A health savings account is marketed as a way to pay for medical bills. Used that way it is fine and mildly useful.

Used the other way, it is the most tax-advantaged account in the American system, and it is the only one that gets a tax break on the way in, on the growth, and on the way out. A 401(k) gets two of those. A Roth IRA gets two. The HSA gets all three, and almost nobody treats it accordingly.

The three advantages #

Contributions are pre-tax. They reduce your taxable income the same way a traditional 401(k) contribution does. If you contribute through payroll deduction, they also escape Social Security and Medicare payroll taxes, which no other retirement account does. That is an extra 7.65% that a 401(k) contribution does not save you.

Growth is untaxed. Invested inside the account, it compounds with no tax drag at all. No annual tax on dividends, no capital gains when you rebalance.

Qualified withdrawals are untaxed. Money spent on qualified medical expenses comes out completely free of federal tax, at any age.

Nothing else does all three. That combination is the whole argument.

The part that turns it into a retirement account #

Most people use an HSA as a pass-through. Money goes in from the paycheck, and it comes out a few weeks later to pay a copay. That captures the tax deduction and none of the compounding, which is where the actual value is.

The alternative is the one worth knowing.

There is no deadline on reimbursing yourself. If you incur a qualified medical expense today, pay for it out of pocket, and keep the receipt, you can reimburse yourself from the HSA at any point in the future. Not this year. Any year. Twenty five years from now, if you like.

So the strategy is: contribute the maximum, invest the balance rather than leaving it in cash, pay current medical expenses out of pocket, and keep every receipt. The account compounds untouched for decades. Later, you have a pile of documented qualified expenses that lets you withdraw a corresponding amount completely tax free, whenever you want it, for any purpose.

You have effectively built a Roth-like account funded with pre-tax dollars, which is a combination that does not otherwise exist.

The receipts are the whole mechanism, so treat them seriously. Photograph every explanation of benefits, every pharmacy receipt, every copay slip, and keep them in one folder in cloud storage with a running spreadsheet of dates and amounts. It is unglamorous and it is the entire strategy.

After 65 it gets easier #

At 65, the rules loosen. You can withdraw for any purpose, not just medical, and pay ordinary income tax on it with no penalty. At that point it behaves like a traditional IRA, with the option of completely tax-free withdrawals for anything medical.

Which matters, because medical expenses in retirement are large. Estimates for what a retired couple spends on healthcare over the rest of their lives run into the hundreds of thousands of dollars, and Medicare premiums, dental, vision, hearing, and long-term care insurance premiums are all qualified expenses. Medicare Part B and Part D premiums in particular can be paid from an HSA, which is a large recurring expense covered tax free.

There is essentially no scenario where a healthy HSA balance goes unused.

Who can actually contribute #

This is where most people fall out, and the requirement is narrow.

You must be enrolled in a high-deductible health plan that meets the IRS definition, and you must not have other disqualifying coverage. That includes a spouse’s non-high-deductible family plan covering you, and it includes a general-purpose health flexible spending account.

You also cannot contribute once you are enrolled in Medicare, and there is a six-month lookback on Medicare Part A enrollment that catches people who work past 65. If that applies to you, look it up properly before contributing.

The minimum deductible and out-of-pocket maximum that define a qualifying plan are set by the IRS and indexed each year.

Contribution limits are also indexed annually, and for 2026 they are in the neighborhood of $4,400 for individual coverage and $8,750 for family coverage, with an additional catch-up amount available from age 55. Check IRS.gov for the current figure rather than relying on those numbers.

The mistake that wastes most of it #

A very large share of HSA money sits in cash earning a nominal interest rate.

The account is a container, not an investment. Most HSA providers let you invest the balance in mutual funds or ETFs once it exceeds a threshold, often somewhere around $1,000 to $2,000, and a lot of people never turn that on because it is buried in the provider’s interface and nobody told them.

Money that is going to sit for thirty years should be invested like any other thirty-year money, which means a low-cost broad index fund. Leaving it in cash gives up the entire middle advantage of the three, and the middle one is where compounding happens.

Keep enough in cash to cover your plan’s deductible, so a bad year does not force you to sell investments at a bad time. Invest the rest.

Where the HSA sits in the order of operations #

It slots into the standard sequence like this:

1. A small emergency buffer.
2. The full employer 401(k) match, which is an immediate 50% or 100% return and beats everything.
3. High-interest debt, because 24% guaranteed is better than any market return.
4. Max the HSA, if you are eligible. This is where it goes, ahead of additional 401(k) contributions and ahead of an IRA, because of the triple advantage and the payroll tax saving.
5. The rest of the tax-advantaged space: IRA, more 401(k).
6. Taxable brokerage.

Some people put the HSA above the match, which I think is wrong, because nothing beats a 100% instant return. Nearly everyone agrees it goes above step five.

The California problem #

If you live in California, one of the three legs is missing, and it is worth being blunt about it.

California does not conform to the federal HSA rules. Contributions are not deductible for California state income tax, and the earnings inside the account are taxable at the state level as they accrue. Withdrawals for qualified medical expenses are not taxed by the state, but the tax-free-growth advantage is gone.

Practically, this means a California resident with an HSA has state tax reporting to do on the interest, dividends, and capital gains inside the account each year, which most brokerages do not make easy because they are not required to produce a state-level statement for it. New Jersey has a similar non-conformity.

Does that kill the strategy? No. The federal advantages are the large ones, and the payroll tax saving is unaffected. But it does two things. It reduces the margin by which an HSA beats a plain 401(k) for a Californian, and it makes very high turnover inside the account expensive, which argues for a simple buy-and-hold index fund rather than active trading.

It also creates a genuinely interesting situation if you contribute while living in California and later retire somewhere without a state income tax, since the state drag stops and the federal advantages carry on. That is the same arbitrage that makes traditional 401(k) contributions attractive in California.

When it is the wrong answer #

Being useful means saying where this does not work.

When the high-deductible plan is a bad fit for your health. This is the big one. An HSA requires an HDHP, and an HDHP means you carry a large deductible before insurance meaningfully kicks in. If you have a chronic condition, take expensive medication, expect a surgery, are pregnant or planning to be, or simply use a lot of care, a lower-deductible plan can be cheaper overall even after the tax benefit. Run the actual numbers on your expected usage against both plans, including premiums, before choosing the HDHP for the HSA. The tax tail should not wag the healthcare dog.

When you cannot afford to pay medical bills out of pocket. The receipt strategy requires having cash to cover current expenses while the HSA compounds. If you would have to raid the account anyway, you get the deduction and not the compounding, which is fine and is not the strategy described here.

When you are not maxing the employer match. Fix that first.

When your HSA provider is bad. Some employer-designated providers charge monthly fees, offer poor fund selections, or require a high cash balance before investing. You can generally transfer HSA money to a different custodian, and it is worth doing if yours is expensive. Contribute through payroll to capture the payroll tax saving, then periodically transfer the balance out to a better provider.

The short version #

Contribute the maximum if you are eligible and an HDHP suits your health. Invest it rather than leaving it in cash. Pay current medical costs out of pocket and keep every receipt. Do not touch the balance for decades.

Done that way it is the best account in the system, and it is the one people most consistently underuse, mostly because it is presented as a spending account rather than an investment account.

Tracking a long-horizon balance you are deliberately not spending is its own problem, since nothing visible happens for years. That is the same reason I built Retire Goals, which holds several goals at once rather than one distant number and shows a projection you can actually watch move. Free, no account, and the data stays on your device, which for something holding your financial details seemed like the only reasonable design.

The healthcare side of this, including what to do if you are uninsured or on a marketplace plan, is in cheap dental, vision, and healthcare in San Francisco. The rest of the arithmetic is in Money, and none of it is financial or tax advice.