Google hopes to make it easier for marketers to pick appropriate websites for their online ads with a new tool called Ad Planner.

The tool is aimed at advertising agencies' media planners, whose job is to decide where to place their clients' ads.

By feeding Ad Planner with their target audience's demographic information, media planners will get a list of sites that should prove effective marketing vehicles.

"You can drill down further to get more detail like demographics and related searches for a particular site, or you can get aggregate statistics for the sites you've added to your media plan," reads the official blog posting.

Ad Planner data can be exported as .csv files to spreadsheet applications or to Google's own DoubleClick MediaVisor ad campaign management tool.

For now, those interested in using Ad Planner need to submit a request to Google.

Last week, Google announced a similar service called Google Trends for websites that is aimed at a general audience.

Since news of Ad Planner emerged on Monday, it has been suggested that Google is moving into the web measurement market to compete against companies like comScore, Nielsen Online, Hitwise and Quantcast.