Most people who have spent any time on Zillow have seen a Zestimate. It sits right at the top of every listing, next to the address and the photo count. For a lot of buyers and homeowners, it's the first number they associate with a home's value. What it actually represents is worth understanding before you put much weight on it.
What Zillow Says a Zestimate Is
Zillow describes the Zestimate as an estimate of a home's market value, not an appraisal. It cannot be used in place of an appraisal, and Zillow says as much directly on its Zestimate page.
The number is generated by a neural network model that pulls from multiple data sources: county and tax assessor records, direct feeds from hundreds of multiple listing services and brokerages, and user-submitted data. According to Zillow, the model incorporates home characteristics like square footage, bedroom and bathroom count, and location; on-market data including listing price, comparable homes, and days on market; off-market data from tax assessments and prior sales; and market trends including seasonal shifts in demand.
Zillow publishes Zestimates for 118 million homes across the country. The model updates multiple times per week.
On-Market vs. Off-Market: Two Very Different Numbers
This is where most people get tripped up, and it's the most important distinction Zillow makes about its own accuracy.
When a home is listed for sale, the Zestimate gets a significant advantage: it can incorporate the listing price, the listing description, and data about comparable active and recently sold homes. That additional signal makes the estimate considerably more precise.
For homes that are not on the market, the model has no listing data to work with. It relies entirely on public records, tax assessments, prior sales, and whatever home facts have been submitted or are on file. Public records are often incomplete, delayed, or missing significant updates like a finished basement, an added bathroom, or a kitchen renovation.
Zillow's own accuracy data, last updated March 6, 2026, highlights the gap.
Nationally: - On-market homes: median error rate of 1.74% - Off-market homes: median error rate of 7.20%
At a national level, more than 95% of on-market homes have a Zestimate within 10% of the final sale price. For off-market homes, only about 62% fall within 10%.
What the Numbers Look Like for Philadelphia and Pennsylvania
The national figures are one thing. The Philadelphia metro and Pennsylvania specifically tell a different story.
Philadelphia metro (on-market): - Median error rate: 2.16% - Within 5% of sale price: 80.30% of homes - Within 10% of sale price: 94.93% of homes
Philadelphia metro (off-market): - Median error rate: 7.88% - Within 5% of sale price: 34.63% of homes - Within 10% of sale price: 58.88% of homes
That means for homes sitting off-market in the Philadelphia area, Zestimates are within 10% of the actual sale price only about 59% of the time. Four in ten off-market Philadelphia homes have a Zestimate that misses the eventual sale price by more than 10%.
Pennsylvania as a whole runs similarly:
Pennsylvania (on-market): - Median error rate: 2.17% - Within 5%: 79.59% - Within 10%: 94.89%
Pennsylvania (off-market): - Median error rate: 9.32% - Within 5%: 30.42% - Within 10%: 52.55%
Statewide for off-market homes, only about half of Zestimates land within 10% of sale price.
Why Off-Market Accuracy Is Lower in Older, Denser Markets
Zillow notes that accuracy is directly tied to the amount of data available for a home and its surrounding area. That dependency matters a lot in a market like Northwest Philadelphia.
Homes in Mt. Airy, Chestnut Hill, Germantown, and the surrounding neighborhoods tend to be older stock with varied histories. A rowhome might have been subdivided and recombined. A Victorian twin might have had a third floor added decades ago that never made it into county records accurately. A home might have a finished carriage house or a significant renovation that never triggered a reassessment. None of that shows up in a Zestimate unless it was reported and recorded.
Zillow acknowledges this directly: if home data is incorrect or incomplete, the Zestimate may not reflect reality. Homeowners can update their home facts on Zillow, and Zillow encourages reporting updates to the local tax assessor so those changes are reflected in public records.
What Happens When a Home Goes On the Market
Zillow addresses this specifically in its FAQ. When a home is listed, new data becomes available to the algorithm, and the Zestimate typically adjusts. In some cases the change is significant, because the model is now incorporating the listing price and comparable active listings rather than relying solely on tax records and historical sales.
One edge case worth knowing: if a home has been listed for a full year without selling, Zillow transitions it back to off-market valuation methods. That transition can produce a sharp difference between the list price and the Zestimate.
What a Zestimate Is Not
Per Zillow, a Zestimate is not an appraisal, cannot be used to secure a loan, and is not designed to set or drive a listing price. Zillow's own guidance recommends supplementing the Zestimate with a professional appraisal, a visit to the home, or a comparative market analysis from a real estate agent.
A CMA is different from a Zestimate in a meaningful way. It's prepared by an agent with direct knowledge of the local market, recent sales that are truly comparable, and firsthand awareness of factors that don't appear in any database. There's no substitute for sitting down and looking at the actual comparables for your street.
If you're buying or selling in Northwest Philadelphia or Montgomery County and want to talk through what a home is actually worth, feel free to reach out.
Henry is a Philadelphia-based REALTOR® serving buyers and sellers in Northwest Philadelphia and Montgomery County, PA. Questions? Get in touch.
