The only triple-tax-free account
An HSA is the single most tax-advantaged account in the US code: contributions are deductible (and skip FICA when made through payroll), growth is tax-free, and withdrawals for qualified medical expenses are tax-free — no other account gets all three. The catch: you must be enrolled in a qualifying high-deductible health plan (HDHP) to contribute.
The stealth-IRA strategy
The advanced move is to pay current medical bills out of pocket, invest the HSA, and keep receipts. There's no deadline on reimbursing yourself, so receipts become tax-free withdrawal coupons redeemable decades later, after the money has compounded. $4,400/year at 7% for 20 years is roughly $180,000 — and Fidelity estimates a 65-year-old couple needs about $330,000+ for healthcare in retirement, so the money will not lack uses. After 65, non-medical withdrawals are simply taxed like a traditional IRA (no penalty).
Common mistakes that forfeit the advantage
Three big ones: leaving the balance in cash (most HSA custodians make you opt in to investing, often above a $1,000-2,000 cash threshold); treating it as a spending account and draining it every year; and forgetting the account is portable — if your employer's HSA provider charges high fees, you can periodically transfer funds to a better custodian like Fidelity, which offers a fee-free HSA.
How to use this calculator
Enter the amount you'll contribute each year (up to the 2026 limits of about $4,400 self-only or $8,750 family), the years you'll keep contributing, an expected investment return, and your marginal tax rate. The calculator projects your invested balance and totals the income tax the deduction saves along the way. To see the account's full power, assume you invest the balance rather than spend it, and pay small medical bills out of pocket — the projection then represents a tax-free medical war chest for retirement. If your plan defaults contributions into cash, remember most custodians require you to opt into investing above a cash threshold, or the growth line never happens.