Are HSA Contributions Prorated? Monthly Rule and Limits

Yes, HSA contributions are prorated any month you are not an eligible individual. The IRS takes your annual limit, divides by 12, and multiplies by the number of months you actually qualified under a High Deductible Health Plan (HDHP). For 2026 the full-year caps are $4,400 for self-only coverage and $8,750 for family coverage, with a $1,000 catch-up if you are 55 or older by year-end.¹² One important exception, the last-month rule, can restore the full annual amount even if your HDHP started late in the year.³

How the Monthly Proration Works

Federal law defines your annual contribution limit as the sum of your monthly limitations for every month you were an eligible individual.² Eligibility is measured on the first day of each month. If your HDHP coverage starts June 1, you get credit for seven months. If it starts June 2, June does not count.³

A 2026 self-only example, coverage beginning June 1:

Annual limit: $4,400
Monthly amount: $4,400 ÷ 12 = $366.67
Months eligible (June through December): 7
Prorated limit: $366.67 × 7 = $2,566.67

The same divide-by-12 approach applies to family coverage and to the $1,000 catch-up amount. Each piece is prorated independently by the number of months you qualified for it.

The Last-Month Rule

If you are an eligible individual on December 1, the IRS treats you as eligible for all 12 months. That means you can contribute the full $4,400 (self-only) or $8,750 (family), plus the catch-up if you are 55 or older, even if your HDHP coverage did not start until October or even late November.³

Your allowed contribution is the greater of the standard month-by-month calculation or the full annual amount based on your December 1 coverage type. Four months of coverage plus eligibility on December 1 still gets you the full-year limit.

The Testing Period You Must Clear

Using the last-month rule commits you to a testing period. It runs from December 1 of the year you used the rule through December 31 of the following year, a total of 13 months, and you must stay an eligible individual with HDHP coverage the entire time.³

Fail the testing period, and the IRS recalculates your limit as if the last-month rule never applied. The difference between what you contributed and what the standard monthly proration would have allowed gets added back to your taxable income, and you owe an additional 10% tax on that amount.⁴ The income inclusion and 10% tax are waived only if you lost eligibility because you died or became disabled. Form 8889 is where you report the failure and calculate what you owe.⁵

When Your Coverage Type Changes Mid-Year

Switching between self-only and family HDHP coverage means running two prorated calculations, one for each tier, and adding them together. The IRS looks at your coverage on the first day of each month and applies the matching monthly amount.³

Say you have self-only coverage January through June in 2026, then family coverage July through December:

Self-only monthly amount: $4,400 ÷ 12 = $366.67
Family monthly amount: $8,750 ÷ 12 = $729.17
Self-only portion (6 months): $2,200.00
Family portion (6 months): $4,375.00
Total prorated limit: $6,575.00

Watch the exact start date. If family coverage begins July 15, July still counts as a self-only month because you did not have family coverage on July 1.

Medicare Ends Your Eligibility

Enrolling in any part of Medicare (Part A, B, C, or D) makes you ineligible to contribute to an HSA. Your limit for that year is prorated based on the months before Medicare took effect.³

The trap is Part A’s retroactive enrollment. When you sign up for Medicare after age 65, Part A is backdated up to six months, though never before the month you turned 65. Any HSA contributions made during those retroactive months become excess contributions. If you turned 65 in January 2026 and enrolled in Medicare in July 2026, Part A can reach back to January, wiping out every eligible month for the year. Stopping HSA contributions at least six months before Medicare enrollment prevents this. Claiming Social Security benefits automatically enrolls you in Medicare Part A, so that timing matters too.

Going Over Your Prorated Limit

Contribute more than your prorated cap and you owe a 6% excise tax on the excess for every year it stays in the account.⁶

You can avoid the penalty by pulling out the excess plus any earnings on it before the due date of your return (including extensions) for the year of the contribution. The withdrawn earnings are taxable in the year of withdrawal.³ Miss that deadline and the 6% applies every year until the excess is either withdrawn or absorbed by under-contributing in a later year. You report the tax on Part VII of Form 5329.⁷

This 6% excise tax is separate from the 10% additional tax for failing the last-month rule testing period. You can owe both in the same year if you over-contributed using the last-month rule and then lost HDHP eligibility.

You Have Until Tax Day to Contribute

HSA contributions for a tax year can be made up to the unextended federal filing deadline. For 2026, you have until April 15, 2027, to deposit funds counted toward the 2026 limit.⁴ That window is helpful when your prorated limit is still in motion late in the year, whether because you are deciding on the last-month rule or your coverage type could still shift. Wait, calculate, then contribute to the exact cap.

1
Internal Revenue Service. IRS Notice: Expanded Availability of Health Savings Accounts
2
Office of the Law Revision Counsel. 26 USC 223 – Health Savings Accounts
3
Internal Revenue Service. Publication 969 – Health Savings Accounts and Other Tax-Favored Health Plans
4
Internal Revenue Service. 2025 Instructions for Form 8889
5
Internal Revenue Service. Instructions for Form 8889
6
Office of the Law Revision Counsel. 26 USC 4973 – Tax on Excess Contributions to Certain Tax-Favored Accounts and Annuities
7
Internal Revenue Service. Form 5329 – Additional Taxes on Qualified Plans and Other Tax-Favored Accounts