SAN FRANCISCO — Private companies like Uber and Snapchat are not legally required to share financial information such as their valuation or the number of shares held by investors. For investors, that lack of transparency is something of a hazard.

Now Equidate, a market for trading shares of private companies, plans to lift the veil on some of that information. The company, based in San Francisco, said on Monday that it would make a host of data about private companies free and open to the public, drawing from corporate filings that include the prices that investors have paid to invest in the companies. Equidate also plans to release tools that shareholders can use to calculate the value of their private-company stock holdings.

“We’re giving away this data to help create a more robust market and to give stakeholders like employees, investors and other shareholders a better way to understand the value of their stock,” said Sohail Prasad, a co-founder of Equidate.

The lack of transparency around private companies, along with a scarcity of shares, has kept the market for start-up stocks fairly small. About $750 million of private shares are actively listed for sale on the Equidate marketplace. By comparison, the total public stock market in the United States includes trillions of dollars of shares.