An HSA follows the tax year. A health FSA follows the plan year. That calendar split is why people miss a December HDHP window or leave FSA cash on the table in March.
If you are an eligible individual on December 1 of a calendar tax year, Publication 969 treats you as eligible for the entire year. The live HSA engine then still shows the full 2026 cap:
- Self-only: $4,400
- Family: $8,750
- Age 55 or older: add $1,000
You generally must stay eligible through the testing period (the next calendar year) or the extra months get recaptured. This page does not invent a prorated engine.
Run the HSA contribution calculator →
Which account to open is a different post: HSA vs FSA. Hub: United States calculators.
A cafeteria health FSA is elected for the plan year. Starting an HDHP on December 1 does not unlock a leftover FSA. Unused FSA dollars are use-it-or-lose-it unless the plan offers a grace period or the 2026 carryover of $680 in Rev. Proc. 2025-32 §2.15. Many plans offer neither.
Direct answer: December HDHP → $4,400 still on the table
- Eligible on December 1, 2026, self-only HDHP, under 55
- Full-year HSA limit = $4,400
- $0 already deposited → room $4,400
- You can still contribute for 2026 until the return due date without extensions (same idea as an IRA, different code)
- A health FSA for 2026 is already closed unless your plan’s grace or carryover says otherwise
limit = $4,400 self-only or $8,750 family + $1,000 if age ≥ 55
remaining = max(0, limit − YTD contributions)
Last-month: eligible on Dec 1 → full limit (testing period applies)
Two clocks
| Account | 2026 cap (this site) | When the year ends |
|---|---|---|
| HSA self-only | $4,400 | Tax year. Last-month can still unlock the full cap. Funding often stays open until the April due date. |
| HSA family | $8,750 | Same tax-year clock. |
| Health FSA | $3,400 (Rev. Proc. 2025-32 §2.15; no live engine) | Plan year. Grace or $680 carryover only if the plan document says so. |
| Dependent-care FSA | $5,000 ($2,500 MFS) | Plan year. Not an HSA. DCAP room. |
HSA deposits from you and your employer share one cap. $1,200 already in a self-only HSA leaves $3,200: HSA room.
A last-month HSA contribution still needs an HDHP that actually qualifies. Pub 969 lists the 2026 deductible and out-of-pocket floors. We do not restate those as a new calculator.
Paycheck timing
HSA payroll deferrals (and many medical premiums) lower FICA as well as FIT. A traditional 401(k) does not. On a $2,000 biweekly stub, $150 of HSA/§125 leaves net $1,530.62; 10% 401(k) leaves $1,475.15 with FICA still $153. See the paycheck calculator.
If you are an eligible individual that day, Pub 969’s last-month rule says the full annual limit is available, subject to the testing period. The live engine will show $4,400 remaining if YTD is $0.
Often yes, until the filing due date without extensions, the same calendar idea as a prior-year IRA. Tell the custodian which tax year the deposit is for. An FSA does not get that extra winter window.
No. The plan must allow carryover. $680 is the 2026 ceiling in Rev. Proc. 2025-32 §2.15, not a promise.
Publication 969 (last-month rule, HSA limits) · Rev. Proc. 2025-32 §2.15 (health FSA / carryover)