NEW YORK — Nielsen, the dominant source of TV ratings, on Tuesday said it had agreed to buy Arbitron for about $1.26 billion to expand into radio measurement.

Arbitron pays 70,000 people to carry around gadgets that register what stations they’re listening to. Since Nielsen also collects cash register data, chief executive David Calhoun said buying Arbitron will let Nielsen be a one-stop shop for advertisers who want to know how the radio advertising they buy affects product sales.