Google Ad Planner will help:
1. Identify websites your target customers are likely to visit
- Define audiences by demographics and interests.
- Search for websites relevant to your target audience.
- Access unique users, page views, and other data for millions of websites from over 40 countries.
2. Easily build media plans for yourself or your clients
- Create lists of websites where you’d like to advertise.
- Generate aggregated website statistics for your media plan.
From Google Ad Planner
Generally, it will let you know how popular a website is and how many users they have.
Unfortunately, they have not published their API yet. If anyone has any update from them regarding the Google Ad Planner API, kindly let us know.