Data Coverage and Zestimate® Accuracy
The home data we have compiled to generate a Zestimate home valuation varies by location. Some counties provide all the data we could hope for, but others are lacking such key things as the number of bedrooms and bathrooms, or, in some cases, the square footage of the home. The more data we have, the more accurate the Zestimate. And, we've made it easier for our users to help us improve accuracy by incorporating edited home facts into our Zestimate calculations. In some areas, we might not be able to produce a Zestimate at all, but we do have some basic information on the homes. The tables below show you where we have Zestimates and other home information.
To measure the accuracy of the Zestimates, we've gone back in time and compared the historic Zestimates with the actual transaction prices of homes that sold. The tables below also provide various measures of that accuracy.
A Zestimate home valuation is Zillow's estimated market value. It is not an appraisal. Use it as a starting point to determine a home's value. Learn more.
- Read about Zillow's Rent Zestimate
|County||Zestimate Accuracy||Homes on Zillow||Homes With Zestimates||Within 5% of Sale price||Within 10% of Sale Price||Within 20% of Sale Price||Median Error|
|Essex||Four Stars||230.9K||227.9K||39.5%||64.5%||89.5%|| 6.5%|
|Hampden||Four Stars||137.2K||136.0K||42.6%||66.1%||84.3%|| 6.9%|
|Norfolk||Four Stars||222.4K||215.4K||40.9%||65.9%||88.6%|| 6.5%|
|Suffolk||Three Stars||215.1K||188.0K||28.9%||59.2%||92.0%|| 8.2%|
|Plymouth||Four Stars||175.5K||173.4K||30.0%||63.3%||86.7%|| 7.0%|
|Hampshire||Four Stars||49.4K||48.1K||51.4%||68.6%||85.7%|| 4.9%|
|Middlesex||Three Stars||475.2K||461.5K||27.6%||61.3%||85.8%|| 8.0%|
|Worcester||Three Stars||256.9K||252.3K||35.8%||60.9%||78.8%|| 8.2%|
|Barnstable||Two Stars||157.4K||155.0K||28.0%||52.0%||76.0%|| 9.3%|
Last updated: February 04, 2015
Note: Zestimate accuracy is computed by comparing the final sale price to the Zestimate on or before the sale date. The data herein is computed for the three-month period ending December 30th, 2014.
Click here to download an Excel spreadsheet of this data.
- Zestimate Accuracy/Star Rating:
- This rating is tied to the Median Error in an area.
The ratings are as follows:
- = Best Zestimate
- = Good Zestimate
- = Fair Zestimate
- = Tax assessor's value, or unable to compute Zestimate accuracy
- 0 stars = No valuation
- 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, 38.7% 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 6.5% of the selling price, and half are off by more than 6.5%.