The HSA Last-Month Rule and Its Testing Period

The HSA last-month rule lets a person who is an eligible individual on the first day of the final month of the tax year generally use the annual contribution limit for the coverage type held on that day. The benefit comes with a testing period: the person must remain HSA-eligible through the end of the following year. If eligibility ends early for a reason other than death or disability, the extra contribution is included in income and generally subject to a 10% additional tax.

Ordinary monthly eligibility comes first

An HSA contribution normally requires HSA eligibility for each month: the individual must have qualifying high-deductible health plan coverage, have no disqualifying additional coverage, not be enrolled in Medicare, and not be claimable as another person’s tax dependent. The regular annual limit is generally calculated month by month, using one-twelfth of the applicable self-only or family limit for each eligible month. An age-55 catch-up amount is also prorated by eligible months.

The last-month rule changes that calculation for a person eligible on December 1 (for most calendar-year taxpayers). The person is treated as eligible for the entire year and generally uses the coverage category in place on that date. This may increase the permitted contribution when eligibility began late in the year or coverage changed from self-only to family. It is an option, not a requirement, and should be compared with the monthly method.

The testing period is the price of accelerated eligibility

A person who relies on the rule must remain an eligible individual throughout the testing period. For a calendar-year taxpayer, that period generally begins December 1 of the year the rule is used and ends on December 31 of the following year. A later loss of HDHP coverage, enrollment in Medicare, or other disqualifying change may end eligibility before the period expires.

Death or becoming disabled are exceptions to the recapture consequence. Other changes generally trigger inclusion of the contribution amount that would not have been allowed under the ordinary monthly calculation. The inclusion is reported in the year eligibility fails and is also subject to a 10% additional tax. This is not a return of the whole account balance; it is a tax adjustment for the extra contribution attributable to the special rule.

Calculate the recapture amount

The calculation compares the actual contribution attributable to the last-month rule with the amount allowed using monthly eligibility. The taxpayer does not automatically include every HSA contribution made during the year. In the simplest case, someone eligible only for December contributed the full-year limit, but the monthly method would have allowed only one-twelfth. The difference is the amount subject to inclusion and the additional tax if the testing period is failed.

Coverage changes make the comparison more detailed. A taxpayer who had self-only coverage for part of the year and family coverage later should use the Form 8889 worksheet to determine the monthly limit. Employer contributions count toward the overall limit. The taxpayer should add employee and employer amounts, include any qualifying catch-up amount, then compare the monthly calculation with the contribution made under the last-month rule.

Example: late-year eligibility followed by a job change

Assume an individual first becomes HSA-eligible on December 1 and uses the last-month rule to contribute the annual limit. In June of the next year, the individual leaves the HDHP for a plan that is not HSA-qualified. The testing period has not ended, so the individual generally reports the portion of the prior-year contribution that exceeded the ordinary one-month limit as income in the year of the eligibility failure, with the additional 10% tax.

If the same person had instead remained eligible through December 31 of the next year, the last-month-rule contribution would avoid that recapture. The taxpayer may still make current-year contributions only for months of current eligibility. Using the rule for the prior year does not grant HSA eligibility in the next year.

Coordinate the rule with Medicare and payroll

Turning 65 does not alone end HSA eligibility, but Medicare enrollment does. Part A can be retroactive for some people who enroll after 65, so applying for Social Security or Medicare may create an ineligible period that was not obvious when payroll contributions were made. Employer contributions, salary reductions, and direct deposits all count toward the limit.

Before using the last-month rule, consider expected job changes, planned retirement, Medicare enrollment, spouse coverage, and any upcoming move to a non-HDHP. HSA funds can remain invested and be used for qualified expenses after eligibility ends; only new contributions require current eligibility. A contribution strategy should distinguish the right to hold and spend the account from the right to add money.

Reporting and practical records

Form 8889 reports HSA contributions, employer funding, qualified distributions, and any amount included in income under the testing-period rule. Retain proof of monthly insurance coverage, the December 1 plan, employer contributions, Medicare effective dates, and the monthly limit worksheet. If excess contributions result, resolve them under the excess-contribution rules rather than assuming the testing-period inclusion corrects the excess automatically.

A taxpayer should compare the monthly method and last-month method before the filing deadline. The last-month rule can be useful when eligibility begins late in the year, but the possible income inclusion and additional tax create downside if coverage is likely to change. In uncertain cases, a smaller monthly contribution can be more predictable than using the full-year limit.

Common mistakes and exam sequence

Common errors include assuming December 31 eligibility is enough, forgetting that the testing period runs into the following year, treating employer contributions as outside the limit, and failing to include Medicare retroactivity. Another error is to include all contributions in income rather than only the amount made possible by the rule.

For an exam problem, establish month-by-month eligibility, determine whether the December 1 rule was used, calculate the normal monthly contribution limit, test continued eligibility through the next year, and apply the income inclusion and additional tax if the period fails.

Additional planning detail

An HSA custodian does not decide whether the taxpayer met the testing period. The account statement may show contributions but will not know when health coverage ended or Medicare began. The taxpayer should make a year-by-year calendar and share the information with the return preparer. If eligibility fails, calculate the rule-based amount on Form 8889 even if the HSA balance remains invested and no money left the account.