Many people think of start-ups as small companies. However, the biggest start-ups in the United States are actually larger than many public companies. Here are the valuations of the five most valuable start-ups in the country, based on their latest funding rounds.

Graph showing market caps of the five most valuable startups in the U.S.

Data sources: Business Insider, CNN, and CNBC. Chart by author.

It isn't surprising that Uber and Airbnb top that list, since they respectively reshaped the ride-hailing and vacation rental industries. Both have faced growing pains over the past year: Uber was stung by a sexual harassment crisis and an executive exodus, and Airbnb was dogged by allegations of racial discrimination. The two have also clashed with regulators in various countries.

SpaceX is often known as Tesla CEO Elon Musk's "other" company. It supplies space transport services to NASA. Palantir conducts data analysis for U.S. intelligence agencies, hedge funds, and banks, while WeWork provides shared workspaces and other services for freelancers, entrepreneurs, and start-ups.

Uber, Airbnb, Palantir, and WeWork have all expressed interest in going public, but SpaceX has flatly rejected the idea, presumably because its growth remains wildly uneven between launches. Investors who want to find the next hot growth stock should keep an eye out for these potential IPOs.

Leo Sun has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Tesla. The Motley Fool has a disclosure policy.