Who can contribute to an HSA, and how the ceiling is set
A health savings account is the only vehicle in the US tax code with three separate tax advantages on the same dollar: contributions are deductible or excluded from wages, growth inside the account is untaxed, and withdrawals for qualified medical expenses are untaxed. That combination is why the contribution limit is policed carefully, and why exceeding it is expensive.
Eligibility under IRC §223(c)(1) requires four things at once. You must be covered by a qualifying high-deductible health plan; you must have no other health coverage that pays before the deductible, with narrow exceptions for dental, vision, disability and specific preventive care; you must not be enrolled in Medicare; and you must not be claimable as a dependent on someone else's return. Eligibility is tested on the first day of each month, which is why the calendar matters more than the number of days.
The ceiling has two tiers, self-only and family, set by the plan you are enrolled in rather than by how you file your taxes. For 2026 they are $4,400 and $8,750. Anyone who reaches age 55 during the year adds a $1,000 catch-up, a figure fixed in the statute and never indexed. Employer contributions count against the same ceiling, so a generous employer reduces your own room dollar for dollar.
The IRS publishes the tier limits, along with the minimum deductible and maximum out-of-pocket figures a plan must meet to qualify, in a revenue procedure each spring. Enter the figures for your year in the statutory limits fields if you are not working with 2026.
Proration, the last-month rule, and the trade between them
The default rule is proration. Take the full-year limit, divide by twelve, and multiply by the months you were eligible on the first day. Six months of family coverage in 2026 gives 8,750 × 6 ÷ 12 = $4,375. The catch-up prorates on the same basis — it is not an all-or-nothing addition — so six months at age 57 gives (8,750 + 1,000) × 0.5 = $4,875.
The alternative is the last-month rule in §223(b)(8). If you are HSA-eligible on 1 December, you are treated as eligible for the whole year and may contribute the full annual limit no matter when your coverage started. Start a job with HDHP coverage on 1 October and you may still put in the whole $8,750 rather than $2,187.50.
It is not free. Electing the last-month rule starts a testing period that runs from 1 December of the contribution year through 31 December of the following year — thirteen months. You must remain HSA-eligible for every month of it. If you fail, for any reason other than death or disability, the difference between what you contributed and what proration would have allowed is added to your gross income in the year you fail, and hit with an additional 10% tax on top.
So the decision is a probability judgement. If you are confident you will hold qualifying coverage throughout next year, the last-month rule accelerates a large tax deduction. If there is a real chance you will move to a non-HDHP plan, enrol in Medicare, or join a spouse's PPO, proration is the safer route and costs only the deduction you defer. Enrolling in Medicare is the version that catches people out, because Medicare Part A enrolment is often backdated up to six months when you claim Social Security after 65.
One more wrinkle for married couples: the family limit is a single household ceiling. Two spouses each holding a family-tier HSA share one $8,750, divided however they agree. But each spouse's $1,000 catch-up must go into an HSA in that spouse's own name — a catch-up cannot be paid into the other spouse's account.
Worked example: family coverage from July, age 45, with employer funding
You join an employer on 20 June 2026 and your family HDHP coverage begins 1 July. Your employer contributes $1,000 to the HSA, you are 45, and 13 biweekly paydays remain in the year.
- Statutory limit. Family tier: $8,750.00.
- Catch-up. Age 45, so $0.00.
- Full-year limit. 8,750 + 0 = $8,750.00.
- Limit per eligible month. 8,750 ÷ 12 = $729.1667.
- Eligible months. You have coverage on the first of July, August, September, October, November and December — 6 months. Coverage starting 1 July counts July; coverage starting 2 July would not.
- Prorated limit. 729.1667 × 6 = $4,375.00.
- Room after employer money. 4,375.00 − 1,000.00 = $3,375.00.
- Per paycheck. 3,375.00 ÷ 13 = $259.62.
Now price the alternative. You are eligible on 1 December, so the last-month rule is available and would raise the limit to the full $8,750.00, leaving $7,750.00 of personal room and $596.15 per paycheck. The extra deduction is 8,750 − 4,375 = $4,375. At a 22% federal and 5% state marginal rate that is worth 4,375 × 0.27 = $1,181.25 in tax this year, plus the 7.65% FICA saving of $334.69 if the money goes in through payroll — about $1,516 in total.
The risk is the mirror image. If you leave for a non-HDHP plan in, say, September 2027, you fail the testing period, and the $4,375 excess is added to your 2027 income and taxed again at 10%: roughly 4,375 × 0.27 + 437.50 = $1,618.75. The two figures are close enough that the decision turns almost entirely on how sure you are about next year's coverage, not on the arithmetic.
Prorated 2026 HSA limits by eligible months
| Eligible months | Self-only | Family | Self-only + catch-up | Family + catch-up |
|---|---|---|---|---|
| 1 | $366.67 | $729.17 | $450.00 | $812.50 |
| 3 | $1,100.00 | $2,187.50 | $1,350.00 | $2,437.50 |
| 6 | $2,200.00 | $4,375.00 | $2,700.00 | $4,875.00 |
| 9 | $3,300.00 | $6,562.50 | $4,050.00 | $7,312.50 |
| 11 | $4,033.33 | $8,020.83 | $4,950.00 | $8,937.50 |
| 12 | $4,400.00 | $8,750.00 | $5,400.00 | $9,750.00 |
Every figure is the full-year limit divided by twelve and multiplied by the month count — for example the 9-month family cell is 8,750 ÷ 12 × 9 = 6,562.50. Employer contributions come out of these amounts, not on top of them.
What to do with the number, and how to avoid an excess
Fund through payroll if you possibly can. A payroll HSA contribution runs through a section 125 cafeteria plan, which exempts it from Social Security and Medicare as well as from income tax. The identical dollar contributed personally and deducted on Form 1040 saves income tax only, so it is worth 7.65% less to you. On a $7,750 contribution that difference is $592.88 — for doing the same thing through a different channel.
Set the per-paycheck figure and then leave headroom. The number this calculator gives fills the limit exactly across the remaining paydays, which is fine if nothing changes and awkward if something does. A mid-year switch from family to self-only coverage, a spouse enrolling you in their PPO, or an unexpected Medicare enrolment all reduce your eligible months retroactively and can turn a perfectly planned contribution into an excess.
If you do over-contribute, the remedy is time-sensitive. Withdraw the excess and the net income attributable to it before the due date of your return, including extensions, and the 6% excise tax under §4973 does not apply — the earnings are taxable but the penalty is avoided. Miss that deadline and the 6% applies for that year and for every subsequent year the excess remains in the account. It is one of the few penalties in the code that recurs annually rather than once.
The account itself has no use-it-or-lose-it feature, which sets it apart from a health FSA. Unspent HSA money rolls forward indefinitely, stays yours when you change jobs, and can be invested. That makes an HSA more like a retirement account than a spending account, and it is why many people fund it to the limit and pay current medical costs from cash instead. Compare it against a health FSA using the FSA payroll deduction calculator, and check whether the underlying plan choice is right using the HDHP vs PPO total cost calculator.
Mistakes that create an excess contribution
- Forgetting that employer money counts. The limit is a combined ceiling. A $1,500 employer contribution leaves $7,250 of personal room on a family plan, not $8,750.
- Treating the family limit as per person. Spouses share one family limit however many accounts they hold. Only the $1,000 catch-ups are individual.
- Counting a partial first month. Eligibility is tested on the first day of the month. Coverage beginning 2 March gives no March eligibility.
- Ignoring a spouse's general-purpose health FSA. A spouse's FSA that can reimburse your expenses is disqualifying coverage for you, even if you are not enrolled in it.
- Missing retroactive Medicare Part A. Claiming Social Security after 65 can backdate Part A enrolment by up to six months, wiping out eligibility for months you already funded.
- Using the last-month rule and then changing plans. The 13-month testing period is unforgiving, and the recapture carries an extra 10% tax.
- Contributing after Medicare enrolment. Eligibility ends the month Medicare begins; contributions after that are excess from day one.
- Assuming the deadline is 31 December. It is not — you may contribute for a tax year up to the unextended due date of that year's return, normally 15 April.
The testing period bites in the following year, not this one
If you use the last-month rule and then lose eligibility during the following calendar year, the recapture lands on the following year's return. The amount added back is the difference between what you actually contributed and what you could have contributed under straight proration, and it is subject to a 10% additional tax on top of ordinary income tax. The only exceptions are death and disability. Because the failure can be caused by something outside your control — a plan change your employer makes, a spouse's open enrolment, a backdated Medicare start — the last-month rule is better suited to people with a stable, self-controlled coverage situation than to anyone whose plan may be changed for them.
How the HSA limit compares with other pre-tax accounts
An HSA is not the only pre-tax health account, and the rules differ in ways that matter. A general-purpose health FSA has a lower limit, belongs to the employer rather than to you, and forfeits unspent balances beyond a small carryover or a short grace period. It is also disqualifying coverage for HSA purposes, which is why employers offering both usually restrict HSA participants to a limited-purpose FSA covering only dental and vision.
A dependent care FSA sits outside all of this with its own statutory limit and its own qualifying expenses; the dependent care FSA tax savings calculator handles that comparison against the child and dependent care credit.
Against retirement accounts, the HSA compares unusually well. A traditional 401(k) deferral avoids income tax now and is taxed on withdrawal; the 401(k) paycheck impact calculator shows what that costs per paycheck. An HSA dollar contributed through payroll avoids income tax, Social Security and Medicare going in, and avoids tax entirely coming out if spent on qualified medical expenses. After age 65 non-medical withdrawals are taxed as ordinary income with no penalty, which makes the account no worse than a traditional IRA in the worst case and considerably better in the expected one.
The practical planning point is ordering. Capture any employer 401(k) match first, because that is an immediate return no tax treatment can beat. Then fill the HSA, because the triple exemption plus the FICA saving on payroll contributions is the strongest per-dollar tax treatment available. Then return to the retirement account. Whether that ordering holds for you depends on your marginal rate, which the marginal vs effective tax rate calculator can pin down.