How I can find the email based on google plus id?

Question

Based on a url like

https://plus.google.com/123456789012345678901 # dummy number

How I can find the gmail username?

Answer 1

If you are developing an app or script, you can use Oauth 2.0 and request the userinfo.email scope.

For details, see https://stackoverflow.com/a/7836385/1354939

This does require that you configure a Google APIs Console project and create a OAuth 2.0 client ID.

Answer 2

You can't. For privacy reasons, having someone in a circle or knowing their Google Plus id should not be enough to get an email address.