A 35-year-old who maxes an HSA and invests the balance — rather than spending it — ends up with roughly $415,000 by age 65, assuming a 7% average annual return on a self-only contribution of $4,400 per year.
That is not a rough estimate. That is a standard annuity calculation on a federal contribution limit. And according to the Employee Benefit Research Institute's 2023 HSA database, only 15 percent of the 14.5 million HSA accountholders were investing their funds in anything other than cash.
The other 85 percent are parking a triple-tax-advantaged account in something that earns roughly nothing, spending it down on co-pays, and watching the account reset to zero each year. It is the most systematically underused tax shelter in the American tax code.
Understanding why requires a small detour through how the account actually works — because most people were told about it in a benefits enrollment meeting and never thought about it again.
Three tax bites, none of them taken
No other account in the US tax code does all three of these things simultaneously.
A traditional 401(k) is pre-tax in — you get the deduction now — but the money is taxable when you withdraw. The IRS deferred the tax, not eliminated it. A Roth IRA flips that: post-tax in, tax-free out. You got no deduction upfront, but everything that compounds belongs to you.
The HSA does something different. Contributions are tax-deductible, the same as a traditional 401(k). Earnings on amounts held in the HSA are not included in your income while they grow. And distributions used to pay qualified medical expenses are entirely tax-free.
Pre-tax in. Tax-free growth. Tax-free out — for the expense category that most reliably eats retirement savings.
For 2026, the contribution limits are $4,400 for self-only coverage and $8,750 for family coverage. If you are 55 or older, you can add a $1,000 catch-up contribution on top of either limit — a provision set in statute and not adjusted for inflation.
To qualify, you need to be enrolled in a high-deductible health plan. For 2026, that means a plan with an annual deductible of at least $1,700 for self-only or $3,400 for family coverage, with out-of-pocket maximums not exceeding $8,500 or $17,000 respectively. If your current plan clears those thresholds, you can contribute.
Starting in 2026, the eligibility rules expanded significantly. Under IRS Notice 2026-05, bronze and catastrophic plans available through the ACA marketplace are now treated as HDHPs, regardless of whether they technically satisfy the general HDHP definition. This means many previously ineligible individuals enrolled in ACA bronze plans can now open and fund an HSA.
The utilization gap
The EBRI data makes the missed opportunity concrete. Among the 14.5 million HSA accountholders tracked at the end of 2023, the average employee contribution was $2,075. The self-only contribution limit that year was $3,850. Half the money that could legally flow into this triple-tax-advantaged account simply did not.
Of all 14.5 million accountholders, only 15 percent had invested any portion of their balance in non-cash assets. The other 85 percent were either spending the money on current medical expenses or holding everything in cash — never letting the account compound.
The behavioral pattern makes sense. An HSA is framed by your employer as a healthcare benefit, not an investment vehicle. You use it to pay for doctor visits and prescriptions. The debit card arrives in the mail. The account name has "health" in it. Nothing about the enrollment process signals: "this is actually the most tax-efficient retirement account you will ever own."
But that is exactly what it is, for the people who use it that way.
The HSA is the only account where a dollar in is worth more than a dollar in a Roth IRA: you get the deduction now, the growth tax-free, and the withdrawal tax-free — if you invest it and pay medical expenses out of pocket.
The shoebox of receipts
Here is the part that changes how you think about medical expenses.
Most people assume that HSA funds must be used in the year the expense occurs. That is not what the IRS says. According to IRS guidance, there is no time limit on when an HSA distribution must occur to reimburse a qualified medical expense — as long as the expense was incurred after the HSA was established, was not previously reimbursed, and was not claimed as an itemized deduction.
The practical implication is significant. Pay a $400 specialist bill out of pocket today. Keep the receipt. Leave the $400 in your HSA invested in a low-cost index fund. In ten, fifteen, or twenty years, take a tax-free distribution for exactly $400 — while the original $400 has potentially grown to $800 or $1,500 inside the account.
The account's effective yield on that delayed reimbursement is your investment return over the holding period, entirely tax-free. No other account structure can produce this result.
The documentation requirements are strict: you must be able to prove the expense was qualified, was not reimbursed from another source, and was not deducted. A folder of EOBs (explanation of benefits) and receipts, organized by year, is enough. The IRS does not require you to submit anything proactively — only to produce records if audited.
A digital folder with scanned receipts and EOBs, organized by tax year, is sufficient documentation for the shoebox strategy. Keep records for as long as you hold the HSA — the IRS requires you to be able to substantiate any distribution taken for a prior-year medical expense if audited.
After 65: the second retirement account you already have
There is a moment, somewhere around age 65, when the HSA quietly transforms.
Before that age, a distribution used for anything other than qualified medical expenses is taxed as ordinary income and hit with an additional 20% penalty. The penalty is the guardrail that keeps the account earmarked for healthcare.
After you reach 65, the 20% penalty disappears. Non-medical distributions from that point forward are taxed as ordinary income — the exact same treatment as a traditional IRA or 401(k) withdrawal. Functionally, the account has become a bonus traditional IRA that you funded with pre-tax dollars.
But here is the asymmetry that makes the post-65 HSA superior to a traditional IRA in one dimension: if you do use the funds for qualified medical expenses — the category that represents one of retirement's largest predictable costs — those distributions remain completely tax-free.
A traditional IRA cannot offer that. Every dollar that comes out of a traditional IRA is ordinary income, regardless of what you spend it on. The HSA, after 65, gives you the choice: ordinary income for non-medical, or fully tax-free for medical. A traditional IRA gives you only the first option.
The implication is that the HSA is nearly always the last account you want to touch before 65 (preserve the triple-tax structure) and potentially the account you want to deplete first for medical expenses after 65, because those withdrawals cost you nothing.
The math of the gap
Return to the EBRI data. Average employee contribution in 2023: $2,075. The self-only maximum: $3,850. The gap was $1,775 per year not contributed — and of what was contributed, most of it stayed in cash.
Run that forward thirty years at 7%. A 35-year-old who contributes the 2026 maximum of $4,400 per year and invests the full balance accumulates roughly $415,000 by age 65. A 35-year-old who contributes the average $2,075 and leaves it in cash ends up with roughly $62,250 in nominal terms — and zero real compounding to show for thirty years of eligibility.
The difference is not investment genius. It is the decision to treat a healthcare account as an investment account, and to use the shoebox strategy to let the funds compound rather than spending them down each year.
For a family, the numbers scale further. The 2026 family contribution limit is $8,750. Maxed and invested for 30 years at 7%, a family HSA generates roughly $826,000 — a sum built entirely inside a tax-advantaged account most families treat as a spending vehicle.
None of this requires stock-picking skill or market-timing. It requires enrolling in an HDHP when it makes actuarial sense, contributing the maximum, investing in a low-cost index fund, paying medical expenses out of pocket when you can, and documenting the receipts.
The account that most people treat as a healthcare spending bucket is, for those who use it correctly, the most tax-efficient wealth-building vehicle available to anyone who qualifies. Five decisions — enroll, contribute, invest, pay out-of-pocket, save receipts — separate the $62,000 outcome from the $415,000 one.
What would it mean for your retirement picture if you had been treating this account differently for the last five years?
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