So, you’re moving. Maybe you took a job in another city? Maybe your spouse’s company is relocating? Maybe you just have the urge to be closer to the Mall of America? (Fun fact: Did you know that more than 5,500 couples have gotten married in the Mall of America’s Chapel of Love wedding chapel? Who knew?)

In any event, you’re taking the leap, and maybe you’re curious about how your salary in your new city is going to stack up? Or about how you should be negotiating your paycheck with your new boss? (Hint: “Just pay me whatever” is not the right approach.)

Enter the salary calculator, stage left. These are the handy online tools that let you input the city you’re moving from, the city you’re moving to, and your current salary, and they’ll tell you what you’d need to make in your new city to equal your current standard of living. “In today’s market, where a lot of people would consider relocating for a job, it’s important to use a cost of living calculator,” says Pamela Villarreal, a senior policy analyst with the National Center for Policy Analysis. “Many times, in large cities, the cost of living is going to be much higher than in, say, the suburbs. These online tools can help you determine if you should accept that job.”

And it’s kind of a big deal, folks. If you’re pulling up roots without considering the basic costs of where you’re putting them down, you could be in for a surprise. The median home price in San Jose, Ca., is still more than $600,000, versus Los Angeles, where it’s about $350,000. (Or Memphis, where it’s $120,000.)

Think about other major cost categories, as well. Phoenix residents spend nearly twice as much money per month on transportation costs as Chicagoans, according to Bundle.com data. And Boston residents spend about $180 more per month on food and drink than folks living in Orlando. In fact, you might be able to significantly improve your standard of living simply by picking a different city. If you still make roughly the same salary but your mortgage payment is half as much, guess what? You can save more for retirement or college—or take an annual family trip to Costa Rica. Your call.

Do a quick online search for “salary calculator” and you’ll find them everywhere—seems like this is a popular train, and every data site worth its salt has hopped on board. So how is it that you get different answers from every single one of them?

Example: According to CNNMoney.com’s calculator, a $75,000 salary in Manhattan would be equivalent to a salary of $31,238 in Dallas, Texas. Bankrate.com’s calculator tells me I’d need $32,862.58. (That seems really specific, no? I mean, 58 cents?) Salary.com’s calculator thinks you’d need to make $45,493. BestPlaces.net believes you could get by on $43,570. Not to nitpick, but that’s a range of more than $14,000 between the top and bottom answers. That’s an awful lot of wiggle room.

One shortcoming is that online calculators gather data from different places. CNNMoney.com’s calculator is based on numbers from the Council for Community and Economic Research (C2ER), for instance, and compares expenses in “dozens of major cities.” BestPlaces.net (the website for Sperling’s Best Places) lists so many data sources that the head spins. Bankrate.com pulls its numbers from the American Chamber of Commerce Research Association, or ACCRA. (Interestingly, ACCRA is produced by C2ER, so CNNMoney.com and Bankrate seem to be pulling data from the same place — which might explain why those numbers are similar.) And Salary.com? The site could not be more vague: “The cost-of-living data is aggregated by our team of market analysts based on publicly available sources.” Translation: “Our team got these numbers from someplace fabulous, but we’re not telling.”

So who do you trust? That depends on who you ask. “One thing you could do is go to more than one source,” says Ken Stewart, an economist with the Bureau of Labor Statistics. “You might tell yourself, ‘Maybe if I move from Point A to Point B, my living costs will go up or down within the range of these estimates.’” In other words, consider it a ball park estimate.

Villarreal (with the NCPA) likes data from the ACCRA, which takes a quarterly survey of more than 300 metropolitan and micropolitan areas. “That’s more areas than the Bureau of Labor Statistics follows,” she says. If you want to go directly to the source, you can hit coli.org and pay $8 for your first city comparison — and $5 for each additional city pair as long as you’re starting from the same city. But frankly, Bankrate’s numbers come from ACCRA as well, and Bankrate’s results display even more information than ACCRA does (including home price, apartment rent, and mortgage rate).

Just for fun, I ran the Manhattan-to-Dallas comparison from ACCRA’s website directly, and the site said I’d need $31,914 to make it in the Lone Star State. That’s only about $1,000 off of Bankrate’s answer. My take: Run the numbers using the tools at CNNMoney and Bankrate and average the results.

You can also take a peek at people’s spending habits on Bundle. For instance, surprisingly, Dallas residents spend more on just about all major categories than New York residents, including $300 more per month on shopping. But that could be because New York inhabitants are spending their hard-earned cash on housing costs, which aren’t represented in Bundle’s data. (According to Bankrate, Dallas rents average about $710 a month, compared to $2,500 in Manhattan. And a Dallas mortgage plus interest runs residents about $769 a month, compared to $4,660 in Manhattan.)

But you can’t stop there. Cost of living calculators include a lot of things, but they don’t reflect one crucial element: taxes. You may think you’re getting a good deal, but without factoring in state income tax, sales tax, and property taxes, you could be missing a big chunk of the picture. Visit the city’s Chamber of Commerce or Visitor’s Bureau website (or give them a call) to track that info down.

Also consider that most cost of living calculators allow you to compare major cities — but costs may differ significantly from that bedroom suburb you’re targeting on the outskirts of town. (Although, arguably, it’s unlikely that costs will be higher in the ‘burbs than within city limits.)

And finally, if you’re relying heavily on data you’re getting from a calculator, keep in mind that you’re looking at data pulled in the middle of a bumpy economy. “Right now the cost of living is down in almost every area,” Villarreal says. “If you go to a city where you might get a really good deal on a house, it’s important to consider the taxes once that housing market starts to appreciate again.” In other words, leave yourself some wiggle room.

One person who found those calculators useful: Elaine Pace, 47, who heads a medical nonprofit in Salt Lake City. “I knew I wanted to run a nonprofit and live near an international airport and mountains,” she says. “It was important for me to be able to live well on a nonprofit salary.” Calculators on salary and cost of living helped her focus on northern Utah, ruling out Denver and other areas, and she wound up in Salt Lake City. Says Pace: “Ten years later I’m still here and completely satisfied with my decision.”

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