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HSA Last-Month Rule Calculator

Started a qualifying high-deductible plan late in the year? If you are HSA-eligible on December 1, the last-month rule can unlock the full annual contribution limit — but it chains you to a 13-month testing period. This calculator sizes the extra room, maps the window, and prices the 10% penalty if you fail it.

Should you use the last-month rule?

Enter your tax year, coverage, and how your next year looks — the timeline below maps your 13-month testing period and the failure math.

Tax year

Limits: 2025 $4,300 / $8,550 · 2026 $4,400 / $8,750 · 2027 $4,500 / $9,000 (self-only / family).

Coverage type

Self-onlyFamily

Use the coverage you hold on December 1 — that is what the rule prices off.

Eligible on December 1 of the tax year?

Eligibility is tested on the 1st of each month. A December 15 start does not count.

Months eligible that year

Count months you were eligible on the 1st. Default 2 — a November start.

I am 55 or older — add the $1,000 catch-up (it prorates and the rule covers it, too)

Your coverage outlook for next year

The testing period runs December 1 of the tax year through December 31 of the next year.

Your marginal tax rate %

Used only to estimate the failure bill: excess income × your rate, plus the 10% tax.

Try:

Use the last-month rule$4,400 availableProrated $733 · rule unlocks +$3,667

December 2026 – December 2027

Eligible monthLost coverage — breaks the test

13 of 13 testing-period months intact — keep coverage through December 2027.

Your 13-month testing period: December of the contribution year through December of the next year. A break anywhere in the strip — a job change, a spouse's FSA, Medicare — fails the test (only death or disability are exempt).

$733Prorated 2026 limit (2/12 months)

$4,400Last-month-rule limit (self-only)

$3,667Extra contribution the rule unlocks

13 moTesting period (2026-2027)

Your last-month rule picture

Tax year · coverage2026 · self-only

Months eligible2 of 12 months eligible on the 1st

Prorated limit (normal rule)$733

Full limit via last-month rule$4,400

Extra contribution at risk$3,667

Testing-period windowDecember 1, 2026 → December 31, 2027

If the test failsNone while you stay eligible through Dec 2027

The rule is worth using: it unlocks an extra $3,667 of tax-advantaged room for 2026. The price is the 13-month testing period — stay HSA-eligible from December 1, 2026 through December 31, 2027, and the full $4,400 stands. Fail it, and the $3,667 the rule unlocked is added to your income in the year you fail, plus a 10% additional tax ($367).

The rule is optional. If a job change, Medicare enrollment, or a spouse's FSA looks likely next year, the safe move is to prorate ($733) instead of gambling on the testing period.

How it works: prorated limit = annual limit × eligible months ÷ 12 (eligibility is tested on the 1st of each month; the $1,000 age-55 catch-up prorates the same way). Full limit via the last-month rule = the full annual limit if you are eligible on December 1. Extra at risk = full − prorated. Fail the 13-month testing period and the extra is added to your income in the failure year, plus a 10% additional tax (Form 8889, Part III).

How the last-month rule changes your HSA limit

The normal HSA rule is monthly math: for every month you were an eligible individual on the 1st of the month, you earn one-twelfth of the annual limit. Start a qualifying high-deductible health plan (HDHP) in November and your 2026 self-only limit is roughly 2/12 of $4,400 — about $733. The last-month rule is the exception Congress wrote for late starters: if you are eligible on December 1 of the tax year, the IRS treats you as if you had been eligible all year, and the full $4,400 is yours — even if December is your first eligible month.

The formulas

prorated limit = annual limit × (eligible months ÷ 12) · full limit via rule = full annual limit (if eligible December 1) · extra at risk = full limit − prorated limit

The annual limits: 2025 is $4,300 self-only / $8,550 family; 2026 is $4,400 / $8,750; 2027 is $4,500 / $9,000. The age-55+ catch-up is $1,000 in every year — it prorates month by month like the base limit, and the last-month rule covers the full $1,000 as well.

Worked example: the November 2026 starter

Became eligible in November 2026 (2 eligible months), self-only, age under 55. Prorated limit: $4,400 × 2/12 = $733. Full limit via the last-month rule: $4,400. Extra at risk: $4,400 − $733 = $3,667. The contributor puts in the full $4,400 — then switches to a non-HDHP plan in March 2027. The testing period fails: the $3,667 is added to 2027 income, plus a 10% additional tax of $367.

The 13-month testing period, in plain terms

The testing period runs from December 1 of the contribution year through December 31 of the following year — 13 months. You must remain an eligible individual for every one of them: qualifying HDHP coverage on the 1st of each month and no disqualifying coverage. The exceptions are narrow — only death or disability. Everything else that ends eligibility breaks the test: switching to a low-deductible plan, a job change with new insurance, enrolling in Medicare, or a spouse enrolling in a general-purpose FSA (that one disqualifies you even if your own plan never changes).

Where this comes in handy

Late-year job starts: a November or December HDHP start is the classic case — the rule can be worth $3,000+ of extra room versus prorating.
Open-enrollment switchers: moving to an HDHP on January 1 never needs the rule, but a mid-year switch to one that lands before December does.
Family planning: a family-limit HDHP on December 1 unlocks the full $8,750 (2026) even with one eligible month — and exposes the biggest recapture if the test fails.
The "probably changing jobs" call: the rule is optional. When next year's coverage is uncertain, prorating is the free insurance policy.

HSA last-month rule — frequently asked questions

Can I contribute the full $4,400 if my HDHP started December 1?

Yes — that is exactly the case the rule was written for. If you are an eligible individual on December 1, 2026 (a qualifying HDHP and no disqualifying coverage), you may contribute the full $4,400 self-only limit for 2026 even if December is your only eligible month. The $1,000 age-55 catch-up is covered the same way. But the 13-month testing period starts that day: you must stay eligible through December 31, 2027.

What happens if I lose my HDHP during the testing period?

The part of your contribution that the last-month rule unlocked — the full-year limit minus what your prorated limit would have been — is added to your gross income in the year you fail, plus a 10% additional tax. Both are reported on Form 8889, Part III. In the worked example above, that was $3,667 of income plus $367 of extra tax.

Can I start coverage December 15 and use the rule?

No. Eligibility is tested on the 1st of each month — you must be eligible on December 1 itself. A December 15 HDHP start means you were not eligible on December 1, so the rule is unavailable and your limit stays prorated (one eligible month at most, for January of the next year — the December year gets nothing from the rule).

Does my spouse's FSA break my HSA?

A general-purpose FSA does. Your spouse's general-purpose FSA covers your medical expenses too, and the IRS treats that as disqualifying coverage for you — you fail the testing period even though your own HDHP never changed. Limited-purpose FSAs (dental and vision only) do not disqualify you. This is the most common "surprise" testing-period failure.

Does enrolling in Medicare end the testing period?

Yes. Medicare enrollment ends your HSA eligibility, which fails the testing period. The only exemptions are death and disability — Medicare, a job change, a move to a low-deductible plan, and a spouse's general-purpose FSA all count as failures. If you are approaching 65 and used the last-month rule, time the Medicare enrollment carefully.

Is the last-month rule optional, or automatic?

Completely optional. Nothing forces you to contribute the full limit — you can always choose the prorated amount instead. Prorating is the safe move whenever next year's coverage is uncertain: you give up the extra room, but there is no testing period to fail and no 10% tax lurking.

What if I was eligible all 12 months — do I still have a testing period?

Effectively no. With 12 eligible months your prorated limit already equals the full limit, so the rule unlocks nothing extra — and the failure penalty only applies to the extra amount the rule unlocked. Zero extra means zero recapture, even if your coverage changes the next year.

Does the $1,000 catch-up get the last-month rule too?

Yes, with the same strings attached. If you are 55 or older and eligible on December 1, you can contribute the full $1,000 catch-up for the year instead of the prorated slice. It is part of the "extra" amount subject to the testing period and the 10% additional tax if the test fails.

Learn the method: the free how the HSA last-month rule works guide walks through the proration math, the 13-month window, the failure traps, and the penalty math — step by step.