2026 limits. $8,750 for family coverage, $4,400 for self-only. Limits are assumed to rise 2% a year, which is a guess; the IRS sets them annually. Catch-up contributions for those 55 and older are not modeled — if you are eligible, add $1,000 per person per year and note that each spouse needs their own HSA to make their own catch-up.

Eligibility. You must be covered by a qualifying high-deductible health plan. For 2026 that means a deductible of at least $1,700 self-only or $3,400 family, with out-of-pocket maximums no higher than $8,500 and $17,000. Medicare enrollment ends eligibility to contribute.

The fourth advantage. Contributions made through an employer's Section 125 cafeteria plan also escape Social Security and Medicare payroll taxes, about 7.65%. Contributions you make directly to an HSA are deductible but do not avoid payroll tax. Set that slider to 0% if you fund your HSA yourself.

Tax savings are shown as plain dollars, never invested. Growing them alongside the HSA would overstate the result, because money outside an HSA is taxed on its earnings along the way. The cumulative figure below the chart is what you kept, undressed — no growth assumed on it at all.

Growth and timing. Contributions are treated as made at the start of each year and compounded annually at the rate you set. Only invested balances grow — most custodians require a cash minimum before investing, and an uninvested HSA earns nearly nothing.

Not modeled: state treatment of HSAs where it differs from federal (California and New Jersey are the notable exceptions), medical spending along the way, employer plan fees, the Social Security wage base above which the 6.2% portion of payroll tax stops, and any change in your tax rate over time.