How much is your house really worth?

There's always a good reason to know the current value of your home. If your property taxes go up, it may be the ammunition you need to appeal the increase. If you need a loan, you'll want to know how much home equity is available to you. If you're thinking of selling your house, knowing its current value can help you decide whether it's time to cash out.

Getting an appraisal will be the most thorough and accurate way to learn the valuation of your home, but it'll cost you several hundred dollars. You can get a ballpark idea without spending a lot of money by going online.

One of the largest sites is run by the National Association of Realtors. It's http://www.realtor.com and it has more than 2 million properties in its database. You can search for free for properties similar to yours using criteria such as price range, location, number of bedrooms, and amenities.

Can you trust home data on the Web?

Regardless of their cost to you, all of these Web sites have their limitations. As Walter Molony, of the National Association of Realtors in Washington, DC, says, "They can give you a rough idea, but they're not taking into account all the specifics of your house. It's not a true market analysis."

So how can you measure the results that you get from one of these sites? Appraiser George Dell of San Diego points out that reliability is based on two things: using good data and a good mathematical model.

"Web valuation sites are dependent on electronic databases," he says. "So the source of the data is very important. If it's coming from a Multiple Listing Service, the data should be good: accurate and timely. If the data are coming from public records, there may be a problem. Some sites use assessed values of homes from public records, which are updated only once a year in some localities, and in a worst case scenario, may take three or four months to get into the database. So the numbers could be 15 or 16 months old by the time you see them. That makes them unreliable if the market's moving up or down quickly."

Mark Lesswing of the Center for Realtor Technology in Chicago, points out: "A site uses a formula to weigh the different criteria, and the formula is custom to each site. Each site is going to give you a different answer. It's an opinion. What used to be an art form has been turned into a computer program. It's just a way to get the information up on the Web."