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OpenTable, the San Francisco company that lets you make online reservations at any of 8,500 restaurants, said it it is seating three million diners a month, which means it’s making some serious cash — in the multiple millions of dollars a month, based on back-of-the-envelope math.

The company also has released mobile version of its site (it’s about time, you’d think, given this service should be ideal for people on the go). See screenshots.

OpenTable is apparently growing quickly. The 3 million seated is up 50 percent from last year.  The company charges a fee of $1 per diner seated on reservations made through its website. It charges 25 cents per diner if it handles reservations made directly from a restaurant’s own website. The company doesn’t say whether users book more through OpenTable or through restaurants directly, but does say that a lot of people book through restaurants.

Let’s do some conservative math: If it books 80 percent through its restaurant partner sites, that means it charges 25 cents on about 2.4 million seated, and then charges $1 on the remaining 20 percent, or 600,000 seated, through its own site. So OpenTable is making $1.2 million a month, or $14.4 million a year – at least. And with location-based services being made possible with new GPS-enabled phones, you’d expect OpenTable to be a growth company — since GPS will let people search more easily for restaurants in their neighborhood, and then book them. So you can see OpenTable headed to $100 million over time.

The company looks strong because it’s up against few competitors. It’s also not just a web site. It licenses both software and hardware to restaurants, and helps those locales manage their clientele lists (other companies sell this customer relations management software, but few have the consumer-facing service as well). OpenTable partners with sites like Yelp and Citysearch, offering those review sites a way for users to make reservations.

OpenTable is sending confidential questionnaires to users requesting their feedback, letting them rate restaurants on food, service and ambiance. This, in turn, lets OpenTable provide “best of” lists, thus starting a slow encroach on the turf those review sites. “It’s the first step to user generated content,” said spokesman Scott Jampol.

Based on this latest interview, I wouldn’t be surprised if OpenTable files to go public next year (Benchmark and other investors have pumped $50 million into the eight-year old company, so they’ll be eager to do this).

The release of the mobile version is a milestone of sorts. OpenTable actually released its first mobile version back in 2000, when everyone released mobile versions because it was the thing to do. OpenTable outsourced the task to Oracle, which had a division called Oracle Mobile –no longer around. Barely anyone had phones that supported the WAP site, though (the Palm VII was one of the few that worked with it). OpenTable then drew back, and stayed on the sidelines this whole time — not seeing any meaningful traction anywhere by transaction-oriented web sites on mobile phones. But with the new phones like the iPhone arriving last year, the company decided to develop the latest version. It has been out for a month, but the company made no announcement until now.

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