The FSA-vs-HSA decision usually is not a choice at all — your health plan generally dictates which one you are eligible for, so the real question is how much to fund. FSA and HSA sound similar — both are pre-tax accounts for medical expenses — but they are structured for very different purposes: an HSA rolls over indefinitely, can be invested, and follows you between jobs, while an FSA is generally use-it-or-lose-it within the plan year. The specifics depend on your plan and situation, and these rules and figures can change.
What is the core difference between an FSA and an HSA?
The core difference is time. An HSA is built to persist — the money generally rolls over indefinitely, can be invested, and follows you between jobs — while an FSA is generally built to be spent inside a single plan year. The second difference is eligibility, and it is usually the one that decides the question for you: per IRS Publication 969, an HSA is generally available only alongside a high-deductible health plan, whereas an FSA is generally available with most non-HDHP plans. Both are pre-tax, so the tax saving in the first year can look similar. What differs is what happens afterward — an HSA balance keeps growing, while unspent FSA money is generally forfeited and does not travel with you when you leave the employer. Your plan's specific terms may differ, and these rules and figures can change. The two side by side:
HSA (Health Savings Account): Per IRS Publication 969, an HSA is generally available only alongside an HDHP. Funds generally roll over indefinitely, can be invested, and follow you between jobs. The IRS describes a triple tax advantage — generally deductible going in, tax-free growth, and tax-free for qualified medical expenses. After age 65, the IRS generally lets you withdraw for any reason (paying ordinary income tax on non-medical use, much like a traditional IRA).
FSA (Flexible Spending Account): Generally available with most non-HDHP plans. The IRS rules generally require funds to be spent within the plan year, though some plans allow a small carryover or grace period. An FSA generally doesn't follow you between jobs — many people who leave a company forfeit unused funds. Your plan's specific terms may differ.
What are the contribution limits and eligibility thresholds?
The IRS sets dollar limits on both accounts and re-indexes most of them year to year — so the useful move is to learn which limits apply to you, then confirm the current figure at the source rather than trusting a number you saw last year. This guide deliberately doesn't print the dollar amounts for that reason. There are generally four limits in play: an annual HSA maximum that differs for self-only and family HDHP coverage, a separate cap on health-FSA contributions, a different cap again for a Dependent Care FSA, and the deductible and out-of-pocket bounds that decide whether a plan counts as an HDHP at all. Two smaller rules sit alongside them — how much unused FSA money your plan lets you carry over, and how a few states treat HSA contributions for state tax. Your plan documents and HR confirm the plan-side pieces; the IRS sets the rest. Which limits apply, and where the current figures live:
- HSA contribution limit — the IRS sets separate annual maximums for self-only vs family HDHP coverage, plus an additional catch-up amount once you reach the qualifying age (commonly cited as 55+). Employer contributions count toward the same limit. Current-year figures are in IRS Publication 969.
- HDHP eligibility thresholds — an HSA is only available alongside a plan the IRS classifies as a High-Deductible Health Plan, meaning a minimum deductible and a capped maximum out-of-pocket within IRS-set bounds. Your plan documents (or HR) confirm whether the plan qualifies; the threshold numbers themselves come from the IRS each year.
- Health FSA contribution limit — the IRS caps annual health-FSA salary-reduction contributions, separate from the HSA. A Dependent Care FSA has its own, different cap. Both are set by the IRS / your plan.
- Limited-Purpose FSA cap — generally follows the health-FSA limit (see the section below); confirm the current amount with your plan.
- Carryover vs grace period — for health FSAs the IRS permits either a limited carryover of unused funds into the next year or a short grace period to spend them, not both, and only if your plan opts in. The carryover dollar cap is re-indexed periodically — confirm the current amount and which option your plan offers.
- State tax treatment — a few states tax HSA contributions at the state level even though they're federally pre-tax (California and New Jersey are the commonly-cited examples). If you're in one, the HSA's tax advantage is federal-only for you — worth checking your state's current treatment.
When should you fund the HSA aggressively?
People generally fund the HSA aggressively when they can afford to leave the money alone. That usually means having an HDHP, since the IRS generally allows HSA contributions only alongside one, and having enough cash flow to pay current medical bills out of pocket rather than drawing the balance back down. From there the reasons are mostly about time: most administrators offer brokerage-style investing once the balance clears a minimum, so someone young and healthy may be buying decades of tax-free growth rather than covering this year's copays. An employer contribution tips it further, since that money arrives on top of anything you put in yourself. The trade-off is liquidity — money in the HSA is money you are choosing not to spend now. Whether that fits depends on your situation, and these rules and figures can change. Some people lean into the HSA when:
- They have an HDHP and can pay current medical bills out of pocket.
- They want to invest the HSA balance for retirement (most administrators offer brokerage-style investing once you hit a minimum balance).
- Their employer contributes — that money is essentially a bonus.
- They're young and healthy and want decades of tax-free growth.
A strategy many people use, when cash flow allows: max the HSA and pay current medical bills with after-tax money, keeping receipts. Because the IRS generally places no deadline on HSA reimbursements, decades later you can reimburse yourself tax-free from the HSA — turning it into a stealth retirement account. Whether this fits depends on your situation.
When should you fund the FSA conservatively?
People generally keep the FSA modest because the money is use-it-or-lose-it. The common approach is to fund only the spending you are confident will happen — prescription refills, planned dental work, glasses, copays — rather than guessing high and hunting for expenses in December. Only a small amount can generally carry over, and anything past that is typically forfeited, which is why estimating closely matters more here than it does with an HSA. The one account people often do fund close to the cap is the Dependent Care FSA, when their employer offers it and there are kids in daycare, because that spend tends to repeat every month and is often treated as a clean win by the people who use it. Your plan's terms may differ, and these rules and figures can change. Other people keep the FSA modest when:
- They have predictable medical spend: prescription refills, planned dental work, glasses, copays.
- They have a Dependent Care FSA option and kids in daycare — many people fund this close to the cap, since it's often a clean win.
- They can estimate their spend within ~$200. The IRS generally lets you carry over a small amount (currently around $640, but it often helps to check the current year), and amounts beyond that are typically forfeited.
A common rule of thumb: many people estimate their floor — the spend they're confident will happen — and fund that, rather than guessing high.
What is a Limited-Purpose FSA — and can you have both?
A Limited-Purpose FSA is an FSA restricted to dental and vision expenses, and it is generally the version people pair with an HSA. If you have an HSA but also expect predictable dental and vision spend, your employer may offer one. Per IRS rules, this generally lets you pay dental and vision costs with FSA dollars while keeping the HSA growing for medical expenses — two pre-tax buckets, no conflict. So the answer to whether you can have both is often yes, in that specific pairing. The practical questions are whether your employer offers the limited-purpose version at all, since plan availability varies, and how much you can put in: the cap generally follows the health-FSA limit, which the IRS re-indexes, so it often helps to confirm the current amount with HR. Your plan's terms may differ, and these rules and figures can change.
What should you ask HR?
The questions worth asking are the ones that decide how much to put in and whether any of it is at risk of being forfeited. The first is whether the FSA has a grace period or a carryover provision, because the IRS generally permits one or the other but not both, and only if your plan opts in — that single answer changes how tightly you need to estimate. The second is whether you can pair a Limited-Purpose FSA with an HSA, which is where predictable dental and vision spend can go without touching the HSA balance. The third is whether the employer contributes to the HSA and when the money lands, since a contribution that arrives in January is not the same as one that arrives in December. The last is the deadline to submit reimbursement claims, which is worth knowing before the plan year ends rather than after. The questions people commonly bring:
- Does the FSA have a grace period or carryover provision?
- If I have an HSA, can I also enroll in a Limited-Purpose FSA?
- Does the employer contribute to the HSA, and when does the contribution land?
- What's the deadline to submit FSA reimbursement claims for the plan year?
The IRS publishes detailed rules on both account types — IRS Publication 969 (HSA, HRA, FSA, and MSA rules) (affiliate link — OffbookHR may earn a commission if you buy through this link. It does not affect ranking.). For which costs the IRS treats as qualified medical expenses, many people also check IRS Publication 502 (Medical and Dental Expenses) (affiliate link — OffbookHR may earn a commission if you buy through this link. It does not affect ranking.). Because these thresholds and figures can change, it often helps to confirm the current rules at the source for your situation.