Nielsen has agreed to purchase radio measurement firm Arbitron for a massive $1.26 billion, the company announced today.

The announcement comes on the heels of yesterday’s big news that Nielsen would offer a new form of television ratings based on the reach of the TV conversation on Twitter. Now Nielsen is taking a major step toward owning radio ratings with its Arbitron buy.

“U.S. consumers spend almost two hours a day with radio. It is and will continue to be a vibrant and important advertising medium,” Nielsen CEO David Calhoun said in a statement. “Arbitron will help Nielsen better solve for unmeasured areas of media consumption, including streaming audio and out-of-home. The high level of engagement with radio and TV among rapidly growing multicultural audiences makes this central to Nielsen’s priorities.”

The Arbitron deal — if it passes regulators — is worth $1.26 billion, or $48 per share. That’s a premium of about 26 percent over Arbitron’s closing price on Monday. The two companies combined do about $6 billion in revenue each year.

With terrestrial radio, TV, and new social TV ratings taken care of, perhaps Nielsen will take a step toward better digital radio measurement or break new ground in web measurement, an area that’s mostly dominated by comScore.

Radio station photo via MDGovpics/Flickr