You can use Zillow to estimate the value of real estate by using Zillow's home value tool, called Zestimate. The Zestimate home value tool only provides an estimated market value, computed using a proprietary formula. A Zestiate is a starting point in determining value, but it is not an appraisal.
The Zestimate tool calculates the value from public and user-submitted data. Your real estate agent or appraiser physically inspects the property, taking into account the property's special features, market conditions and location to determine a more accurate value. Zillow encourages its users to obtain more information when determining the value of property, such as getting a comparative market analysis from a real estate agent, getting an appraisal from an appraiser and visiting the property.
Zillow's estimate tool also produces a forecast as to how much the property may be worth in a year's time, based on the current home and market information. You cannot use Zillow's estimator tool to obtain a federally guaranteed loan, because by law an appraisal is required from a professional appraiser. The amount estimated by Zillow's tool is an estimate of fair market value, and it's helpful if you want to judge market trends or make calculations for your own personal purpose.