Imagine for a moment that you had a magical account that allowed you to bypass paying taxes on money you knew you had to spend on some of life's least fulfilling expenses, like an emergency root canal or that expensive but essential blood pressure medication.

It's not fantasy for some workers whose companies offer a benefit called a flexible savings account. And if your company offers the benefit, this may be your chance to sign up. Fall marks the open enrollment season for most employers, the time when they let employees add, revise, or drop their benefits for the coming year.

Flexible spending accounts work like this: You, as an employee, ask that a portion of your salary be set aside from each paycheck to pay for a certain amount of health and child care expenses. The money gets put aside in an account before taxes. When you spend money on qualified health or child care expenses, you're reimbursed from the account. Essentially, you're paying those costs with untaxed dollars and increasing the purchasing power of your paycheck.

Here's the rub: You have to estimate your annual costs for health and child care expenses at the beginning of the year. If you don't use all the money before the end of the year, it becomes the property of the employer, commonly called the "use it or lose it" rule. (You knew this sounded a little too good to be true, right?)

That quirk makes the benefit kind of tricky, but still worth looking at.

It's a little easier for people who want to use a flexible savings account for child care expenses. Anyone who has children in day care or a regularly babysitter probably knows their annual expenses and can pretty easily estimate them for the coming year.

It gets more complicated with health care expenses, because no one without a working crystal ball can anticipate the broken leg suffered while trying to clean the leaves out of the gutter. It can be a little difficult to guess how much you'll spend in the coming year on medical, dental, and vision costs.

One way is to tally up your health care spending for the current year. That can give you a good place to start, and maybe even end. It may pay to be a little conservative with this estimate. You don't want to leave a lot of unused money in the account.

Another way to begin thinking about health care costs is to look at the list of allowed expenses and start estimating your annual cost for each. The list can be pretty broad, and you probably have one in that pile of information that Human Resources handed out. If not, many companies rely on a list produced by the IRS.

Before you declare yourself young and invincible and ignore this whole idea, consider the range of things that count as qualified medical expenses. The IRS allows reimbursement for everyday expenses like contact lenses and cleaning solution, aspirin, and bandages. If you're planning laser eye surgery, that may be covered.

Reimbursement may also be allowed for everything from insurance co-payments and prescription fees to acupuncture and fertility treatments.

Take a couple of minutes to add your potential medical costs, and now imagine saving taxes on all that money. It can make slogging through all that math worthwhile.

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