HSA Last-Month Rule: Full $4,400 Now, 13-Month Test After

The decision in one paragraph

You enrolled in a high-deductible health plan (HDHP) partway through 2026 — new job, an open-enrollment switch, or a mid-year move off a spouse’s plan. Normally your HSA contribution is prorated: one-twelfth of the annual limit for each month you were eligible on the first of the month. Start an HDHP on October 1 and you would be capped at roughly 3/12 of $4,400 = $1,100 self-only. The last-month rule rewrites that. If you are HSA-eligible on December 1, 2026, IRC §223(b)(8) treats you as eligible for the entire year, so you can contribute the full $4,400 (self-only) or $8,750 (family). The price of admission: you must stay HSA-eligible through December 31, 2027 — a 13-month testing period.

What IRC §223(b)(8) actually says

The Internal Revenue Code sets the HSA contribution limit month by month. Section 223(b)(1)–(2) computes your annual maximum as the sum of the monthly limits for each month you were an “eligible individual” on the first day of that month. Miss months, contribute less.

Section 223(b)(8) — the last-month rule — overrides that arithmetic. It says: if you are an eligible individual on the first day of the last month of the tax year (December 1 for calendar-year taxpayers), you are treated as having been eligible every month of that year. The proration disappears. You get the full limit.

But §223(b)(8)(B) attaches a recapture rule. If you do not remain an eligible individual through the testing period, the amount you contributed above what you could have contributed without the rule is added to gross income, and an additional 10% tax applies to that recaptured amount. The code calls this the “failure to maintain high deductible health plan coverage.”

The 2026 limits the rule lets you reach

Source: IRC §223(b); IRS 2026 cost-of-living adjustments. Note the catch-up nuance — the $1,000 age-55 catch-up is per HSA-holder, so a married couple with two HSAs can each add $1,000, but only into their own accounts.

Worked example: Maria, October HDHP start, self-only

Maria is 41, single, and files as a single taxpayer in Georgia. She left a non-HDHP employer and started a new job with an HDHP effective October 1, 2026. Her marginal federal rate is 24% (single, taxable income in the $103,351–$197,300 band for 2026). Georgia’s flat income tax is 5.39%.

Without the last-month rule (proration)

Eligible on the first of October, November, and December — three months. Her limit is 3/12 × $4,400 = $1,100. At a combined 24% federal + 5.39% Georgia rate, that $1,100 deduction saves about $323.

With the last-month rule

Because she is eligible on December 1, 2026, she contributes the full $4,400. The extra $3,300 is now deductible too.

The last-month rule puts an extra $970 in Maria’s pocket this year and gets $3,300 more into a triple-tax-advantaged account. Her obligation: keep the HDHP — with no disqualifying coverage — through December 31, 2027. She intends to stay at this job, so the rule is a clean win.

The 13-month testing period, drawn out

People trip on the name. The “testing period” is just the IRS checking that you kept the plan for roughly a year after you used the rule. Concretely, for a 2026 contribution:

1. Start: December 1, 2026 (the last month of the contribution year).
2. End: December 31, 2027 (the last day of the following year).
3. Requirement: remain an eligible individual the entire stretch — covered by an HDHP and no disqualifying coverage (general-purpose FSA, Medicare, a spouse’s non-HDHP family plan, etc.).

That span is 13 months, which is where the shorthand comes from. You do not have to contribute again in 2027 — you just have to stay eligible. You can switch HSA custodians, change jobs, or even let the account sit; what matters is that your health coverage keeps qualifying you.

What failing the test costs you

Say Maria drops the HDHP in June 2027 to join a spouse’s richer PPO. She fails the testing period. The recapture under §223(b)(8)(B) works like this:

• Recaptured amount: the contribution she made using the rule minus what she could have contributed without it. Here: $4,400 − $1,100 = $3,300.
• Income tax: that $3,300 is added to her 2027 gross income and taxed at her 2027 marginal rate.
• Additional 10% tax: a flat $3,300 × 10% = $330 penalty on top, reported on Form 8889.

The recapture is reported in the year eligibility is lost (2027), not the year of the contribution. Two exceptions stop the recapture: death and disability. Nothing else — not a layoff, not a better job, not a move to Medicare — gets you off the hook.

Who should skip the last-month rule

The rule is a bet that your coverage stays put for the next full calendar year. Decline the bet when:

• You will turn 65 and enroll in Medicare during the next year. Medicare enrollment ends HSA eligibility and triggers recapture. Prorate instead, and stop HSA contributions six months before you file for Social Security — Medicare Part A back-dates up to six months.
• You expect to switch off the HDHP — planned job change into non-HDHP coverage, marriage onto a spouse’s PPO, or an employer dropping the HDHP option.
• Your HDHP enrollment feels shaky — contract or seasonal work, a startup whose benefits could change, or a plan you only took to bridge a gap.
• You would owe more in recapture tax than the proration deduction is worth. If you only saved a few hundred dollars by front-loading, the 10% penalty plus ordinary tax on the recapture can erase the benefit and then some.

In all of these, the safer move is to prorate — contribute one-twelfth of the limit per eligible month. You lose the front-loaded deduction but you carry zero recapture risk.

What most people miss

Three things quietly sink last-month-rule users:

• A general-purpose FSA breaks eligibility — even your spouse’s. If your spouse has a general-purpose health FSA at their job, that coverage can disqualify you from being an eligible individual, because the FSA can reimburse your medical costs. A limited-purpose (dental/vision) FSA is fine. People use the rule, then a spouse’s FSA quietly fails the testing period.
• The funding deadline and the eligibility test are different dates. You can be eligible on December 1, 2026 and still fund the contribution as late as the April 2027 tax-filing deadline. But the 13-month testing period is fixed by the December 1 eligibility date — funding late does not shorten it.
• Mid-year coverage-tier changes use a special averaging rule, not the last-month rule. If you switch between self-only and family coverage during the year, §223(b)(8) still lets December 1 set your full-year limit at whatever tier you hold that day — but if you do not use the last-month rule, you average the monthly limits instead. Decide which tier you hold on December 1 before you assume the family limit.

Family-coverage version: Maria’s neighbor Devon

Devon, 47, married with two kids, switched onto a family HDHP on November 1, 2026 when his employer dropped its PPO option. Without the rule he is eligible for two months — 2/12 × $8,750 = $1,458. With the last-month rule, December 1 eligibility unlocks the full $8,750. At a 22% federal marginal rate (MFJ, taxable income in the $96,951–$206,700 band for 2026) the extra $7,292 of deduction saves him about $1,604 in federal tax this year — money that would have evaporated under proration. His testing period runs December 1, 2026 through December 31, 2027, and because neither spouse is near 65 and neither carries a general-purpose FSA, the bet is low-risk. Had Devon’s wife signed up for a general-purpose health FSA at her job, that single choice would have disqualified the family and clawed back the $7,292 excess as 2027 income plus a $729 additional tax (10% of $7,292).

Reporting it on Form 8889, line by line

The last-month rule is not something you elect with a checkbox and forget. You report it on Form 8889, the HSA form filed with your 1040:

• Line 1: coverage tier on December 1 — self-only or family. This is the single date that sets your limit.
• Line 3: the full annual limit the rule lets you claim — $4,400 or $8,750 for 2026 — rather than the prorated figure the worksheet would otherwise compute.
• Line 13: your HSA deduction, which flows to Schedule 1 and reduces adjusted gross income directly (an above-the-line deduction, so you get it even if you take the $15,750 standard deduction).
• Part III (lines 18–21): the recapture section. You only touch this in the year you fail the testing period — it adds the excess to income and computes the flat 10% additional tax.

Because the HSA deduction is above-the-line, the last-month rule helps even taxpayers who do not itemize. A single filer taking the $15,750 standard deduction still subtracts the full $4,400 on top, which can also nudge them below thresholds for other phase-outs.

How to use the rule cleanly

1. Confirm December 1 eligibility. On December 1 of the contribution year, you must be covered by a qualifying HDHP with no disqualifying coverage. That single day controls the full-year limit.
2. Pick your tier deliberately. Self-only locks in $4,400; family locks in $8,750 (2026). The tier you hold December 1 sets the ceiling.
3. Fund by the tax deadline. You have until the April 2027 filing deadline (not extensions) to deposit the 2026 contribution and still deduct it for 2026.
4. Report on Form 8889. Check the last-month-rule box and complete the contribution lines. If you later fail the test, you report the recapture and 10% additional tax on the same form for the failure year.
5. Guard the testing period. Through December 31 of the following year, avoid Medicare enrollment, a disqualifying FSA, and any non-HDHP primary coverage.

The decision lever

The last-month rule turns on one question you can answer today: am I keeping this HDHP, with no disqualifying coverage, through the end of next year? If the honest answer is yes — stable job, no Medicare on the horizon, no spouse-FSA conflict — use the rule, capture the full $4,400 or $8,750 deduction now, and bank the extra $900–$1,000 of current-year tax savings. If the answer is “probably not” or “I’m not sure,” prorate by eligible months and skip the recapture exposure entirely. The penalty for guessing wrong — ordinary income tax on the excess plus a flat 10% — is steep enough that certainty about next year is the whole game.