It’s easy to fall into the trap. You do a little web surfing, end up on, and start looking at the site’s valuation of your home. More often than not, it’s much higher than you’d expect and visions of dollar signs begin dancing inside your head.

We hear this often from clients who are ready to put their homes on the market. Online estimate can make clients overvalue their home’s worth. It’s a difficult conversation to have with clients. But one that’s important to have before listing time. An overpriced house will sit on the market. Eventually, the price will have to drop if it’s going to sell.

So how do these online estimates work and why are they often incorrect? For starters, Zillow and Redfin are both based across the country from Pittsburgh. They use information from property sales and municipal offices to assess a home’s worth—never visiting the actual house or neighborhood. According to the Zillow website, all information is gathered from public records.

But you only need to look at a competing sites to see that online sources give very different values on the same home. For example, Redfin values a 4-bedroom home in McCandless at a range of $324-$358K. Zillow lists the same home at $316K. That’s a $42K price swing. Valuations are closer for recently-sold homes. But many people who are looking to sell—and are being influenced by these online values—have had their home long enough to make it difficult for Zillow or Redfin to give a truly accurate assessment.

The takeaway is that online valuations can start the conversation about a home’s worth. But local sources will get you to the price that’s competitive for your market.

Forbes adds that hiring an appraiser or trusted real estate agent is the only way to know the true value of your home. “It could save you time, money and the heartbreak of slowly realizing that you trusted the wrong estimate and your home sale is going nowhere until you make a change.”