Zillow claims their "Zestimates" are accurate within 5% on the 110 Million+ homes their system evaluates nationwide. I believe this means their Zestimate on a US home with a median price of about $229,000 could fall in the the range of 5% low at $217,550 to or 5% high at $240,450 - which do you think is right or will you just access the average?
With an average price of new home in the United States at about $368,000 as of April 2017, the range could be $349,600 to $386,400. That is quite a range, and a group of suburban Chicago home builders have filed a class-action law suit against Zillow arguing home buyers were confusing a "Zestimate" as a formal appraisal.
The problem with complex mathematical calculations based on public records or algorithms used by companies like Zillow.com and HouseValues.com is they don't calculate "Real Home Value Differences"> For example in the same neighborhood one home has an old kitchen with Formica counter tops and another has been updated with a Gourmet Kitchen with Silestone counters. Same is true with new homes even on the same street: one has a boat dock and a lake view and the other in on a corner lot with the best view of neighborhood traffic.
There is no substitute for walking through a few homes yourself, and getting a good Realtors opinion and a Comparative Market Analysis and/or a Professional Appraisal.