What are cities worth? More than big private companies, as it turns out: The value of housing in the nation’s 50 largest metropolitan areas ($22 trillion) is more than double the value of the stock of the nation’s 50 largest corporations ($8.8 trillion).

The market capitalization—or “market cap,” as it is commonly called—is computed as the current share price of a corporation multiplied by the total number of shares of stock outstanding. In theory, if you were to purchase every share of the company’s stock at today’s market price, you would own the entire company.

In roughly similar fashion, we can compute the market capitalization of cities—or at least of their housing stock. We start with Zillow’s estimate of the market value of owner-occupied housing in each of the nation’s largest metropolitan areas which is computed by estimating the current market price of each house in a metropolitan area, and sum that value over all of the owner occupied houses. We also estimate the value of rental housing. For rented units we use a commonly accepted technique of estimating current values based on the income generated from rent. (Americans paid about $535 billion in rent in 2015, according to data compiled by Zillow; we can use this data and some financial formula to estimate the value of rental housing. Details of this calculation are explained below.) Then we add together the value of all owner-occupied housing and the value of rental housing to compute the total market cap of housing in each metropolitan area in the US.

Together, the 50 largest publicly traded private corporations in the United States had a market capitalization of $8.8 trillion at the end of 2015. The total market value of housing in 2015 in the 50 largest metropolitan areas was $22 trillion. For reference, the gross domestic product—the total value of all goods and services produced in the US in 2015—was estimated at $18 trillion. It’s hard to find things measured in trillions of dollars, so we’ve juxtaposed GDP against the market cap of housing and businesses. Keep in mind that the GDP is a flow (trillions of dollars per year) while the value of corporations and housing is a stock (trillions of dollars in value at one-point in time).