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HSA Prorated Contribution Limit: Month by Month

HSA eligibility is tested on the first day of each month. Your limit is the annual amount times eligible months divided by twelve, catch-up included.

published 2026-09-07 · figures checked 2026-09-07

You are HSA-eligible for a month only if you had qualifying coverage on the first day of that month. Your limit is the annual amount times eligible months divided by twelve. The age-55 catch-up prorates the same way.

The HSA contribution limit calculator does that arithmetic for one year at a time. This page explains the rule behind it, and where the rule surprises people.

Eligibility is a monthly test, not an annual one

IRS Publication 969 lists four conditions for being an eligible individual. You are covered by a qualifying high deductible health plan “on the first day of the month”. You “have no other health coverage except what is permitted”. You “aren’t enrolled in Medicare”. And you cannot be claimed as a dependent on someone else’s return.

The coverage condition names the date: the first of the month. A plan that starts on March 2 gives you no March. A plan that starts on March 1 gives you all of March, even if you cancel it on March 3.

The formula

prorated limit = annual limit × eligible months ÷ 12

The Instructions for Form 8889 do the same thing in a different shape. The Line 3 Limitation Chart asks you to write an amount on a line for each of the twelve months, then “add them all together and divide by 12”. A month you were not eligible gets zero. A month with self-only coverage gets the self-only annual amount. A month with family coverage gets the family annual amount.

That is why the chart handles a mid-year switch without a special rule. Each month carries its own coverage type into the average.

The 2026 annual amounts are $4,400¹ for self-only coverage and $8,750² for family coverage. The dollar figure for each month count is tabulated on the 2026 HSA limits page.

Switching between self-only and family coverage

There is no single “coverage type” for the year. A month in which you had family coverage on the 1st is a family month, and a month in which you had self-only coverage on the 1st is a self-only month.

Example. A hypothetical taxpayer has self-only coverage on the first of January through June, and family coverage on the first of July through December. Six months carry the self-only amount and six carry the family amount. The Line 3 chart adds all twelve entries and divides by twelve.

The result is not the family limit, and it is not the self-only limit. It sits between them.

The catch-up prorates on the same clock

The age-55 catch-up is $1,000³, and it is not a flat addition to a partial year. The Additional Contribution Amount Worksheet in the Instructions for Form 8889 multiplies the catch-up by the number of months you were an eligible individual, then divides by twelve.

The age test itself is annual: you qualify if you are 55 or older at the end of the tax year. The proration that follows is monthly. The age-55 catch-up rules work through the combination.

A month with Medicare is a zero month

Publication 969 is blunt about it: “Beginning with the first month you are enrolled in Medicare, your contribution limit is zero.” The Instructions for Form 8889 say to “enter 0 on the line below for the month” for any month you were enrolled.

Medicare enrollment can be backdated, which turns months you thought were eligible into zero months after the fact. That is set out in Medicare and your HSA.

How do I prorate my HSA contribution?

Count the months in which you were an eligible individual on the first day of the month. Multiply the annual limit for your coverage type by that count and divide by twelve.

Which day of the month decides whether it counts?

The first day. Publication 969 requires HDHP coverage “on the first day of the month”, and the same date governs the other three eligibility conditions.

What if I switched from self-only to family coverage?

Each month keeps its own coverage type. The Line 3 Limitation Chart in the Instructions for Form 8889 records a separate amount for each of the twelve months and averages them.

Does the age-55 catch-up get prorated?

Yes. The Additional Contribution Amount Worksheet multiplies the catch-up by eligible months and divides by twelve, exactly like the base limit.

Does an employer contribution reduce my prorated limit?

Yes. The prorated limit is the ceiling on everything that goes into the account for the year, including payroll deductions, your own deposits and employer money.

When is the deadline to make a prorated contribution?

Contributions for the 2026 tax year may be made until April 15, 2027⁴. Publication 969 states that a person who stops being an eligible individual during the year may still contribute for the months they were eligible, up to that deadline.

Do I lose the whole year if I was only eligible for part of it?

No. Partial eligibility gives a partial limit, not a zero limit. The one exception runs the other way: the last-month rule can give a full annual limit to someone who was eligible only on December 1.

Information, not advice. This is the reading version; the interactive calculator and the full page are at https://thresholds.pages.dev/hsa/proration/.