How This Comparison Is Calculated
This calculator applies IRC section 223 (HSAs) and IRC section 125 (cafeteria plans / health FSAs), using the 2026 inflation-adjusted limits in IRS Rev. Proc. 2025-19 (HSA) and Rev. Proc. 2025-32 (health FSA). It runs the same set-aside amount through both accounts and compares the net tax benefit after forfeiture risk. For the full statutory background, see our HSA vs FSA Guide.
Step 1: Cap the Contribution to Each Account Confirmed
The HSA contribution is capped at $4,400 (self-only) or $8,750 (family) for 2026, plus $1,000 if you are age 55 or over, minus any employer HSA contribution. The health FSA contribution is capped at the $3,400 salary-reduction limit. If your set-aside amount exceeds an account's cap, the calculator uses the cap and flags the difference.
Step 2: Federal Income Tax Savings Confirmed
Income tax savings = contribution × estimated marginal federal rate. The marginal rate is derived from your gross income minus the 2026 standard deduction (single/MFS $16,100, MFJ $32,200, HOH $24,150) under 2026 brackets per Rev. Proc. 2025-32. Both accounts reduce federal taxable income, so on the same dollar the income tax break is identical.
Step 3: FICA Savings Confirmed
Contributions routed through an employer Section 125 cafeteria plan also escape FICA - Social Security 6.2 percent plus Medicare 1.45 percent, a 7.65 percent additional saving. A health FSA is by definition a cafeteria-plan benefit, so it always earns the FICA saving. An HSA earns the FICA saving only when funded through payroll; a direct HSA contribution outside payroll saves income tax but not FICA. Above the Social Security wage base ($184,500 for 2026) only the 1.45 percent Medicare portion applies - the calculator uses a flat 7.65 percent and discloses this simplification.
Step 4: Forfeiture Risk (the FSA penalty) Confirmed
An HSA never forfeits - unused balance rolls over in full and can be invested. A health FSA is subject to the use-it-or-lose-it rule of IRC section 125: any amount left over at year end above the permitted carryover (up to $680 for 2026) or grace-period spending is forfeited to the employer. The calculator estimates FSA money at risk as the FSA contribution minus your expected eligible spending minus the carryover cap, floored at zero. That forfeiture is subtracted from the FSA's tax savings to produce its net tax benefit, which is what the recommendation compares.
Why the HSA usually wins: On the up-front tax break the two accounts are close (both pre-tax, both FICA-advantaged through payroll). The HSA pulls ahead on everything that happens after the contribution: no forfeiture, tax-free investment growth, portability across jobs, a higher limit, the age-55 catch-up, and post-65 access as a de facto traditional IRA. The FSA's one structural advantage is that your full annual election is available on January 1, before you have funded it - useful for a large, certain, early-year expense.
Assumptions
- 2026 HSA limits per IRS Rev. Proc. 2025-19; 2026 health FSA limit and $680 carryover per Rev. Proc. 2025-32.
- 2026 federal brackets and standard deductions per Rev. Proc. 2025-32 (2025 per Rev. Proc. 2024-40).
- HDHP qualification (for HSA eligibility) is confirmed by your insurer; the calculator does not validate plan design.
- FICA saving uses a flat 7.65% and assumes wages below the Social Security wage base.
- Forfeiture risk assumes a plan carryover of up to $680; plans offering a grace period instead, or neither, will differ.
- State income tax savings are not modeled (CA and NJ do not conform to HSA treatment).
- A dependent care FSA is a separate account and is not part of this HSA-vs-health-FSA comparison.