The HSA last-month rule can treat an eligible person on December 1 as eligible for the full contribution limit, but the testing period generally runs through December 31 of the following year. Losing HSA eligibility during that period can make the extra contribution taxable and subject to an additional 10 percent tax, unless an exception applies.
A September 2026 forum question shows the exact confusion: whether changing coverage level fails the testing period and how an FSA affects eligibility. IRS Publication 969 and Form 8889 instructions control the result.
Start here: a practical action plan
- Open the official source and confirm the rule, deadline, or plan term for the exact facts.
- Collect the notices, statements, forms, invoices, and confirmations named in the guide.
- Replace the worked-example figures with actual amounts and check every calculation.
- Record the next deadline and save written proof after each action is completed.
- Relying on a forum answer instead of the governing form, plan document, policy, or agency source.
- Spending disputed money or a reserve before the final amount is confirmed in writing.
- Losing dated records needed to prove timing, calculation inputs, or a correction request.
Compare the monthly limit with the full-year limit
HSA eligibility is normally measured on the first day of each month. Count eligible months, apply the coverage level for each month, and include the age-55 catch-up when it applies. Then compare that prorated limit with the full-year amount allowed by the last-month rule.
The extra amount created by the last-month rule is the exposure during the testing period. Employer deposits count toward the same limit. A payroll label does not override disqualifying coverage such as a general-purpose health FSA.
Worked example: four eligible months create an $2,833 difference
Example: assume a $4,250 self-only annual limit and eligibility beginning September 1. Four eligible months create a normal prorated limit of $1,416.67: $4,250 multiplied by 4 divided by 12. The last-month rule could allow $4,250, which is $2,833.33 more.
If eligibility ends the next July for a reason outside an exception, the extra $2,833.33 can be included in income and face a $283.33 additional tax at 10 percent. This example isolates the last-month amount and does not calculate ordinary income tax.
| Item | Example | Why it matters |
|---|---|---|
| Annual limit assumed | $4,250 | Example input |
| Eligible months | 4 | September through December |
| Monthly method | $1,416.67 | $4,250 x 4 / 12 |
| Extra last-month amount | $2,833.33 | $4,250 minus $1,416.67 |
Put the testing period on a coverage calendar
For a contribution based on December eligibility, mark every month through the end of the following December. Save plan documents for the high-deductible health plan and any FSA, HRA, Medicare, or other coverage. Recheck eligibility before an open-enrollment change.
A change from family to self-only HDHP coverage does not automatically mean all HSA eligibility ends, but it can change the contribution limit. Losing eligibility entirely is different. Use Form 8889 Part III when the testing period fails.
Use the rule only when the future coverage is reasonably stable
The last-month rule creates earlier tax-advantaged room, but it also creates a commitment. If a job change, Medicare enrollment, spouse's FSA, or plan change is likely, the monthly method may be easier to defend and less expensive to unwind.
Do not withdraw money merely because eligibility changed. The testing-period consequence is a tax-return calculation, not automatically an excess-contribution correction. Confirm the facts before asking the custodian to move funds.
Frequently asked questions
What is the HSA last-month rule?
It can treat an eligible person on December 1 as eligible for the full year when calculating the HSA contribution limit.
How long is the HSA testing period?
For the last-month rule, it generally runs from December 1 of the contribution year through December 31 of the following year.
What happens if I lose HSA eligibility during the testing period?
The extra amount allowed by the rule can become taxable and may face a 10 percent additional tax unless an exception applies.
Does moving from family to self-only coverage fail the test?
Not by itself if the new coverage remains HSA eligible, but the contribution limit and family-level amount need separate review.
Can a spouse's FSA affect the testing period?
Yes. A general-purpose FSA that can reimburse your expenses can be disqualifying coverage even when the FSA is in a spouse's name.
Where is a testing-period failure reported?
Form 8889 Part III is used to calculate income and additional tax when a testing period is not satisfied.
Sources and review notes
The calculations and scenarios in this guide are original BreadWinnr worked examples. We recalculated each example and checked that its parts match the stated total. Official sources support the financial concepts. Forum links document the reader problem that prompted the guide and are not treated as financial authority.
- IRS Publication 969, HSA last-month rule
- IRS Instructions for Form 8889
- Reddit: HSA last-month rule testing period
Read our research, testing, corrections, and source policy.
This guide provides general financial education. It does not account for your complete financial, tax, legal, or contractual situation.
Make the example yours.
Use the sinking fund calculator to try a different starting point.