Two reports are out today on the size of the Internet advertising market. The Interactive Advertising Bureau has a preliminary estimate of $21.1 billion for U.S. Internet ads in 2007, a 25 percent increase over 2006. (For the fourth quarter of 2007, it is estimating $5.7 billion for the size of the industry, up from $5.2 billion in the third quarter).

Meanwhile, the Kelsey Group puts U.S. Internet advertising at $22.5 billion for 2007 (IDC, as previously reported, is at the high end with $25.5 billion).

The Kelsey Group also provides a global estimate of $45 billion for Internet advertising, which is 7.4 percent of the total $600 billion global advertising market. That compares to a 6.1 percent share of global advertising for online ads in 2006. And for what it’s worth (not much) it is forecasting global internet advertising to reach $147 billion in 2012. These forecasts are always wrong, but the 2007 numbers are helpful.