, the annual statutory contribution limit for self-only coverage is $4,400 (exclusive of catch-up contributions) – therefore, the monthly limit is $366.67 (1/12 of $4,400 = $366.67). Assume Mary is HSA eligible only for the months of January and February of 2026 and is not HSA eligible on December 1. She may only contribute to her HSA for 2 months in 2026 for a total of $733.34 (based on self-only coverage = $366.67 x 2 months = $733.34).
Full Contribution Rule
As long as you are HSA eligible on December 1, you are treated as an eligible individual for the entire year. If you have QHDHP coverage in place on December 1, you may contribute up to the full annual statutory limit. However, to take advantage of this rule, you must remain HSA eligible during the 13-month “testing period.” This requires that you remain HSA eligible through the month of December of the applicable tax year plus the entire following calendar year (December plus an additional 12 months = 13 months). If you fail to remain HSA eligible during the “testing period,” the contributed amount over the allowed monthly contribution (i.e., the months you were not HSA eligible) is included in income, taxed and subject to a 10% penalty.
For example, Chris, age 53, becomes an HSA-eligible individual on December 1, 2026. He has single QHDHP coverage on that date. Under the full contribution rule, he contributes $4,400 to his HSA for 2026. Chris loses HSA eligibility in June 2027. Because he does not remain HSA eligible during the 13-month testing period, he must include as income the contribution that could not have been made except for the full contribution rule, $4,033.33 ($4,400 – $366.67, his one month of HSA eligibility), plus a 10% penalty.