MySQLI STMT Num_rows

I've been having a problem with the prepared statements in MySQLI OOP. The problem is that I don't know how to use the num_rows() method correct. If I use var_dump to see the result of the variables I will get NULL because there is no data.

 $stringMail    = trim($_POST['mail']);
 /* 
 .....
 */
 $stmt = $mysqli->prepare("SELECT email FROM users WHERE email = ? ");
 $stmt->bind_param('s', $stringMail);
 $stmt->execute();
 $stmt->store_result();
 $stmt->bind_result($usersMail);
 $iaMail = $stmt->num_rows;
 $stmt->close();

Sorry for the bad English, I'm Dutch and my English isn't the very best... Thanks for helping!

You don't need it. As you have $usersMail already.