The Health Savings Account carries the only triple tax advantage in the US tax code: contributions dodge income tax going in, growth compounds tax-free, and withdrawals for medical costs come out tax-free too. No 401(k) or IRA manages all three. Yet most account holders treat the HSA as a spending wallet for this year's prescriptions instead of what it can be -- a stealth retirement account for healthcare costs decades away.

This calculator projects the long game. Enter your annual contribution -- $4,150 for individual coverage is a common figure -- plus your current balance, expected return, and the years until you need the money. It compounds the balance, subtracts your expected annual medical withdrawals, and separately tallies the income tax you avoid each year at your marginal rate. A 30-year-old contributing the max for 20 years at 6 percent, pulling $1,000 a year for care, can watch the balance climb past $150,000 while banking tens of thousands in tax savings.

The catch is eligibility: you must carry a high-deductible health plan to contribute. If you do, the winning move is paying small medical bills from your checking account, saving the receipts, and letting the HSA compound untouched -- you can reimburse yourself from those receipts years later, tax-free.

With the pre-filled defaults — Annual Contribution $4,150, Current HSA Balance $8,000, Expected Annual Return 6%, Years to Grow 20, Marginal Tax Rate 24%, Annual Medical Withdrawals $1,000 — the HSA Calculator returns Total Contributions $83,000.00, Projected HSA Balance $436,976.04, Annual Tax Savings $996.00, Total Tax Savings $19,920.00, Net Balance (After Withdrawals) $436,976.04. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.