How Accurate is a Zillow "zestimate"?

Without actually going inside your home and seeing all the upgrades and improvements you have made over the years, all Zillow can do to arrive at its "zestimate" is compare your home to others that have sold in your area. 

A REALTOR® on the other hand, can compare 'apples to apples' by looking at similar homes, with similar improvements, square footage and age, that have sold in the last 6 months and hone in on your home's value in the current market far more accurately. 

Zillow's website has some startling statistics and by their own admission, fall short more often than not. 

By Zillow’s Own Admission:

_____________________________________________________________

Broome County, NY
Homes on Zillow:                                                77,419
Homes with Zestimates:                                      64,492
Zestimates within 5% of Sales Price:                    9.6%
Zestimates within 10% of Sales Price:                  24.7%
Zestimates within 20% of Sales Price:                  54.3%

Median Error:                                                     18.7%

_____________________________________________________________________

Definitions

Homes on Zillow:
This indicates the percentage of homes for which we have data (e.g., number of bedrooms or bathrooms) in a particular locale. These are the homes you can find via maps or search on Zillow.com.

Homes With Zestimates:
We can only calculate Zestimates for homes where we have certain data, including transactions. This column indicates the percentage of homes in an area with Zestimates.

Within 5% of Sale Price:
This is the percentage of transactions in a location for which the Zestimate was within 5% of the transaction price. For example, in the Portland area, 33.5% of the time the Zestimate was within 5% of the selling price.

Within 10% of Sale Price:
This is the percentage of transactions in a location for which the Zestimate was within 10% of the transaction price.

Within 20% of Sale Price:
This is the percentage of transactions in a location for which the Zestimate was within 20% of the transaction price.

Median Error:
Half of the Zestimates in an area were closer than the error percentage and half were farther off. For example, in Seattle, Zestimates for half of the homes are within 8.2% of the selling price, and half are off by more than 8.2%.