● Money & small business
HSA limit for a partial year: month by month, or the last-month rule (2027 tables)
The HSA limits, $4,500 for self-only and $9,000 for family coverage in 2027 ($4,400 and $8,750 in 2026), assume you are eligible all year. If you aren't, there are two ways to work out your limit, and you can use the larger one. Every figure below comes from our HSA contribution limit calculator, which gives the same results as the examples in IRS Publication 969 and was checked against an independent calculation of the Form 8889 Limitation Chart in 2,005 cases.
Month by month
For each month you are an eligible individual on the first day (covered by an HDHP, with no other health coverage that rules out an HSA, and not enrolled in Medicare), you get a twelfth of that month's limit: $375 a month for self-only coverage and $750 for family coverage in 2027. From age 55, add a twelfth of the $1,000 catch-up for each of those months.
The last-month rule
If you are eligible on December 1, you are treated as eligible for the whole year with the coverage you had on December 1. So starting an HDHP late in the year can still give you the full limit. The catch is the testing period: you must stay eligible from December 1 until December 31 of the following year. If you don't, other than because of death or disability, whatever you put in above the month-by-month limit becomes taxable income in the year you stop being eligible, plus a 10% additional tax.
Starting an HDHP during 2027
| Covered from | Self-only: month by month | Family: month by month | Last-month rule | Taxable if you fail the testing period (self / family) |
|---|---|---|---|---|
| January 1 | $4,500 | $9,000 | $4,500 / $9,000 | none |
| February 1 | $4,125 | $8,250 | $4,500 / $9,000 | $375 / $750 |
| March 1 | $3,750 | $7,500 | $4,500 / $9,000 | $750 / $1,500 |
| April 1 | $3,375 | $6,750 | $4,500 / $9,000 | $1,125 / $2,250 |
| May 1 | $3,000 | $6,000 | $4,500 / $9,000 | $1,500 / $3,000 |
| June 1 | $2,625 | $5,250 | $4,500 / $9,000 | $1,875 / $3,750 |
| July 1 | $2,250 | $4,500 | $4,500 / $9,000 | $2,250 / $4,500 |
| August 1 | $1,875 | $3,750 | $4,500 / $9,000 | $2,625 / $5,250 |
| September 1 | $1,500 | $3,000 | $4,500 / $9,000 | $3,000 / $6,000 |
| October 1 | $1,125 | $2,250 | $4,500 / $9,000 | $3,375 / $6,750 |
| November 1 | $750 | $1,500 | $4,500 / $9,000 | $3,750 / $7,500 |
| December 1 | $375 | $750 | $4,500 / $9,000 | $4,125 / $8,250 |
The taxable amount assumes you contributed the full last-month-rule limit; the 10% additional tax is on top, $825 in the family December case. The IRS's own example (Publication 969, 2025 limits) is the same case a year earlier: family coverage from December 1, 2025, $8,550 contributed, eligibility lost in June 2026, so $8,550 − $712.50 = $7,837.50 goes on the 2026 return as income, plus the 10% tax. If you might change jobs, drop the HDHP or go on Medicare next year, the month-by-month limit is the safe one.
Switching from self-only to family coverage
A marriage or a baby often moves you to family coverage mid-year. In 2027, with self-only coverage before the switch:
| Family coverage from | Month by month | Last-month rule | Taxable if you fail the testing period |
|---|---|---|---|
| April 1 | $7,875 | $9,000 | $1,125 |
| July 1 | $6,750 | $9,000 | $2,250 |
| October 1 | $5,625 | $9,000 | $3,375 |
The IRS example (2025): self-only from January, family from November 1: $4,300 × 10 + $8,550 × 2 = $60,100, divided by 12 = $5,008.33 month by month, against $8,550 under the rule. Losing eligibility the next March made $3,541.67 taxable.
Leaving an HDHP or going on Medicare
If you aren't eligible on December 1, the last-month rule doesn't apply and only the months you were eligible count. Family coverage for the first six months of 2027 and none after: $4,500. From the first month you are enrolled in Medicare, your limit is zero. Self-only coverage all year, age 55 or older, Medicare from:
| Medicare from | 2027 HSA limit |
|---|---|
| February | $458.33 |
| April | $1,375.00 |
| July | $2,750.00 |
| October | $4,125.00 |
| December | $5,041.67 |
That's $5,500 ($4,500 + $1,000) divided by 12 for each month before Medicare: the IRS's example is the same calculation at 2025 limits, $2,650 for Medicare from July. Medicare Part A can start retroactively if you enroll after 65 (the Social Security Administration backdates it up to six months, but not before the month you turned 65), and Publication 969 treats contributions made for months of retroactive coverage as excess contributions, so the safe course is to stop contributing six months before you apply.
Married couples
If either spouse has family HDHP coverage, both are treated as having family coverage, and the family limit is shared between your HSAs: equally unless you agree on a different split. In 2027 that's $4,500 each, or for example $6,300 and $2,700 for a 70/30 split. The $1,000 catch-up isn't shared: each spouse who is 55 or older adds it to their own HSA, so with an equal split and both 55+, each can put in $5,500. Your employer's contributions count toward your share: with family coverage from July 1 and $1,000 from your employer, the last-month rule allows $9,000, of which $8,000 is left for you (unmarried, or with the whole family limit allocated to you).
Try your own months in the HSA contribution limit calculator. For how much an HSA saves in tax and grows to, see the HSA calculator and the 2027 open enrollment limits.