How does an FSA work?
Last updated August 2026
Short answer
An FSA is a spending account rather than a savings account, and almost every mistake with one comes from treating it as the second thing.
The mechanics
You elect an annual amount before the plan year begins. It is deducted from pay in equal instalments, before income and payroll taxes.
The tax saving is immediate and proportional to your bracket. Someone at 24% federal plus payroll tax keeps roughly a third more of that money than if it were spent after tax.
Reimbursement happens through a plan debit card or by submitting receipts, depending on the administrator.
Full availability from day one
The whole election is accessible on the first day of the plan year, before you have contributed most of it.
Under the uniform coverage rule, an employee who spends the full amount early and then leaves generally keeps the benefit, with the employer absorbing the difference.
That asymmetry is real and rarely mentioned, and it makes an FSA useful for a known expense scheduled early in the year.
Use it or lose it
Unspent money is forfeited at the end of the plan year unless the plan offers relief.
Two forms of relief exist: a carryover of up to $680 for 2026, or a grace period of up to two and a half months. A plan may offer one, not both.
Underestimating is therefore the safer error. The cost of electing too little is paying for some expenses with after-tax money; the cost of electing too much is losing it outright.
Try it in Walnut
Walnut connects to brokerage accounts and analyses what you hold. An FSA is not an investment account, which is one of the clearer differences from an HSA.
What it covers
Qualified medical expenses under the IRS definition: deductibles, copays, prescriptions, dental and vision costs, and a wide list of over-the-counter items.
Premiums for your health insurance are generally not eligible, which surprises people every year.
Publication 502 is the reference list, and the administrator applies it, so a rejected claim is usually a definition question rather than an error.
How it differs from an HSA
An FSA has no high-deductible health plan requirement, so more employees can use one.
It does not invest, does not compound, and does not follow you to a new employer.
An HSA does all three, which is why an HSA is a retirement account wearing a medical label and an FSA is a way to buy this year's healthcare more cheaply.
Who it suits
Employees with predictable medical costs and no access to a high-deductible plan, where the FSA is the only pre-tax option available.
Anyone facing a known expense early in the plan year, since the full election is available before it has been funded.
It suits long-term savers less well, because nothing compounds and nothing follows you to the next employer.
Sources
The 2026 salary reduction limit of $3,400 and the $680 carryover maximum are from Rev. Proc. 2025-32. Eligible expenses are listed in IRS Publication 502. Walnut is informational and is not an investment adviser. This guide is educational and not personalized tax advice.
FAQ
How does a health FSA work?
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You elect an amount before the plan year, it comes out of your pay in equal instalments before tax, and you spend it on qualified medical expenses. The full elected amount is available from the first day, even though you have not yet contributed it.
How much can I put in for 2026?
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$3,400 in salary reduction contributions. If the plan permits carryover, up to $680 of unused funds may roll into the following year.
What happens to money I do not spend?
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It is forfeited, unless your plan offers either the carryover of up to $680 or a grace period of up to two and a half months. Plans can offer one of those, not both.
Is an FSA the same as an HSA?
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No. An FSA belongs to the employer's plan, does not require a high-deductible health plan, does not invest, and does not travel with you when you leave. An HSA is yours, can be invested, and rolls over indefinitely.
Why is the whole amount available on day one?
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Because the employer bears the risk. If you elect $3,000, spend it in January and leave in February, you generally keep the benefit and the employer absorbs the shortfall. That uniform-coverage rule is a genuine feature.
What is a dependent care FSA?
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A separate account for childcare and dependent care costs, with its own limit set by statute rather than by the health FSA figure. It follows different rules and the money is not interchangeable with the health FSA.
Can I change my election mid-year?
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Only after a qualifying life event such as marriage, divorce, a birth, or a change in employment status. Otherwise the election is locked for the plan year, which is why the estimate at open enrolment matters.
Does an FSA reduce my payroll taxes too?
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Yes. Salary reduction contributions avoid federal income tax and generally Social Security and Medicare tax as well, which is why the effective saving is larger than your income tax bracket alone suggests. It also slightly reduces the wages your Social Security benefit is later calculated on.