When Zillow debuted in 2006, the fledgling site bore little resemblance to the real-estate behemoth it is now. There were no options to find an agent, get a mortgage, or request a tour — the search portal couldn't even tell you which homes were actually for sale. There was, however, the Zestimate: a "free, unbiased valuation" for 40 million houses around the US, based on a proprietary algorithm. Half the single-family homes in America suddenly had a dollar figure attached to them, and anyone could take a peek. Zillow's site crashed within hours as a million people raced to ogle at the results.

The initial rush was a sign of things to come. Nowadays, the Zestimate is arguably the most popular — and polarizing — number in real estate. An entire generation of homeowners doesn't know life without the algorithm; some obsessively track its output as they would a stock portfolio or the price of bitcoin. By the time a seller hires a real-estate agent, there's a good chance they've already consulted the digital oracle. For anyone with even a passing interest in the housing market, the Zestimate is a breezy way to take the temperature. Keep tabs on mortgage rates all you want, but they can't tell you that your house has appreciated 20% over the past year, or that your annoying coworker's property is worth more than yours.

Many industry insiders, however, regard the number as a starting point at best and dangerously misguided at worst. Real-estate agents recount arguments with sellers who reject their pricing advice, choosing instead to take the Zestimate as the word of God. One meme likens its disciples to adults who still believe in Santa. Zillow itself lost hundreds of millions of dollars during the pandemic when it relied on its algorithm to buy homes at what turned out to be inflated prices, part of an ill-fated attempt to flip homes at scale.

The Zestimate is just one of a slew of automated valuation models that are increasingly used by banks, investors, and laypeople to estimate the value of homes. No other model, however, has wormed its way into our culture like the Zestimate. The model, like other consumer-facing AVMs, is prone to errors that render it more of an amusement than a serious pricing tool. But while the algo's price-guessing skills may be suspect, it's undeniably elite at one thing: luring people to Zillow-dot-com.

The Zestimate is both everywhere and an enigma. About 104 million homes, or 71% of the US housing stock, have a little dollar figure hovering above them on Zillow's website. One of them is the house in Austin where I was raised until the age of 10. It's not for sale, but right underneath the address, in bold, is the Zestimate. Next to it is a "Rent Zestimate," or the amount the owner could probably charge a tenant each month. You can click to see a graph of its Zestimate over the past decade — the Zillow-fied value of my childhood home rose a staggering 72% from May 2020 to its peak in May 2022 but has since dropped 24% from that top tick thanks to the chill running through the Austin market. In just the past 30 days, the Zestimate has dropped by $4,455. Ouch.

Just how accurate are those numbers, though? Until the house actually trades hands, it's impossible to say. Zillow's own explanation of the methodology, and its outcomes, can be misleading. The model, the company says, is based on thousands of data points from public sources like county records, tax documents, and multiple listing services — local databases used by real-estate agents where most homes are advertised for sale. Zillow's formula also incorporates user-submitted info: If you get a fancy new kitchen, for example, your Zestimate might see a nice bump if you let the company know. Zillow makes sure to note that the Zestimate can't replace an actual appraisal, but articles on its website also hail the tool as a "powerful starting point in determining a home's value" and "generally quite accurate." The median error rate for on-market homes is just 2.4%, per the company's website, while the median error rate for off-market homes is 7.49%. Not bad, you might think.

But that's where things get sticky. By definition, half of homes sell within the median error rate, e.g., within 2.4% of the Zestimate in either direction for on-market homes. But the other half don't, and Zillow doesn't offer many details on how bad those misses are. And while the Zestimate is appealing because it attempts to measure what a house is worth even when it's not for sale, it becomes much more accurate when a house actually hits the market. That's because it's leaning on actual humans, not computers, to do a lot of the grunt work. When somebody lists their house for sale, the Zestimate will adjust to include all the new seller-provided info: new photos, details on recent renovations, and, most importantly, the list price. The Zestimate keeps adjusting until the house actually sells. At that point, the difference between the sale price and the latest Zestimate is used to calculate the on-market error rate, which, again, is pretty good: In Austin, for instance, a little more than 94% of on-market homes end up selling for within 10% of the last Zestimate before the deal goes through. But Zillow also keeps a second Zestimate humming in the background, one that never sees the light of day. This version doesn't factor in the list price — it's carrying on as if the house never went up for sale at all. Instead, it's used to calculate the "off-market" error rate. When the house sells, the difference between the final price and this shadow algorithm reveals an error rate that's much less satisfactory: In Austin, only about 66% of these "off-market Zestimates" come within 10% of the actual sale price. In Atlanta, it's 65%; Chicago, 58%; Nashville, 63%; Seattle, 69%. At today's median home price of $420,000, a 10% error would mean a difference of more than $40,000.

Without sellers spoonfeeding Zillow the most crucial piece of information — the list price — the Zestimate is hamstrung. It's a lot easier to estimate what a home will sell for once the sellers broadcast, "Hey, this is the price we're trying to sell for." Because the vast majority of sellers work with an agent, the list price is also usually based on that agent's knowledge of the local market, the finer details of the house, and comparable sales in the area. This September, per Zillow's own data, the typical home sold for 99.8% of the list price — almost exactly spot on. That may not always be the case, but the list price is generally a good indicator of the sale figure down the line. For a computer model of home prices, it's basically the prized data point. In the world of AVMs, models that achieve success by fitting their results to list prices are deemed "springy" or "bouncy" — like a ball tethered to a string, they won't stray too far. Several people I talked to for this story say they've seen this in action with Zillow's model: A seller lists a home and asks for a number significantly different from the Zestimate, and then watches as the Zestimate moves within a respectable distance of that list price anyway. Zillow itself makes no secret of the fact that it leans on the list price to arrive at its own estimate.

Other sites have their versions of the Zestimate, too — there are actually about 25 different AVMs in the market, says Lee Kennedy, the founder and managing director of AVMetrics, a company known for independently testing these models. Realtor.com will show you three estimates, each from a different AVM provider. Redfin, a Zillow competitor, also has its own model. Kennedy has been studying AVMs for more than three decades, but it wasn't until the advent of Zillow that the masses became aware of them. Consumer-facing AVMs, like the Zestimate or the Redfin Estimate (Restimate?) are meant to be used informally, he says, as casual starting points before consulting real experts. They're not supposed to be used for real pricing, which should be left to the big guys — the "business-to-business" AVMs used by banks, investors, and the government-sponsored enterprises Fannie Mae and Freddie Mac. Lauryn Dempsey, a real-estate agent in the Denver area, gives similar advice to her clients.

"They're tools that provide information," Dempsey says, "but they should not be used in a vacuum to make decisions."

The business-to-business models are so costly to develop, Kennedy tells me, that they'll probably never be offered to regular people for free. But his testing indicates they're much more reliable. His firm has unveiled blind testing that looks at how models perform before taking into account the list price, a method that penalizes those aforementioned bouncy algorithms. The standard measurement breaks down how often the model can get within 10%, in either direction, of the actual selling price. In a highly urbanized area with lots of housing transactions, some of the models can correctly get close to the final selling price about 80% to 90% of the time — "not bad," Kennedy says. AVMs of all kinds work best in areas with a lot of homes that look and feel roughly the same. Cookie-cutter suburbs are heaven; areas with a wide range of home styles and ages, like Boston, pose a greater challenge. The value of a ranch home in the middle of nowhere is even tougher to peg.

So the Zestimate isn't exactly unique, and it's far from the best. But to the average internet surfer, no AVM carries the weight, or swagger, of the original. To someone like Jonathan Miller, the president and CEO of the appraisal and consulting company Miller Samuel, the enduring appeal of the Zestimate is maddening. "When you think of the Zestimate, for many, it gives a false anchor for what the value actually is," Miller says.

Miller is no unbiased observer. Given that he's an appraiser who estimates the value of homes for a living, it should come as no surprise that he's siding with the humans over the robots. But he raises real issues, highlighting the disconnect between the public's continued use of the Zestimate and its actual track record.

I could say that I virtually stalked my childhood home for "research," but let's be real: By the time I scrolled to the bottom of the page, I had fully surrendered to the voyeuristic urges that draw millions of visitors to the Zillow website each month. It's been almost two decades since I've stepped inside the house, and I can only imagine the changes its new owners have made to my old room (sadly, no pics of the interior). But with the aid of Zillow, my trip down memory lane was lined with data: I walked away with intimate knowledge of the home and its occupants. Prior to 2006, no regular person had this kind of power.

The launch of Zillow spawned a whole genre of internet snooping that, if anything, has only intensified in the years since. When I call up John Wake, a former economist and real-estate agent who now writes the newsletter Real Estate Decoded, he reveals that he, too, looked up his childhood home only a few months ago. "That part is really fun," he tells me. Keeping tabs on your own Zestimate, though, can provide less of a thrill. In December 2022, after interest-rate hikes tamped down home prices, Wake shared with his followers on X that his Zestimate was down 18% from May: "YIKES!" In a 2020 column, the Wall Street Journal editor Kris Frieswick opened up about the difficulty of quitting the algorithm: "My self-worth is defined by my Zestimate. Each day I approach Zillow.com filled with hope, and fear." The column reads mostly as tongue-in-cheek, but plenty of people take their number very seriously. As Frieswick pointed out, at least several disgruntled homeowners have actually sued Zillow over Zestimates they said were inaccurate.

Looking up other people's houses, by comparison, is a mostly harmless pastime. Bosses, neighbors, lovers, and exes — all are fair game in the all-seeing eyes of the tool. During the heat of the 2021 homebuying frenzy, a "Saturday Night Live" sendup of a Zillow ad declared: "The pleasure you once got from sex now comes from looking at other people's houses." The skit, which featured a lot of moaning and sultry mood lighting, was mostly about the fantasies of browsing homes for sale on Zillow — as one YouTube commenter observed, "They didn't even get into the naughty pleasure of looking up all your friends' Zestimate values." This kicked off a thread of others chiming in with "guilty!" and lots of cry-laughing emojis. "OMG I thought this was just my kink," another person replied. I imagine all of these people at a raucous dinner party, bonding over their exploits on zillow.com. And here I am, the buzzkill in the corner talking about median error rates.

Virtual spelunking aside, the hazards of the Zestimate are most obvious when a seller actually decides to list their home. Francine Carstensen, a real-estate agent in Alabama, says those in her line of work have a complicated relationship with the Zestimate: "We love it, and we hate it." A lofty estimate might jolt a homeowner into action — "I could sell my house for what?!" — and drive more business her way. But the number can also make it hard to do her job. A few times, she tells me, she's lost clients over a pricing disagreement involving the Zestimate. It can be difficult enough to pry a seller away from their unrealistic expectations without a number on a screen confirming their hopes for a bigger payday.

"I hate it when they tell me, 'Well, this is what Zillow tells me my house is worth,'" Carstensen says. "Because it's very rarely accurate."

Accuracy may not even be the point. It didn't appear to be in 2006, when the beta version of the Zestimate launched. "The Zestimate started out fairly inaccurate, but it didn't matter," Rich Barton, a Zillow cofounder who was then its CEO, recalled in a 2021 podcast episode. "It was provocative." Spencer Rascoff, another cofounder and former CEO, sold his own home in 2016 for 40% less than its Zestimate. The next year, the company offered $1 million to whoever could improve the Zestimate algorithm the most. The winning team, a group of three data scientists working remotely from the US, Canada, and Morocco, beat the Zillow benchmark by 13%.

No misstep appeared more damning, however, than the implosion of Zillow's homebuying business. In 2018, the company launched Zillow Offers, making all-cash offers to sellers looking to move quickly and seamlessly. In theory, Zillow could then turn around and offload the home in short order for a modest fee, plus however much the home had appreciated. The company used a combination of internal algorithms and human analysts to value the home and predict what it could sell for in a few months — in some cases, homeowners could get an immediate cash offer based on their Zestimate with just a few clicks. But the company's forecasts turned out to be way off base. Zillow Offers squandered $422 million in the third quarter of 2021 alone — a Business Insider investigation found that almost two-thirds of the homes listed by Zillow in Atlanta, Phoenix, Dallas, Houston, and Minneapolis were being marketed at a loss. Amanda Pendleton, a Zillow spokesperson, tells me it was the volatility of the market, not the Zestimate, that really led to the program's downfall. Once the losses came to light, the company swiftly shuttered the division and laid off a quarter of its staff.

I remember wondering whether this would be the death knell for the Zestimate, a kind of algorithm-has-no-clothes moment. I was wrong. Zillow and its best-known creation haven't gone anywhere — the company continues to highlight its progress, providing periodic updates as its data scientists tinker away at the formulas. As search portals like Homes.com and Redfin jockey with Zillow for dominance, the Zestimate is too valuable of an asset to give up. People still flock to Zillow for those little numbers next to each home, for the thrill of feasting their eyes upon something that, like salaries, is considered taboo to talk about in person. For Zillow, that's an unequivocal win.

"It's 100% a marketing tool," says Mike DelPrete, a scholar-in-residence at the University of Colorado Boulder who studies the intersection of tech and real estate. "Like, not even 99%. It's a marketing tool."

James Rodriguez is a senior reporter on Business Insider's Discourse team.

Evan Duby had been a real-estate broker for just a few years when he decided to try an unusual method of selling a home. Five buyers were offering to pay similar amounts for one of his listings, a one-bedroom co-op unit in a leafy Brooklyn neighborhood with an asking price of $485,000. Rather than instructing the house hunters to submit their best offers and cross their fingers — as is customary in the US — Duby, with his client's permission, convened an auction. The buyers gathered on a conference call, where they signaled their willingness to pay as Duby raised the price in $5,000 increments. The home sold for $505,000. The sellers walked away satisfied, as did the winning buyers; the losing bidders were disappointed, but at least they knew where they stood. For Duby, it was a revelation.

"I don't know what possessed me," Duby tells me. "I was just sort of trying to see, is there a better way to do this?"

Trying to buy a house can feel like playing a game of poker in which one player holds all the cards. When the seller's agent tells you they're weighing another bid, or even 30 other bids, there's no way to tell if their claim is bluster or fact. When you lose, you may not know whether another $10,000 would have sealed the deal or if your insistence on an inspection tanked your chance. The nagging uncertainty isn't limited to buyers. Even in a hot market, a seller may leave the closing table unfulfilled. Did their request for "highest and best" offers actually yield the highest and best? It's hard to say.

It doesn't have to be this way. In Australia, about a third of homes sell via auctions that wrap up in a matter of minutes. Sellers get to see exactly how far buyers are willing to go to nab their dream home; buyers gain a clear picture of what it takes to win in the market. The openness and simplicity stand in sharp contrast to America's system, in which buyers write blind offers and then pray theirs meets the mark.

In the US, open auctions are usually reserved for swanky mansions or, more often, distressed properties facing foreclosure or extensive repairs. Mention an auction and people are likely to ask what's wrong with the house, or how lavishly expensive it is — or, simply, why? Real-estate agents haven't been too keen on making the process more transparent: Conventional wisdom says that asking buyers to submit their best and final offers will elicit the highest price. A FOMO-filled buyer, the thinking goes, may unknowingly blow the competition out of the water and deliver a windfall for the seller.

Despite all that, the idea of open auctions is more tantalizing than ever. Buyers and sellers are exhausted from years of opaque bidding wars — even these days, with the market substantially cooler than it was a couple of years ago, a lack of inventory means homes may still draw multiple offers. The real-estate industry is notoriously resistant to change, but recent class-action lawsuits have rewritten the rules about how buyers pay their agents and opened the door for more overhauls down the line.

Capitalizing on this feeling, a small cadre of companies are trying to bring versions of the Australian model to the States. They face an uphill battle. Duby, who recently started GoEx, a venture-capital firm focused on real-estate tech, is squarely in the ripe-for-change camp, but even he'll admit it's hard to shift the status quo. For roughly a decade after that first auction, he continued to broker deals the conventional way — for American sellers, Aussie-style auctions were a tough sell. But just because they haven't caught on in the States doesn't mean they're destined to remain a pipe dream. Duby imagines a not-so-distant future in which prequalified buyers bid for homes online as if they were picking up a rare watch on eBay.

"I don't see why we don't do that," Duby tells me. "I don't see how that doesn't help."

The man could be mistaken for a pastor: crisp gray suit, arms stretched toward the heavens, a crowd gathered before him. But he's here to sell real estate, not religion.

"Reflect on this absolutely fantastic opportunity in front of you," he booms in a distinctly Aussie drawl. "Not me — the house!"

With that, the auction begins. The property at stake is a quaint one-story home surrounded by a white picket fence in a suburb of Melbourne. The bidding starts at 1.26 million Australian dollars, but the price climbs as the auctioneer needles buyers to dig deeper: "You know you want to!" In the end, it sells for more than 1.5 million Australian dollars, or about $980,000. The whole thing takes less than half an hour.

I watched all this unfold on TikTok, where the account @AuctionReporters maintains a steady stream of these strangely addictive dispatches from Australia's real-estate market. Every now and then a video like this will go viral among Americans who balk at the ritual. An open auction is so vastly different from the secretive practice here in the States that it can break our brains — in a reply to a similar video on X, someone posted, "this is real???"

Real-estate auctions are a time-honored tradition in Australia, dating back more than 200 years to its days as a British colony. Their popularity varies based on location and the strength of the market: In boom times for home sales, more sellers turn to the gavel — a study by Kenneth Lusht, a professor at Pennsylvania State University, found that in some pockets of Melbourne, auctions accounted for as much as 80% of home sales during periods of particularly strong demand. A later study of sales from 2011 to 2019 in the states of New South Wales and Victoria, home to more than half of the country's population, found that 30% to 40% of listings went to auction during that period.

Auctions are risky, to be sure. Homeowners publicly disclose the terms and privately set a reserve price, or the minimum amount they'd accept — if the bidding doesn't reach that figure, the house doesn't sell. A study published in 2022 described such a house as carrying a "stench of failure" and found that it was more likely to sell at a discount later. An auction is basically impossible to stop once it's in motion, and sellers may not always be pleased with the results. But in times of healthy demand for homes, auctions can deliver benefits for sellers looking to ride out the frenzy. The study on the risks of auctions also found significant upsides: Successful sales tended to achieve prices that were 1.2% higher than comparable "private treaty" sales, in which sellers set an asking price and then wait for bids to roll in. A separate 2010 study of a broad swath of Australian home sales also found that auctions tended to yield higher selling prices than the alternative.

It's hard to imagine regular homes in the US trading hands this way. But a handful of companies have proposed a middle ground between the public spectacle of Australia's auctions and America's behind-closed-doors strategy. Final Offer, in Massachusetts, is one online marketplace that mediates auction-ish sales. A real-estate agent can list their seller's property on the platform, specify their asking price and their terms, and input a "final offer price" and specific terms of sale, or the amount a buyer can agree to pay in order to stop the bidding and win outright (similar to eBay's "Buy It Now" feature for auctioned items). When a buyer makes a qualifying offer, the clock starts ticking: The seller can choose to reveal the price and terms of any offer in contention, and interested buyers can try to exceed the bids before the window closes.

Here's a real example: Late this summer, the owner of 5818 Ipswich Road, a two-bedroom home built in 1951 in Bethesda, Maryland, listed it on Final Offer for $650,000. Buyer 1 submitted an offer of $658,125, and the seller agreed to take it if no better offers came in over the next three days. Other buyers soon entered the picture: Buyer 2 bid $661,500. Buyer 1 responded by going $3,000 higher. Over the next two days, Buyers 3, 4, and 5 threw their hats in the ring, and the price climbed above $800,000. At the eleventh hour, Buyer 6 emerged with a bid of $810,573. Then came the kicker: Buyer 1 made the "final offer" of $850,000, ending the bidding process. The entire saga is available for anyone to see on Final Offer's website.

In real estate, it turns out, a little transparency goes a long way. "You're giving buyers information they've never had before about what the seller really wants," says Tim Quirk, who cofounded Final Offer and serves as its chief strategy officer. In a typical sale, spurned buyers rarely walk away knowing what would have won — maybe with that knowledge they would have been willing to up their price or adjust their terms, perhaps waiving an inspection and agreeing to buy the home as is. And sellers, even when they take a deal, never quite know if someone might have gone even higher if they knew what they were up against. "What ultimately ends up happening is you get remorse on both sides of the table," Quirk tells me.

Final Offer is still small — Quirk said that a little more than 1,000 homes had sold on the platform in the two years since it started. Sellers on the site don't have to disclose the prices and terms of offers that come in and can opt to let buyers see only that other offers have been made. But Quirk tells me more than 80% of sellers choose to make the bids public.

SparkOffer, which last year was acquired by Auction.com, is another platform that aims to give buyers a sense of their competition. The site shows buyers how many offers they're competing with and is beta testing a new feature that assigns each bid a score based on its price and proposed terms; when other bids come in, buyers get to see how their score stacks up. They don't know the exact details, but they'll at least have a sense of where they stand and what it might take to climb the ranks. Sellers get to outsource the messy back-and-forth of negotiations to a platform that prods buyers to sweeten their offers — while preserving the possibility that a buyer may unwittingly overbid.

Neither Quirk nor Mike Russo, the founder of SparkOffer, thinks of his platform as an auction exactly. For one, home sales aren't just about price. When a buyer makes an offer, they can propose contingencies, or conditions that need to be met in order for the sale to close. They might request an inspection and ask the seller to make repairs if something comes up. They could retain the right to back out of the deal if an appraiser values the home lower than expected. The buyer could also ask for concessions, or some money back from the seller to cover stuff like closing costs. At the height of the pandemic-era homebuying frenzy, some desperate buyers threw caution to the wind and waived these contingencies — they were simply tired of losing. Bottom line, not all buyers are equal in the eyes of a seller.

To reach widespread acceptance in the US, an auction-esque model would have to let sellers choose the terms that work best for them. Most important, new marketplaces would have to convince American sellers — and their agents — to turn away from tradition. That kind of cultural shift is a tall order. "I don't think the American consumer is ready for auctions yet," says Rob Hahn, the CEO of Decentre Labs, a company he started in 2022 to bring online real-estate auctions to the masses.

But it's unclear who really benefits from the system we have. Buyers rarely get feedback that could help them make stronger offers in the future. Maybe as a seller you'll get lucky and a buyer with blinders will overpay for your house — but that approach is shortsighted, since most sellers have to turn around and purchase another home. The biggest beneficiaries, Duby tells me, may just be real-estate agents: When buyers and sellers are shuffled through this murky process, they'll look for a professional guide.

Australia isn't a perfect analog for the US. The country doesn't have a system of local databases where most homes are listed — as we do in the States with the multiple listing services — which makes it tougher for buyers to find homes. And while most sellers are represented by agents, most buyers are not. But still, hardly anyone is proposing we copy and paste this Australian tradition. The country's real-estate market does tell us, however, that another way is possible.

American consumers may not be ready for auctions yet, Hahn told me. "But it doesn't mean that it's not going to happen at some point."

James Rodriguez is a senior reporter on Business Insider's Discourse team.