FSA vs HSA: which should you choose during open enrollment

Open enrollment (October 15 to December 7, 2026) often puts a health FSA and an HSA-eligible HDHP on the same set of screens, and it's easy to elect both the way you've always elected benefits. Usually you can't have both: IRS Publication 969 treats a general-purpose health FSA as "other health coverage" that disqualifies you from contributing to an HSA, even if you're otherwise on a qualifying HDHP. The fix, where your employer offers it, is a limited-purpose or post-deductible FSA instead of the general one. Every rule below is quoted directly from Publication 969; the dollar figures are the 2026 limits used throughout this site's open enrollment guides.

The rule: you usually can't have both

Publication 969 states it plainly, under "Other employee health plans": "An employee covered by an HDHP and a health FSA or an HRA that pays or reimburses qualified medical expenses can't generally make contributions to an HSA." The same publication's broader rule on coverage is just as direct: "If you (and your spouse, if you have family coverage) have HDHP coverage, you can't generally have any other health coverage." A general-purpose FSA -- the kind that reimburses any qualified medical expense, which is what most people have had at every past employer -- counts as that other coverage. "Can't generally" is as plain as the IRS gets here: this is the default outcome of electing both the ordinary way, not a rare edge case.

The workaround: limited-purpose or post-deductible

Publication 969 carves out specific kinds of FSA that don't cause this problem. An employee can contribute to an HSA while covered by an HDHP and one of these:

• Limited-purpose health FSA or HRA. Pub 969: these arrangements "can pay or reimburse the items listed earlier under Other health coverage except long-term care" -- that list is accidents, disability, dental care, vision care, and telehealth and other remote care -- "because they can be paid without having to satisfy the deductible." Most employers that offer one restrict it to dental and vision, the two biggest predictable costs on that list, but the IRS text itself is broader.
• Post-deductible health FSA or HRA. Pub 969: these arrangements "don't pay or reimburse any medical expenses incurred before the minimum annual deductible amount is met," though "the deductible for these arrangements doesn't have to be the same as the deductible for the HDHP."

Many employers that offer an HSA-eligible HDHP also offer a limited-purpose FSA specifically so employees don't have to give up one benefit for the other. Check your open enrollment materials for that exact phrase -- "limited-purpose" or "limited" -- not just "FSA" or "flexible spending."

The 2026 numbers

That's the tradeoff sitting underneath the eligibility rule, not just a technicality. A health FSA is use-it-or-lose-it by design -- see how much to put in your FSA for the carryover math in full. An HSA holds money indefinitely, is portable between jobs, and can be invested. Losing HSA eligibility by habit, because you re-elected the FSA you've always had, trades a permanent, investable account for a few hundred dollars of pretax dental and vision money you could usually have gotten anyway through a limited-purpose FSA.

Worked example: the habit that costs $900

Jordan is eligible for an HSA this year: a qualifying self-only HDHP, with the employer putting in $500. Jordan plans to add $3,000 of their own money, $3,500 total, comfortably under the $4,400 self-only limit, and expects to save 30% of it in tax: $900. During open enrollment Jordan also checks the box for the general-purpose health FSA -- the same $1,200 election made every year for dental and vision -- without connecting it to the HSA election a few screens earlier.

Because that FSA is general-purpose, Jordan isn't an HSA-eligible individual for any month it's in effect. Publication 969's definition of excess contributions covers what happens next: "You will have excess contributions if the contributions to your HSA for the year are greater than the limits discussed earlier," and the Form 8889 Limitation Chart only counts money for months you were an eligible individual -- so a month covered by the general FSA contributes zero to the limit, making anything put in during that time excess. Excess contributions "aren't deductible," so the $900 of expected tax savings doesn't happen, and Publication 969 adds: "Generally, you must pay a 6% excise tax on excess contributions... The excise tax applies to each tax year the excess contribution remains in the account" -- $210 a year here (6% of the $3,500 Jordan and the employer put in), for every year it isn't withdrawn. The one way out: "You withdraw the excess contributions by the due date, including extensions, of your tax return for the year the contributions were made."

The fix costs nothing. Jordan's employer also offers a limited-purpose FSA. Switching the election to that -- same $1,200, same dental and vision claims -- keeps Jordan HSA-eligible all year. The $3,500 goes in, the $900 of tax savings happens, and there's no excise tax.

Which one should you choose?

1. Not on an HSA-eligible HDHP this year? The HSA question doesn't apply to you -- elect the general-purpose FSA. Size it with how much to put in your FSA and the FSA calculator.
2. On an HSA-eligible HDHP and want the HSA? Skip the general-purpose FSA. If your employer offers a limited-purpose or post-deductible FSA, electing it alongside the HSA is usually free dental-and-vision money on top of the HSA -- check the exact plan name in your enrollment materials, since "FSA" alone isn't enough to tell which kind it is.
3. Not sure the HDHP is worth it, or your coverage changes mid-year? See HDHP vs PPO for whether the HDHP wins at all, and the partial-year HSA guide if your coverage starts, changes, or ends mid-year.

Run your own numbers in the FSA/HSA eligibility calculator: tell it whether you're on an HDHP, what kind of FSA (if any) covers you, and how much you planned to put in the HSA, and it shows whether you're eligible and, if not, the tax benefit you'd be giving up. If you're also near 65 and weighing Medicare, the HSA eligibility rules there are different again -- see Medicare Advantage vs. Original Medicare and the HSA/Medicare excess contribution calculator. For the rest of the HSA contribution math -- starting an HDHP mid-year, switching coverage, the last-month rule -- see the HSA contribution limit calculator.

The FSA/HSA incompatibility and the limited-purpose and post-deductible exceptions are quoted directly from IRS Publication 969; the 2026 FSA and HSA limits match the figures used throughout this site's open enrollment guides. This explains the mechanics, not personalized advice -- check your own plan documents, since not every employer offers a limited-purpose or post-deductible option, and some plans use a grace period instead of a carryover.