HSA Last-Month Rule and Testing Period: How to Contribute the Full Annual Limit When You Gain HDHP Coverage Mid-Year

The HSA Last-Month Rule Explained: The Full Contribution Access for Mid-Year Starters

The Last-Month Rule, codified under IRC Section 223(b)(8), is an elective provision that allows an individual who is HSA-eligible on December 1 of a tax year to treat themselves as having been eligible for the entire year, contributing the full annual HSA limit rather than the prorated amount based on actual months of eligibility. The rule is named for the fact that December is the qualifying “last month” of the calendar year.

The rule is particularly valuable for individuals who gain HDHP eligibility late in the year, through a new job, a switch from a non-HDHP employer plan, or a life event that changes health coverage. Without the Last-Month Rule, a person who gains HDHP coverage on October 1 is eligible for only 3/12 of the annual contribution limit, a small fraction that provides limited immediate tax benefit. With the Last-Month Rule, they can contribute the full annual limit as long as they meet the testing period requirement.

The Last-Month Rule election is made by simply contributing the full annual limit rather than the prorated amount. There is no formal election form or IRS filing required to invoke the rule, the election is implicit in the contribution amount. However, the testing period requirement binds the taxpayer whether or not they understood they were electing it at the time. This implicit election structure means that taxpayers who contribute more than their prorated limit based on actual months of HDHP eligibility are automatically subject to the testing period requirement, whether they intended to elect the Last-Month Rule or not.

The Standard Prorated Contribution Alternative: When Less Is Actually More

The default rule for mid-year HSA eligibility is simple prorating: the annual contribution limit is multiplied by the number of months of HDHP eligibility and divided by 12. A person who becomes HDHP-eligible on May 1 and remains eligible through December 31 has 8 months of eligibility, their contribution limit is 8/12 of the annual limit. For 2026, that is 8/12 of $4,300 for self-only coverage, or approximately $2,867.

For the prorated calculation, a partial month of eligibility counts as a full month if the individual was HDHP-eligible on the first day of that month. If HDHP coverage begins on May 15, May does not count, the eligible months begin in June, producing 7/12 of the annual limit. If HDHP coverage begins on May 1, May counts as a full eligible month, producing 8/12 of the annual limit. The day-of-month start date therefore has a meaningful impact on the contribution limit for mid-year coverage starts.

The prorated contribution approach requires no testing period and carries no risk of retroactive taxation. For individuals whose future HDHP eligibility is uncertain, those planning to retire, expecting a job change, or anticipating a potential switch to a spouse’s non-HDHP plan, the prorated approach is always the safer choice. The additional contribution opportunity from the Last-Month Rule is only worth the testing period risk when future HDHP eligibility is highly reliable.

The Testing Period: The 13-Month Commitment That Comes with the Last-Month Rule

Every taxpayer who invokes the Last-Month Rule, by contributing more than their prorated limit, is subject to a testing period that extends from December 1 of the year the rule is invoked through December 31 of the following year. The full testing period covers 13 months. During this entire 13-month window, the taxpayer must remain an eligible HSA contributor, continuously enrolled in an HDHP and not subject to any disqualifying coverage.

The testing period requirement is enforced through the tax return for the year in which the testing period ends. On Form 8889 (Health Savings Accounts), the taxpayer reports whether the testing period was satisfied. If the testing period fails, because the taxpayer lost HDHP eligibility at any point during the 13-month window, the additional amount contributed under the Last-Month Rule (the difference between the full annual limit and the prorated amount) is added back to income and subject to a 10 percent additional tax in the year the failure occurred.

The testing period can be failed by any event that terminates HSA eligibility during the 13-month window: enrollment in Medicare Part A or Part B; enrollment in a non-HDHP health plan (including through a new employer, a spouse’s plan, or a government plan); voluntary termination of HDHP coverage; or loss of health coverage entirely. Disability and death are exceptions to the testing period failure rules, a taxpayer who fails the testing period due to death or disability does not incur the penalty, though the additional amount may still be included in income.

Consequences of Testing Period Failure: Tax and Penalty Quantified

If the testing period fails, the excess contribution, the amount contributed under the Last-Month Rule above what the prorated limit would have allowed, is included in the taxpayer’s gross income for the year the failure occurred, and a 10 percent additional tax is assessed on the same excess amount. This consequence is equivalent to treating the excess contribution as if it had never been made and retroactively imposing an excise tax on the overage.

The financial quantification of a testing period failure is instructive. An individual who becomes HDHP-eligible on October 1, contributes the full $4,300 individual limit for the year (rather than the prorated $1,433 for 3 months), and then joins a non-HDHP plan on March 1 of the following year (failing the testing period in Month 4 of the required 13) must include $2,867 in gross income for the failure year and pay a 10 percent additional tax of $287 on that amount. For a taxpayer in the 32 percent federal bracket plus 9.3 percent state bracket, the total tax on the inclusion is approximately $1,185 ($2,867 × 41.3%). Combined with the $287 penalty, the total cost of the testing period failure is approximately $1,472, a significant cost but substantially below the full tax benefit that would have been realized if the testing period had been satisfied.

When to Use vs When to Avoid the Last-Month Rule

The Last-Month Rule is beneficial only when the probability of completing the testing period is high. Any meaningful uncertainty about HDHP eligibility during the 13-month testing window argues against invoking the rule. Specific situations where the Last-Month Rule is appropriate and situations where it should be avoided are worth making explicit.

Use the Last-Month Rule when: the new HDHP coverage is through a stable, long-term employer where job changes are unlikely; the testing period window does not include an anticipated Medicare enrollment (the account holder is under 64 at the start of the testing period); no job change or benefits change is expected during the 13-month window; and the additional contribution opportunity represents a meaningful absolute tax saving (the difference between full and prorated limits is large because the coverage start date was early in the year). For a person who gains HDHP coverage on February 1, the Last-Month Rule adds only one additional month’s contribution to an already-large prorated limit, the risk may not be worth the modest additional benefit.

Avoid the Last-Month Rule when: the new HDHP coverage is through a short-term, contract, or uncertain employment arrangement; the account holder is 64 or will turn 65 before December 31 of the testing period year; a planned retirement, job change, or marriage that changes health coverage is expected within 13 months; or the employer plan is under evaluation for modification that might eliminate the HDHP option. The testing period failure cost, income inclusion plus 10 percent penalty, is modest relative to the total contribution amount but is a real cost that should not be incurred unnecessarily when the prorated contribution is freely available without testing period risk.

New Job Mid-Year: The Classic Last-Month Rule Scenario

The most common scenario for Last-Month Rule consideration is a job change mid-year where the new employer’s health plan is an HDHP and the former employer’s plan was not. An employee who leaves a non-HDHP employer in September and joins a new employer with HDHP coverage effective October 1 would ordinarily be limited to 3/12 of the annual HSA contribution limit. The Last-Month Rule allows them to contribute the full annual limit, provided they remain on the HDHP through December 31 of the following year.

For a new employee evaluating the Last-Month Rule in this scenario, the key risk factors are: how confident are they in their continued employment at the new employer through December 31 of the following year? Does the new employer offer only an HDHP, or could a plan change at open enrollment bring a non-HDHP alternative that the employee might prefer? If the new employer is stable and HDHP-only, the Last-Month Rule is strongly appropriate, the testing period is effectively guaranteed to be satisfied by continued employment. If the new employer offers HDHP and non-HDHP options and the employee plans to reconsider the plan choice at the next open enrollment, the Last-Month Rule should be avoided because open enrollment into a non-HDHP plan in Year 2 would fail the testing period.

Marriage and Divorce Mid-Year: HSA Coverage Changes and Contribution Effects

Marriage and divorce mid-year create complex HSA eligibility interactions because they can change both the type of HDHP coverage (from self-only to family or vice versa) and the disqualifying coverage analysis (a spouse’s general-purpose FSA can disqualify HSA contributions). Understanding these interactions is essential for the year of any significant relationship change.

Marriage mid-year changes the available contribution limit from December 1 if one spouse has been on a self-only HDHP and switches to a family HDHP to add the new spouse. The family limit of $8,550 becomes available from the date of the coverage change, but the full annual calculation must account for the months under self-only limit vs months under family limit. If the Last-Month Rule is invoked based on December 1 family coverage, the testing period requires continued family HDHP eligibility through December 31 of the following year.

A spouse’s general-purpose Health FSA is the most common disqualifying coverage trap in married households. If a spouse has a general-purpose FSA that covers both spouses, and the employee switches to an HDHP, the HDHP spouse may be HSA-ineligible because the spouse’s general-purpose FSA constitutes disqualifying coverage. Converting the spouse’s FSA to a limited-purpose FSA (dental and vision only) eliminates the disqualification. Couples making an HDHP switch should review both spouses’ health benefit elections to confirm no cross-disqualification exists.

The Medicare Enrollment Trap in the Testing Period

For individuals approaching age 65, Medicare enrollment is the most common and most costly cause of testing period failure. The HSA Last-Month Rule testing period extends through December 31 of the year following the year the rule was invoked. For an individual who invokes the Last-Month Rule in November 2026, the testing period extends through December 31, 2027. If that individual turns 65 in April 2027 and enrolls in Medicare, terminating HSA eligibility from April 1, 2027, the testing period fails in Month 5 of the required 13, triggering the income inclusion and penalty.

The practical rule is simple: any individual who will turn 65 during the 13-month testing period should avoid the Last-Month Rule entirely. The prorated contribution based on actual months of HDHP eligibility before Medicare enrollment is the appropriate, and risk-free, contribution calculation for this population.

State HSA Tax Non-Conformity: California, Alabama, and New Jersey

Three states, California, Alabama, and New Jersey, do not conform to the federal HSA tax treatment. In these states, HSA contributions are not deductible for state income tax purposes, investment earnings inside the HSA are taxable for state income tax, and withdrawals are taxable for state income tax (qualified medical expense withdrawals are exempt from state tax in California for the withdrawal itself, but not for the earnings accrued inside the account). This non-conformity significantly reduces the effective HSA benefit for residents of these states, though the federal tax advantage remains intact.

For California residents in the 9.3 to 13.3 percent state tax bracket, the state tax non-conformity costs approximately $400 to $570 per year in additional state taxes on a maximum individual HSA contribution of $4,300. Over 20 years of contributions and investment growth, this cumulative state tax cost is meaningful but does not eliminate the substantial federal tax advantage. California HSA holders should track their HSA investment earnings separately on the California Schedule CA to correctly report the state taxable income from their HSA each year, a compliance requirement that adds complexity to state tax preparation for California HDHP enrollees.

5-Step Last-Month Rule Decision Protocol

Case Study: Software Engineer Evaluates Last-Month Rule After September Job Change

A 38-year-old software engineer in Austin, Texas left a large technology employer in August and joined a startup in September. The former employer offered an HMO with no HDHP option. The new employer offers only one health plan, a qualifying HDHP with a $1,800 deductible and $7,500 out-of-pocket maximum. HDHP coverage began September 1.

The engineer’s situation is well-suited to the Last-Month Rule. The new employer offers only an HDHP, no non-HDHP plan is available at open enrollment, so there is no risk of voluntarily switching to disqualifying coverage. He is 38, Medicare enrollment is not a concern during the 13-month testing period. No job change is anticipated, the startup has adequate funding, and his salary makes the position stable through the testing period. The additional $688 federal tax saving on the Last-Month Rule excess contribution is real and low-risk given his circumstances. He contributed the full $4,300 to his HSA before December 31, set a calendar reminder to verify continued HDHP eligibility through December 31 of the following year, and reported the contribution correctly on Form 8889. No testing period issue arose and the full contribution was preserved.

Frequently Asked Questions