Job 1 for Working Moms: Running the Numbers

That second income sounds great, until you start to add up what it will cost you.

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Can you afford to go back to work? The extra family income sounds great, but many women returning to the workforce forget to account for a lot of expenses. The truth is that working costs money, from child-care expenses to dry-cleaning bills to maybe even hiring a dog walker.

For many women, it's worth it: Bringing home a paycheck is empowering and probably even good for your marriage, since money is the top thing most couples fight about. It can be intellectually stimulating to reconnect with the outside world, and benefits like healthcare coverage and retirement contributions can make a prolonged difference in your quality of life.

But plan carefully, and don't overlook the small stuff. Start by calculating your immediate cash-flow needs and creating a budget, and find inventive ways to cut spending wherever possible. Determine how much bottom-line cash your second income will add to the family coffers once all the hidden costs are tallied up. Here are some of the budget items you'll need to consider, with monthly minimums that full-time workers are likely to spend:

Child-care costs. For moms with young kids, this will most likely be your biggest expense. Annual preschool care for a 4-year-old can range from roughly $3,000 in Alabama to more than $8,000 in New York, according to the National Association of Child Care Resource & Referral Agencies. If you opt for a one-on-one approach, a babysitter or nanny could cost from $400 to $1,000 per week. For older kids in school full time, it will be less, but part-time sitters or nannies still charge $10 to $15 an hour, and they often bolt for a higher-paying or full-time job. If possible, share a nanny with a friend who's in the market for child care as well.

Tip: Take advantage of tax breaks. The child-care credit is based on what you pay for the care of dependent children under the age of 13. If you're paying for the care of one child, the credit is up to $3,000, double that for two, though it depends on your adjusted gross income. If your household income is more than $43,000, for example, the credit could save you up to $600 a year for one kid, $1,200 for two or more.

Your employer might offer a child-care flexible spending account that allows you to set aside pretax money for child-care expenses, up to $5,000 a year per household. For a middle-income family, the bottom-line tax savings could amount to about $2,000. Your company might also offer discounts for certain local child-care providers. Finally, check to see if your employer is open to flex hours, job sharing, or telecommuting. If so, you may be able to use these to juggle your schedule and cut your child-care costs.

Minimum monthly expense, altogether: $600

Commuting costs. Unless you're sharing a ride with your husband, you'll have to pay for transportation. These out-of-pocket expenses are all over the map, of course, depending on where you live. A ballpark figure, if you take public transportation in a big city, comes to anywhere from $100 to $250 a month, or up to $3,000 a year. If you're one of the vast majority who drive to work, your travel expenses will easily exceed that conservative estimate, once you calculate the cost of gasoline, additional car maintenance, parking, and tolls. Some employers do offer subsidized perks of, say, $35 a month for reduced parking or bus and subway fare.

Minimum monthly expense: $200

Eating out. You might start out by vowing to pack a lunch—but with a busier schedule than you've been used to, that will get old fast. Plus, there are incidentals. Start with that cup of coffee on the way to the office, say, $4. Then add in maybe $5 to $10 a day for lunch. How about sneaking late-afternoon treats or sodas out of the vending machine? Guesstimate, all told: $200 a month. Eating costs could go up at home, too. With both parents working, you'll probably eat out more, or order take-out, because you're too whipped to fix home-cooked meals or don't have time. Minimum monthly expense: $400

Work clothes. No more jeans and cargo pants. You'll need to spring for a fresh wardrobe, hair care, and accessories you might not otherwise buy. At the low end, you'll probably spend $500 for a starter wardrobe, plus $125 per month on various clothing expenses. Remember, if you fall prey to buying work clothes that require dry cleaning, you readily spend upwards of $5 per shirt or $10 to have a pleated skirt cleaned and pressed.