When Josh Lowensohn moved into a new two-bedroom apartment recently, he was nervous that he might be paying too much for it. Then he went to Rentometer.com, where he typed in his rent, address, and the number of units in his building, and found he was paying a reasonable market rate.

The Rentometer “made me feel a little better, because we’re definitely paying more than my last place,” Mr. Lowensohn said.

RELATED: Check Your Rent on ‘Rentometer’.

The Web site, which provides users with colorful charts that mark an apartment’s rent compared with others in the neighborhood on a spectrum of high, medium, and low, has attracted 2 million unique visitors since launching in January. It is part of an online property management service, Rentomatic, which last week announced plans to expand its services in 10 states, including New York. The company is geared toward part-time landlords, and enables tenants to pay their rent online, record their rental history, and send messages to their landlords.

The Rentometer provides a glimpse into rental data rarely seen by consumers, the co-founder of real estate Web site PropertyShark.com, Ryan Slack, said. “It’s very hard to get clear information on what rents are,” he said.

The Web site was the natural response to questions landlords would often pose to the company, the co-founder of Rentomatic, Allison Atsiknoudas, said. “They regularly asked us, how much should I be charging for rent?” she said.

When the company launched the Rentometer, it wasn’t prepared for the avalanche of users that followed a mention on the Web site Digg.com. “We got bombed,” Ms. Atsiknoudas said. “We weren’t expecting that much traffic that quickly.”

The site is popular because it provides “one stop for comparison,” Mr. Lowensohn, who found out about the site when he reviewed it for the Web site Webware.com, said.

The Rentometer works by aggregating real estate listings data from landlords who are members of the site, as well multiple other real estate Web sites, Ms. Atsiknoudas said. The data is then “cleansed” by an algorithm. She said the company also has worked with a company called Location Inc. to help integrate neighborhood boundaries into data sets.

A new version of the Rentometer, which should be ready later this year, will include more analysis, historical trends, and an easier way to browse listings, she said.

Still, the site’s popularity will depend on the accuracy of its data. For New York City, in particular, data may be skewed by the large number of rent-controlled apartments, the director of the tenants advocacy organization Met Council on Housing, Jenny Laurie, said.

“I don’t know how accurate it is,” Ms. Laurie said, but added that in concept, the service could help tenants get a realistic picture of rents in their neighborhood. “The more information the consumer has, the better decision they can make,” she said.

In addition, the Rentometer may result in higher rents, as landlords grow better informed of what their neighbors are charging in rent, Mr. Slack said.

The information, while useful, is not completely new. There is some transparency in rents on Craigslist.com, Mr. Slack said.

“After five minutes on Craigslist, you pretty much know what is and isn’t a good rent,” he said, but added that the site would have been useful when he first arrived in New York.

Ms. Laurie agreed. “Tenants who are moving into New York from out of town are diving into a shark pool,” she said. “The more information they have to help them in the real estate market, the better.”