The HSA last-month rule can let you contribute a full year’s HSA amount even if you were HSA-eligible for only part of the year. It is useful, but it comes with a catch: you must remain eligible during a testing period.
For 2026, the HSA limits are $4,400 for self-only HDHP coverage and $8,750 for family HDHP coverage. See HSA Contribution Limits 2026 for the full table.
The Basic Rule
Normally, your HSA contribution limit is prorated by month. If you are HSA-eligible for 6 months of the year, your basic limit is 6/12 of the annual limit.
The last-month rule says that if you are an eligible individual on the first day of the last month of your tax year, usually December 1, you can be treated as eligible for the entire year.
For most taxpayers, that means if you are HSA-eligible on December 1, 2026, you may be able to contribute the full 2026 HSA limit.
The Testing Period
The last-month rule is not free. You must remain HSA-eligible during the testing period. For calendar-year taxpayers using the last-month rule for 2026, the testing period generally runs from December 1, 2026 through December 31, 2027.
If you fail the testing period, the extra amount you contributed because of the last-month rule generally becomes taxable income and may be subject to an additional tax.
Example 1: Coverage Starts July 1, 2026
Morgan starts self-only HSA-eligible HDHP coverage on July 1, 2026 and remains eligible on December 1.
Without the last-month rule, Morgan’s prorated limit would be:
$4,400 x 6 / 12 = $2,200
With the last-month rule, Morgan may contribute the full $4,400 for 2026, but must stay HSA-eligible through the testing period ending December 31, 2027.
Example 2: Coverage Starts December 1, 2026
Taylor starts family HDHP coverage on December 1, 2026.
Without the last-month rule, Taylor’s prorated family limit would be:
$8,750 x 1 / 12 = $729.17
With the last-month rule, Taylor may be able to contribute the full $8,750 for 2026. But if Taylor switches to non-HDHP coverage in 2027, the extra contribution can become taxable.
Example 3: You Know You Will Lose Eligibility
If you know you will enroll in Medicare, switch to a non-HDHP, or gain disqualifying coverage in 2027, using the last-month rule may be a bad idea.
In that case, the prorated contribution is often safer. The last-month rule is most useful when you are confident you will remain HSA-eligible for the entire testing period.
How Age 55 Catch-Up Works
The $1,000 age 55 catch-up can also be affected by monthly eligibility. If you are 55 or older and use the last-month rule, you may be able to include the full catch-up if you are eligible on December 1 and satisfy the testing period.
Married couples should remember that each spouse’s catch-up must go into that spouse’s own HSA. For examples, see Can Both Spouses Have an HSA in 2026?.
Last-Month Rule vs Employer Contributions
Employer contributions count toward the HSA limit. If your employer contributes based on your enrollment, include those deposits before deciding how much to add yourself.
Example: Your full 2026 self-only limit is $4,400, and your employer contributes $800. Your remaining room is $3,600 if you are eligible for the full limit.
Bottom Line
The HSA last-month rule can turn partial-year HSA eligibility into a full-year contribution limit, but only if you satisfy the testing period. For 2026, that usually means being eligible on December 1, 2026 and remaining eligible through December 31, 2027.
Use the rule when you expect stable HSA eligibility. Use prorating when your future coverage is uncertain.
Related Reading
- HSA Contribution Limits 2026
- Can Both Spouses Have an HSA in 2026?
- FSA Limit 2026 and Carryover Limit