HSA Prorated Contribution Calculator
Enrolled in an HDHP mid-year? Calculate your prorated HSA contribution limit, or apply the last-month rule to contribute the full annual amount — with the IRS testing-period requirement spelled out.
IRS counts a month if you were HDHP-eligible on its first day.
Age 55+ unlocks the $1,000 catch-up.
Your Contribution Limit
$2,1506/12 prorated
Full Annual Limit
$4,30012-month baseline
Excess Contribution
$2,1506% excise ≈ $129/yr
⚠ You are $2,150 over your allowed limit. Excess contributions face a 6% excise tax per year (estimated $129) until withdrawn (with earnings) by the tax-filing deadline.
Prorated limit: $4,300 × 6/12 = $2,150.
Default (prorated) rule: HSA limit = annual limit × months HDHP-covered ÷ 12. IRS counts a month if you were HDHP-eligible on the first day of that month.
Last-month rule (IRC §223(b)(8)): If you are HSA-eligible on December 1, you can contribute the full annual limit regardless of how many months you were covered. But you must remain HSA-eligible through December 31 of the following year (the testing period).
2025 annual limits: Self-only $4,300 · Family $8,550 · Catch-up $1,000 (age 55+). The catch-up is also prorated under the default rule.
Excess contributions: 6% excise tax per year until withdrawn (with earnings) by the tax-filing deadline including extensions. After the deadline, you generally must either withdraw with earnings or leave it and pay the 6% each year the excess remains.
Sources: IRS Publication 969, Form 8889 instructions, IRC §223 and §4973.
Frequently asked questions
How does HSA proration work for mid-year enrollment?
Under the default rule, your HSA contribution limit is the annual limit times the number of months you were HDHP-eligible on the first day of the month, divided by 12. A July 1 HDHP start gives you 6/12 of the annual limit — $2,150 for self-only or $4,275 for family in 2025.
What is the HSA last-month rule?
The last-month rule (IRC §223(b)(8)) lets you contribute the full annual HSA limit if you are HDHP-eligible on December 1 of the tax year, regardless of months covered earlier. In exchange, you must remain HSA-eligible every month through December 31 of the following year — the testing period. If you drop eligibility during the testing period, the extra contributions (above the prorated amount) become taxable income plus a 10% additional tax.
What is the 2025 HSA contribution limit?
For 2025, the HSA limit is $4,300 for self-only HDHP and $8,550 for family. Age 55+ gets an extra $1,000 catch-up. All limits are prorated for mid-year eligibility unless you elect the last-month rule.
What happens if I contribute too much to my HSA?
Excess HSA contributions face a 6% excise tax per year until withdrawn (with earnings) by the tax-filing deadline including extensions. After the deadline, the 6% continues each year the excess remains. Report on Form 5329.