A Flexible Spending Account lets you set aside pre-tax dollars from your paycheck to cover qualified medical expenses, but the rules around it trip up millions of workers every year. Understanding how does an FSA work can save you hundreds or even thousands of dollars annually in taxes alone. According to the Bureau of Labor Statistics, roughly 43% of private-sector workers have access to an FSA through their employer, yet many either skip enrollment entirely or fail to use their full balance before the deadline.
The concept is straightforward: money goes in before federal income tax, Social Security tax, and Medicare tax are calculated, which lowers your taxable income. You then use those funds throughout the year to pay for eligible healthcare costs. But the details around contribution limits, the use-it-or-lose-it rule, and what actually qualifies as an eligible expense are where things get complicated.
What Is a Flexible Spending Account?
A Flexible Spending Account is an employer-sponsored benefit that allows you to contribute a portion of your salary on a pre-tax basis to pay for out-of-pocket healthcare expenses. The IRS governs FSA rules under Section 125 of the Internal Revenue Code, which covers cafeteria plans. Your employer sets up the account, and you elect a contribution amount during your annual open enrollment period.
Unlike a health savings account (HSA), an FSA does not require you to be enrolled in a high-deductible health plan. Most traditional PPO and HMO plans are compatible with FSAs. This makes them accessible to a broader group of employees. However, FSAs are tied to your employer. If you leave your job, you generally lose access to remaining funds unless you elect COBRA continuation coverage.
How FSA Contributions Work
When you enroll in an FSA, you choose how much to contribute for the plan year. For 2025, the IRS cap on healthcare FSA contributions is $3,300 per employee. Your employer divides your annual election evenly across your paychecks throughout the year. If you elect $2,400, for example, $200 comes out of each monthly paycheck before taxes.
Here is where how does an FSA work gets interesting from a cash flow perspective. Your full annual election amount is available on the first day of the plan year, even though you have not yet contributed the full amount. If you elect $3,300 and have a $3,000 medical bill in January, you can use your FSA to pay for it immediately, even though only a fraction has been deducted from your pay so far. If you leave your employer mid-year, you are generally not required to pay back the difference between what you used and what you contributed.
The Tax Advantage Explained
FSA contributions reduce your taxable income, which means you save money on federal income tax, state income tax (in most states), Social Security tax (6.2%), and Medicare tax (1.45%). The combined savings typically range from 25% to 40% of your contribution amount, depending on your tax bracket.
Consider a worker in the 22% federal tax bracket who contributes $3,000 to an FSA. The federal tax savings alone total $660. Add Social Security and Medicare savings of $229.50, and the total comes to nearly $890 in tax savings for the year. That is real money back in your pocket for expenses you would have paid anyway. For a deeper look at the tax implications, see our healthcare costs guide.
What Can You Spend FSA Money On?
The IRS defines eligible expenses in Publication 502. Generally, any medical expense that qualifies as a tax deduction under Section 213(d) of the tax code is also FSA-eligible. Common eligible expenses include:
- Doctor visit copays and coinsurance
- Prescription medications
- Over-the-counter medicines (since the CARES Act of 2020)
- Dental treatments including cleanings, fillings, and orthodontics
- Vision care including eye exams, glasses, and contact lenses
- Mental health services and therapy
- Medical equipment like crutches, blood pressure monitors, and first aid supplies
Since the CARES Act, menstrual care products including tampons and pads are also eligible. Over-the-counter medications like pain relievers, allergy medicine, and melatonin no longer require a prescription for FSA reimbursement.
The Use-It-or-Lose-It Rule
This is the biggest pitfall of FSAs. Unlike HSAs, FSA funds generally do not roll over from year to year. If you do not spend your balance by the end of the plan year, you forfeit the remaining money. The IRS does allow employers to offer one of two relief options, but not both.
The first option is a grace period of up to 2.5 months after the plan year ends. If your plan year follows the calendar year, this gives you until March 15 of the following year to incur expenses against the previous year’s balance. The second option is a carryover provision that lets you roll over up to $660 (for 2025) into the next plan year. Check with your HR department to find out which option, if any, your employer offers.
Smart planning is essential. Review your previous year’s medical spending before choosing your FSA election amount. Factor in predictable expenses like annual checkups, prescription refills, glasses or contacts, and any planned procedures.
FSA vs HSA: Key Differences
People often confuse FSAs and HSAs because both offer tax advantages for medical spending. The differences matter significantly. HSAs require enrollment in a high-deductible health plan, while FSAs work with any employer-sponsored health plan. HSA funds roll over indefinitely, while FSA funds are subject to the use-it-or-lose-it rule. HSAs are individually owned and portable between jobs, while FSAs are employer-owned and generally end when employment ends.
HSAs also offer a triple tax advantage: contributions are pre-tax, growth is tax-free, and withdrawals for qualified expenses are tax-free. FSAs provide only the upfront tax deduction. However, the FSA front-loading benefit, where your full annual election is available on day one, is something HSAs do not offer. You can only spend HSA money you have actually deposited. For more details, check our breakdown of FSA vs HSA differences.
Frequently Asked Questions
Can I change my FSA contribution mid-year?
Generally, no. FSA elections are locked in for the plan year once open enrollment closes. However, certain qualifying life events such as marriage, divorce, birth of a child, or loss of other coverage may allow you to adjust your election mid-year. Check with your employer’s benefits department for specific rules.
Can I use my FSA for my spouse or dependents?
Yes. FSA funds can be used to pay for qualified medical expenses incurred by you, your spouse, and your tax dependents, even if they are not covered under your employer’s health plan. This includes children up to age 26 for health coverage purposes, though FSA dependent rules follow tax dependent definitions.
What happens to my FSA if I leave my job?
In most cases, you lose access to your FSA when your employment ends. Any remaining balance is forfeited. However, you can use your FSA to reimburse eligible expenses that were incurred before your termination date. Some employers offer COBRA continuation for FSAs, but it is rarely cost-effective because you lose the tax benefit and must pay the full contribution amount plus an administrative fee.
Can I have both an FSA and an HSA?
You cannot have a general-purpose healthcare FSA and an HSA at the same time. However, you can pair an HSA with a limited-purpose FSA (LPFSA), which restricts eligible expenses to dental and vision costs only. This combination lets you maximize tax savings while keeping your HSA funds invested for the long term.
Making the Most of Your FSA
The key to maximizing your FSA is accurate forecasting. Tally up your expected out-of-pocket costs for the coming year including copays, prescriptions, dental work, and vision expenses. If you tend to underestimate, bump your number up slightly rather than risk losing money at year-end. Set calendar reminders for September or October to check your balance and schedule any remaining appointments or purchases before the deadline.
Keep every receipt. Most FSA administrators require documentation for reimbursement, and the IRS can request proof that expenses were qualified. Many administrators now offer mobile apps that let you snap photos of receipts, submit claims instantly, and track your balance in real time. Taking five minutes to understand how does an FSA work before open enrollment can easily translate into hundreds of dollars saved over the course of the year.